IRIG-B Time Code

ELI5

Imagine a clock on the wall of a control room — not just any clock, but one connected to GPS satellites so it is accurate to within a millionth of a second. Now imagine every device in the building — every relay, every meter, every controller — has a wire running from that clock so they can all set their own watches.

When something goes wrong and alarms fire from five different devices within the same millisecond, the investigators can reconstruct exactly what happened first, what happened second, and what caused what. Without that clock, five devices say “something happened” but nobody can prove the order.

IRIG-B is that wall clock. It is a dedicated timing signal — a pulse train on a coaxial cable — that keeps every device honest. The protection system works without it, but you cannot prove what it did.

Outsider’s Guide

What It Does

IRIG-B is a serial time code that distributes GPS-accurate time to every intelligent electronic device (IED) in a substation or data center power system. It encodes the current time as a repeating 1-second frame of 100 pulses, transmitted over dedicated coaxial cable or fiber optic from a central GPS receiver.

Every protection relay, RTU, meter, and controller receives this signal and uses it to timestamp its internal event logs. When a fault occurs and protection operates, the Sequence of Events (SOE) records from all devices can be correlated to reconstruct exactly what happened, in what order, with sub-millisecond precision.

Where It Sits in a DCA Architecture

   GPS Satellites (L1/L2 signals)
            |
     [ GPS Antenna ] ← rooftop, clear sky view
            |  coax
     [ GPS Receiver ] ← SEL-2488, Arbiter 1094B, Microchip GridTime 3000
            |
     [ IRIG-B Distribution Amplifier ]
        /   |   |   \
     coax  coax coax coax ← star topology, one cable per device
       |    |    |    |
     [Relay][Relay][RTU][Meter]  ← each IED has an IRIG-B input (BNC or terminal block)

One GPS receiver per substation. One coax cable per IED. Sub-millisecond accuracy everywhere.

Why It Matters to a PM

Time sync is invisible infrastructure — nobody thinks about it until a fault investigation fails because the timestamps do not line up. A protection event that trips five breakers in 8 milliseconds is unanalyzable without synchronized clocks. IRIG-B is the accountability layer: it does not make the protection work, but it proves what the protection did.

The cost is physical: dedicated coaxial cable from the GPS receiver to every IED, a GPS antenna with clear sky view, and a receiver that costs several thousand dollars. The alternative — inaccurate or missing timestamps — costs far more during a regulatory investigation or insurance claim.

Red Flags in Design Submittals

When reviewing a subcontractor’s design package, watch for these indicators that time sync has been underscoped or forgotten:

• No GPS antenna on the roof plan. IRIG-B starts with a GPS receiver, and the receiver starts with an antenna. If the antenna is not on the architectural drawings, nobody has scoped the roof penetration, antenna cable routing, lightning protection, or surge suppression at building entry. The entire timing system is missing from the design.
• “NTP is sufficient for protection-grade timestamping.” NTP achieves 1–10ms accuracy under ideal conditions, but network congestion, switch queuing, and asymmetric routing make it unreliable for SOE correlation. IRIG-B provides ±1μs accuracy independent of the data network. Any proposal that substitutes NTP for IRIG-B on protection IEDs is underspecifying the timing architecture.
• IRIG-B distribution is a single line item. A facility with 30+ IEDs needs a distribution amplifier tree — GPS receiver feeding distribution amplifiers, each amplifier feeding a subset of IEDs. Budget for the receiver, amplifiers, coax home runs, BNC connectors, and cable routing. A single line item means the distribution architecture has not been designed.
• No IRIG-B format specification. Multiple IRIG-B formats exist — AM modulated, DC Level Shift (DCLS), with or without IEEE 1344 extensions. SEL relays require DCLS with IEEE 1344 for year and time quality data. A submittal that says “IRIG-B” without specifying the format and extensions is setting up a commissioning mismatch.

Visual Explanation

1. Time Distribution Architecture

How IRIG-B fits into a dual-path DCA timing architecture — GPS-sourced IRIG-B for legacy IEDs alongside PTP for modern Ethernet-native devices:

                         GPS Constellation
                        /        |        \
                   L1/L2 signals to ground
                        |                 |
               [ GPS Antenna A ]   [ GPS Antenna B ]     ← redundant, separate roof locations
                        |                 |
               [GPS Receiver A]    [GPS Receiver B]      ← SEL-2488, Arbiter 1094B
                   |       |          |       |
              IRIG-B    PTP GM   IRIG-B    PTP GM        ← dual outputs: analog + Ethernet
                |          |        |          |
           [ Dist Amp A ]  |   [ Dist Amp B ]  |         ← IRIG-B distribution amplifiers
            /  |  |  \     |    /  |  |  \     |
          coax runs    PTP      coax runs    PTP         ← star IRIG-B / switched PTP
           to IEDs   network     to IEDs   network
               |          |        |          |
          [ Legacy    [ Modern    [ Legacy    [ Modern
            IEDs ]      IEDs ]      IEDs ]      IEDs ]

Primary path (left): GPS Receiver A provides IRIG-B for legacy relays and PTP for modern devices. Backup path (right): GPS Receiver B with independent antenna provides automatic failover. Key principle: no single failure — antenna, receiver, cable, or network — desynchronizes the entire facility.

2. IRIG-B Signal Waveform

DC Level Shift (DCLS) encoding — the dominant format in modern substations. Each bit is a pulse with width encoding its value:

  Voltage
  High ─┐     ┌─┐   ┌───┐ ┌─────────┐ ┌─┐
        │     │ │   │   │ │         │ │ │
        │     │ │   │   │ │         │ │ │
  Low ──┘     └─┘   └───┘ └─────────┘ └─┘
        |     |     |     |           |
        2ms   2ms   5ms   8ms         2ms
        "0"   "0"   "1"   Reference   "0"
                          (Position ID)

  Pulse Widths:
  ┌──────────────────────────────────────────────┐
  │  Logic 0:     2ms high, 8ms low  (20% duty)  │
  │  Logic 1:     5ms high, 5ms low  (50% duty)  │
  │  Reference:   8ms high, 2ms low  (80% duty)  │
  │  Position ID: 8ms high, 2ms low  (same as ref)│
  └──────────────────────────────────────────────┘

  AM (Amplitude Modulated) — 1kHz carrier version:
  ┌──────────────────────────────────────────────────┐
  │  Same pulse width encoding, but each pulse is a   │
  │  burst of 1kHz sine wave. Amplitude ratio:         │
  │  High = full amplitude, Low = 10-30% amplitude.    │
  │  Detected by envelope demodulation.                │
  │  Accuracy: microsecond-class (vs nanosecond DCLS)  │
  │  Distance: up to 150m coax (vs 40m for DCLS)       │
  └──────────────────────────────────────────────────┘

3. One-Second Frame Format

100 bits at 100 pulses per second = exactly 1 frame per second. BCD-encoded time fields with position identifiers marking boundaries:

  Bit:  0    9   19   29   39   49   59   69   79   89  99
        |    |    |    |    |    |    |    |    |    |    |
  P0   Sec  P1  Min  P2  Hour P3  Day  P4  Day  P5  CF1 P6
  ref  BCD  ref  BCD  ref  BCD  ref BCD  ref (H)  ref     ref

  ┌─────────────────────────────────────────────────────────────┐
  │ Bits  0-9:   P0 (ref) + Seconds (BCD: 0-59)                │
  │ Bits 10-19:  P1 (ref) + Minutes (BCD: 0-59)                │
  │ Bits 20-29:  P2 (ref) + Hours (BCD: 0-23)                  │
  │ Bits 30-39:  P3 (ref) + Day of Year low digits (BCD: 0-99) │
  │ Bits 40-49:  P4 (ref) + Day of Year hundreds (BCD: 1-3)    │
  │ Bits 50-58:  P5 (ref) + Control Function 1 (CF1)           │
  │              CF1 includes: year (BCD), leap second, DST     │
  │ Bits 59-68:  P6 (ref) + Control Function 2 (CF2)           │
  │ Bits 69-79:  P7 (ref) + TFOM + reserved                    │
  │ Bits 80-97:  P8 (ref) + Straight Binary Seconds (SBS)      │
  │ Bits 98-99:  P9 (ref) + P0 of next frame                   │
  └─────────────────────────────────────────────────────────────┘

  Position Identifiers (P0-P9): 8ms reference pulses that mark
  the start of each 10-bit group. P0 of one frame immediately
  follows P9 of the previous frame — the two consecutive 8ms
  pulses form the "on-time" marker for the 1-second epoch.

4. Accuracy Cascade

Where accuracy is gained and lost from satellite to IED internal clock:

  GPS Satellite             ← +/-10 ns (atomic clock)
       |
       | L1/L2 signal propagation
       v
  GPS Antenna               ← signal quality depends on sky view, multipath
       |
       | coax (antenna cable loss: 3-5 dB per 30m)
       v
  GPS Receiver              ← +/-40 ns avg, +/-100 ns peak (SEL-2488)
       |                       disciplined oscillator maintains accuracy
       |                       during brief GPS outages
       |
       |── IRIG-B DCLS ──→  IED (DCLS input)     ← +/-100 ns to +/-1 us
       |   (coax, 40m max)   well-defined edges, constant propagation delay
       |                     accuracy depends on cable length and termination
       |
       |── IRIG-B AM ────→  IED (AM input)        ← +/-1 us to +/-10 us
       |   (coax, 150m max)  envelope detection introduces demodulation jitter
       |                     adequate for T1 class (+/-1ms) with large margin
       |
       |── PTP/1588 ─────→  IED (Ethernet)        ← +/-100 ns to +/-1 us
       |   (network switches) accuracy depends on switch TC/BC support
       |                     and network asymmetry compensation
       |
       └── NTP ──────────→  Host (Ethernet)       ← +/-1 ms to +/-10 ms
           (any network)     adequate for HMI, historian, non-protection

  ┌─────────────────────────────────────────────────────────────────┐
  │ IEC 61850-5 Accuracy Classes:                                   │
  │ T1: +/-1 ms   — SOE timestamping (IRIG-B, PTP both qualify)    │
  │ T2: +/-100 us — Zero-crossing detection, synchrocheck           │
  │ T3: +/-25 us  — Instrument transformer synchronization          │
  │ T4: +/-4 us   — High-accuracy measurement                      │
  │ T5: +/-1 us   — Merging units, synchrophasors (PTP required)   │
  └─────────────────────────────────────────────────────────────────┘

Cheat Sheet

Format Code Reference

IRIG-B format codes follow the pattern Bxyz where B = 100 pps rate, x = modulation, y = carrier, z = data content.

Code	Modulation	Carrier	Data Content	Substation Use
B000	DCLS	None	BCD TOY + CF + SBS	Legacy installations
B001	DCLS	None	BCD TOY + CF	Older IEDs
B002	DCLS	None	BCD TOY only	Minimal data
B003	DCLS	None	BCD TOY + SBS	No control functions
B004	DCLS	None	BCD TOY + Year + CF + SBS	Modern standard
B005	DCLS	None	BCD TOY + Year + CF	Modern standard
B006	DCLS	None	BCD TOY + Year	Minimal with year
B007	DCLS	None	BCD TOY + Year + SBS	No control functions
B120	AM	1 kHz	BCD TOY + CF + SBS	Long cable runs
B121	AM	1 kHz	BCD TOY + CF	Legacy AM
B122	AM	1 kHz	BCD TOY only	Minimal AM
B123	AM	1 kHz	BCD TOY + SBS	AM without CF

BCD TOY = Binary Coded Decimal Time of Year (seconds, minutes, hours, day of year). CF = Control Functions (IEEE 1344 extensions: year, leap second, DST, UTC offset, TFOM). SBS = Straight Binary Seconds (17-bit count of seconds since midnight). Recommendation: Use B004 or B005 for all new installations. Includes year and IEEE 1344 extensions.

Cable Specifications

Parameter	RG-58 Coax (DCLS)	RG-58 Coax (AM)	Fiber Optic	Shielded Twisted Pair
Impedance	50 ohm	50 ohm	N/A	~100 ohm
Connector	BNC	BNC	ST/SC	Terminal block
Max distance	~40m (130 ft)	~150m (500 ft)	2-10 km	~300m (AM)
Propagation delay	3-5 ns/m	3-5 ns/m	~5 ns/m	~5 ns/m
Termination	50 ohm at load	50 ohm at load	N/A	Match impedance
Typical part	RG-58A/U	RG-58A/U	MM 62.5/125	Belden 9841
SEL cable	SEL-C953	SEL-C953	SEL-C863	—

Termination rule: every DCLS cable run must be terminated with a 50-ohm resistor at the receiving end. Unterminated cables cause reflections that corrupt the signal. AM cables are more tolerant but still benefit from proper termination.

IEC 61850-5 Time Accuracy Classes

Class	Accuracy	Application	Typical Source
T1	+/-1 ms	SOE timestamping, event logging	IRIG-B (DCLS or AM), PTP
T2	+/-100 us	Zero-crossing detection, synchrocheck	IRIG-B DCLS, PTP
T3	+/-25 us	Instrument transformer sync	PTP with hardware timestamping
T4	+/-4 us	High-accuracy measurement	PTP with TC/BC switches
T5	+/-1 us	Merging units, synchrophasors (SV)	PTP with dedicated profile

Key insight: IRIG-B DCLS comfortably meets T1 and T2 — sufficient for SOE and most protection applications. T3 through T5 require PTP. This is the primary driver for PTP adoption in modern process-bus substations.

GPS Receiver Quick Reference

Feature	SEL-2488	Arbiter 1094B	Microchip GridTime 3000
GNSS	Multi-constellation (GPS + GLONASS + Galileo + BeiDou)	GPS (12-channel)	Multi-constellation (184-channel)
IRIG-B outputs	DCLS + AM	DCLS + AM	DCLS + AM
PTP support	Optional PTP Grandmaster	No	Yes (10x 1GbE)
NTP server	Yes	No	Yes
Accuracy	+/-40 ns avg, +/-100 ns peak	+/-100 ns (250 ns spec)	Vendor spec (check datasheet)
Anti-spoofing	Dual-constellation validation	Basic	BlueSky technology
Operating temp	-40 to +85C	-20 to +60C	-40 to +70C
PRP support	No	No	Yes (dual Ethernet)
Key advantage	SEL ecosystem integration, AcSELerator config	Simple, proven, low cost	All-in-one timing platform

Pulse Width Encoding

Pulse Type	Duration	Duty Cycle	Meaning
Logic 0	2ms high, 8ms low	20%	Binary zero
Logic 1	5ms high, 5ms low	50%	Binary one
Reference	8ms high, 2ms low	80%	Position identifier (P0-P9)
On-time marker	Two consecutive reference pulses	—	Start of 1-second frame (P9 + P0)

Oscilloscope verification: Set trigger on rising edge, timebase to 2ms/div. A healthy DCLS signal shows clean 2ms, 5ms, and 8ms pulses with sharp transitions. Rounded edges or ringing indicate cable termination problems.

Best Practices

1. Design the Antenna First

The GPS antenna determines the accuracy ceiling for the entire time distribution system. Everything downstream — receiver, distribution, IED sync — is bounded by the quality of the GPS signal.

Requirements: Clear sky view with no obstructions above 15 degrees elevation. Minimum 160-degree hemisphere. Mount on the roof, away from HVAC equipment, lightning rods, and large metal surfaces that cause multipath reflections.

Why it matters: A partially obstructed antenna intermittently loses satellite lock. The receiver falls back to its internal oscillator, which drifts. If the outage is brief, nobody notices until a fault investigation reveals timestamps that are off by several milliseconds. If the antenna was designed into the building from the start, this does not happen.

Lightning protection: The antenna cable is a direct path from the roof to the equipment room. Install a coaxial surge protector at the building entry point. Ground it to the facility ground bus per IEEE 1692.

2. Use DCLS for Accuracy-Critical IEDs

Format B004 or B005 (DCLS with year and control functions) is the modern standard. DCLS provides nanosecond-class accuracy because the receiver decodes well-defined digital edges rather than demodulating an analog envelope.

When to use AM instead: Cable runs longer than 40m where DCLS signal degrades, or legacy IEDs that only accept AM input (rare in current-generation equipment). Some GPS receivers output both DCLS and AM simultaneously — use DCLS for nearby IEDs and AM for distant ones.

3. Redundant GPS Receivers with Independent Antennas

A single GPS receiver is a single point of failure. Receiver failure, antenna damage (storm, bird strike, UV degradation), or cable fault desynchronizes every IED in the facility simultaneously.

Design pattern: Two GPS receivers, two antennas on separate roof locations, two distribution amplifiers. Each IED receives IRIG-B from both paths. The IED selects the primary input and automatically fails over to the secondary if the primary signal quality degrades (TFOM increase or signal loss). Many modern relays (SEL-4xx, SEL-7xx series) have dual IRIG-B inputs for exactly this purpose.

4. Terminate Every Cable Run

IRIG-B DCLS is a baseband digital signal on 50-ohm coaxial cable. An unterminated cable reflects the signal at the open end, creating standing waves that distort pulse widths and corrupt timing accuracy.

Rule: Install a 50-ohm termination resistor at every receiving IED. If the IED has an internal termination option, enable it. If not, use an external BNC T-adapter with a 50-ohm terminator. Verify with an oscilloscope — a properly terminated signal has clean transitions with no ringing or overshoot.

5. Route IRIG-B Cables Away from Power Conductors

IRIG-B coax carries a low-voltage digital signal. Electromagnetic interference from power cables, bus bars, or switching transients can corrupt the signal, especially on longer runs.

Rules: Separate IRIG-B cables from power cables by at least 300mm. Use shielded coax (RG-58A/U). Route in dedicated cable trays or conduit, not bundled with power wiring. Ground the cable shield at one end only (typically at the GPS receiver) to prevent ground loops.

6. Monitor Time Sync Health

Time sync failures are silent. The protection system continues operating — it just stops producing accurate timestamps. You discover the problem weeks later when a fault investigation reveals that SOE records do not correlate.

What to monitor: GPS lock status (satellite count, PDOP, TFOM), IRIG-B signal presence on each distribution output, IED sync status (most relays report sync/unsync via a status bit readable over MMS or DNP3). Configure SCADA alarms for: GPS lock loss, TFOM degradation (above threshold), any IED reporting unsynchronized.

Distribution Architecture

GPS receiver selection drives the distribution design. Evaluate receivers on output count (how many IRIG-B outputs before needing amplifiers), format flexibility (simultaneous AM and DCLS outputs for mixed IED fleets), holdover stability (how long the internal oscillator holds accuracy after GPS lock loss), and protocol support (NTP/PTP alongside IRIG-B for devices that need Ethernet-based time). The receiver is the root of the timing tree — its capabilities constrain every downstream choice. In SEL-heavy installations, the SEL-2488 Satellite-Synchronized Clock is common because it shares the rack-mount form factor and engineering tool ecosystem. In multi-vendor fleets, the Arbiter 1094B is vendor-neutral and supports a wider range of IRIG-B format codes — useful when different relay families expect different formats (IRIG-B with IEEE 1344 extensions vs. unmodulated IRIG-B).

For facilities with more than 8 IEDs, use a two-tier distribution architecture: GPS receiver outputs feed distribution amplifiers, and each amplifier feeds a cluster of IEDs. Co-locate amplifiers with the IEDs they serve to keep coax runs short (under 40m for DCLS, or the manufacturer’s maximum length for longer runs — typically 300m for RG-58 coax). Size each distribution frame with at least 25% spare capacity (a 24-output frame for 18 IEDs) so expansion doesn’t require a hardware upgrade. Each IED gets a dedicated output — never parallel two IEDs on a single IRIG-B output. Paralleling causes impedance mismatch, signal degradation, and an ambiguous failure domain where one IED’s input fault can corrupt the signal for the other. Surge protection at the building entry point (between the GPS antenna and the receiver) is non-negotiable — a lightning strike on an unprotected antenna destroys the receiver and every IRIG-B input downstream.

Strengths, Weaknesses & When to Choose an Alternative

Strengths

1. Sub-microsecond accuracy with DCLS — IRIG-B DCLS exceeds IEC 61850-5 Class T1 (+/-1ms) by orders of magnitude. Typical accuracy is +/-100ns to +/-1us, providing comfortable margin for SOE timestamping. You never worry about whether your timestamps are “good enough.”

2. Hardware-level signal — No network stack, no software jitter, no packet prioritization, no switch queuing delays. The signal travels at the speed of light through copper. The only variable is cable length, which is constant. This determinism is why time-obsessed industries (power, aerospace, defense) trust IRIG-B.

3. Universal IED support — Virtually every protection relay, RTU, and digital meter manufactured in the last 30 years has an IRIG-B input. BNC connector on the front or back panel, or terminal block on a rear connector. No protocol configuration, no IP addressing, no network integration — plug in the cable and the IED synchronizes.

4. Deterministic and predictable — Cable propagation delay is constant (3-5 ns/m through RG-58 coax). A 30m cable adds exactly 90-150ns of delay, every time, forever. No jitter, no congestion, no asymmetry compensation. The delay can be calibrated out if needed.

5. Proven over decades — IRIG-B has been deployed in substations since the 1980s, in military test ranges since the 1960s, and in aerospace since the original IRIG standard in 1960. Failure modes are well-understood. There are no surprises.

6. Independent of the data network — IRIG-B runs on its own physical infrastructure. When the Ethernet network fails — switch reboot, broadcast storm, fiber cut — time sync continues unaffected. This independence is a security property: an attacker who compromises the data network does not automatically compromise time synchronization.

Weaknesses

1. Dedicated cabling — Every IED needs a physical coax cable from the distribution amplifier. In a facility with 40 IEDs, that is 40 coax runs, 40 BNC connectors, 40 cable labels. PTP delivers time over the same Ethernet cables that carry data — zero additional cabling.

2. Single point of failure without redundancy — A single GPS receiver, one antenna, one distribution amplifier creates a single point of failure that affects every IED simultaneously. Redundancy (dual receivers, dual antennas, dual distribution paths) doubles the already-significant cabling cost.

3. No authentication — The IRIG-B signal carries no security whatsoever. If an attacker gains physical access to the distribution amplifier or any cable, they can inject a false time signal that silently corrupts every timestamp in the facility. The corruption is undetectable until someone tries to use the timestamps for analysis.

4. Limited distance — DCLS signals degrade beyond approximately 40m of coax without repeaters. AM extends to approximately 150m. Large facilities may need multiple distribution amplifiers or fiber-optic conversion, adding cost and complexity.

5. No feedback channel — The GPS receiver broadcasts time to all IEDs but has no way to confirm that any IED is actually receiving and using the signal. An IED with a broken BNC connector or misconfigured input silently runs on its internal clock, drifting without alarm unless the IED itself reports unsynchronized status via SCADA.

When to Use

• Protection-grade SOE timestamping is required (IEC 61850-5 Class T1 or better)
• IEDs lack PTP/IEEE 1588 support (legacy relays, older RTUs, older meters)
• Time sync must be independent of the Ethernet data network for reliability or security
• Regulatory or contractual requirements mandate hardware-based time distribution
• Brownfield installations where IRIG-B infrastructure already exists and works

When NOT to Use

• All IEDs support PTP natively and the network has PTP-capable switches (Transparent Clock or Boundary Clock). PTP delivers equivalent accuracy over existing Ethernet infrastructure with zero additional cabling. But verify PTP accuracy end-to-end before decommissioning IRIG-B.
• NTP-level accuracy is sufficient for the application. HMI, historian, and non-protection SCADA hosts do not need sub-millisecond timestamps. NTP (+/-1-10ms) costs nothing beyond existing network infrastructure.
• Cable routing is impractical — retrofit installations where running 40 coax cables through existing cable trays is physically impossible or prohibitively expensive. PTP over existing Ethernet becomes the only viable option.
• Process bus with Sampled Values — IEC 61850-9-2 SV requires T3/T4/T5 accuracy (under 25us) that IRIG-B can provide, but the SV merging units need Ethernet-based time distribution (PTP) anyway because they are Ethernet devices.
• Greenfield with PTP-native IEDs — If every device in the design supports PTP, adding IRIG-B infrastructure is redundant cost. But consider keeping one IRIG-B distribution path as a diverse backup.

Common Misconception

“IRIG-B is obsolete now that PTP exists.”

IRIG-B and PTP coexist in most modern substations. They serve different populations of devices and provide diverse time distribution paths:

• IRIG-B serves legacy IEDs (which will be in service for 20-30 years), provides network-independent time sync, and has zero dependency on Ethernet switch behavior.
• PTP serves modern Ethernet-native IEDs, process bus merging units, and devices that need T3-T5 accuracy classes unachievable over IRIG-B AM.

Ripping out working IRIG-B infrastructure to go all-PTP is rarely justified. The installed base of IRIG-B IEDs will be in service for decades. The prudent approach is hybrid: maintain IRIG-B for legacy devices and add PTP for new ones. IEEE 2030.101-2018 explicitly describes this coexistence architecture.

Comparison Table

Dimension	IRIG-B (DCLS)	PTP (IEEE 1588v2)	NTP
Accuracy	+/-100 ns to +/-1 us	+/-100 ns to +/-1 us	+/-1 ms to +/-10 ms
Infrastructure	Dedicated coax/fiber (star)	Existing Ethernet (requires TC/BC switches)	Existing Ethernet (any)
IED support	Universal (30+ years of devices)	Growing (current-generation IEDs)	Universal (any IP device)
Network dependency	None — independent physical layer	Full — Ethernet path quality determines accuracy	Full — any IP connectivity
Cabling cost	High (one coax per IED)	None (uses data network)	None (uses data network)
Authentication	None	Annex K, MACsec (emerging)	NTS (Network Time Security)
Feedback channel	None (broadcast only)	Yes (two-way delay measurement)	Yes (client-server)
Failure mode	Cable/receiver failure	Switch failure, network congestion, asymmetry	Server/network failure
Determinism	Excellent (constant cable delay)	Good with TC/BC (variable with software timestamps)	Poor (network jitter)
Typical DCA role	Primary SOE time source for protection IEDs	Emerging for process bus and modern IEDs	Non-critical hosts (HMI, historian)

Biggest Pitfalls

1. Obstructed GPS Antenna Sky View

What goes wrong: The GPS antenna is installed in a location with partial sky obstruction — behind a parapet wall, under HVAC equipment overhang, near a lightning rod mast, or in a recessed mounting location. The receiver intermittently loses satellite lock as satellites move through the obstructed zone.

Why it happens: Antenna placement is often an afterthought — the electrical designer specifies “GPS antenna on roof” without a site survey. The contractor installs it wherever is convenient, not wherever has the best sky view.

How to prevent: Site survey before installation. Verify minimum 160-degree hemisphere of clear sky. Use a GPS planning tool or smartphone compass app to check satellite positions at the proposed location. Specify antenna placement on the roof plan with minimum clearance requirements from obstructions.

How to detect: GPS receiver reports low satellite count (under 4), high PDOP (above 6), or intermittent TFOM degradation. If the receiver loses lock at the same time every day, the obstruction corresponds to a specific satellite orbital position.

2. Cable Length Exceeding DCLS Limits

What goes wrong: DCLS IRIG-B signal degrades beyond approximately 40m of RG-58 coax. The digital pulse edges become rounded, reducing timing accuracy from nanoseconds to microseconds — or worse, causing intermittent sync loss on the most distant IEDs.

Why it happens: Cable schedule specifies DCLS output, but the cable routing from the GPS receiver to the farthest IED exceeds 40m. The installer routes through cable trays that take indirect paths, adding length. Nobody measures the actual installed cable length against the DCLS distance limit.

How to prevent: Calculate cable lengths during design from the GPS receiver location to every IED. If any run exceeds 35m (leaving 5m margin), either relocate the distribution amplifier, add a secondary distribution amplifier closer to the distant IEDs, or use AM output for those runs (AM tolerates 150m+).

How to detect: Oscilloscope at the IED BNC input shows rounded pulse edges, reduced amplitude, or ringing. The IED may report intermittent sync loss or increased time error. Compare waveform quality at the nearest IED (good reference) versus the distant IED (potential problem).

3. Grounding Loops in IRIG-B Cable Shields

What goes wrong: The coax cable shield is grounded at both ends — at the distribution amplifier and at the IED. The two ground points are at slightly different potentials (common in facilities with multiple ground buses). Current flows through the cable shield, inducing noise on the center conductor that corrupts the IRIG-B signal.

Why it happens: Standard practice for some cable types is to ground shields at both ends. Installers apply this rule to IRIG-B coax without understanding the ground loop issue. The problem may not appear during initial commissioning but develops over time as ground potentials shift with load changes.

How to prevent: Ground the IRIG-B cable shield at one end only — typically at the GPS receiver or distribution amplifier (source end). Leave the shield floating at the IED end. Document this in the cable installation specification. If the IED chassis ground connects to the BNC shell, use an isolating BNC adapter.

How to detect: Noise on the IRIG-B signal visible on an oscilloscope — 60 Hz hum or switching noise superimposed on the pulse waveform. Signal quality degrades as facility load increases. Temporarily disconnect the shield at one end — if the noise disappears, it is a ground loop.

4. Single GPS Receiver for the Entire Substation

What goes wrong: The single GPS receiver fails — power supply, firmware crash, antenna cable damage, or GPS constellation issue. Every IED in the facility simultaneously loses time synchronization. The IEDs fall back to their internal oscillators and begin drifting.

Why it happens: Cost pressure. A GPS receiver costs $3,000-$8,000. A second receiver with independent antenna doubles that cost plus requires a second antenna installation, second set of coax runs, and IEDs with dual IRIG-B inputs. The project scope says “provide GPS time synchronization” — singular.

How to prevent: Specify redundant GPS receivers in the design basis. Two receivers, two antennas on separate roof locations (so a single event like lightning or debris cannot damage both), two distribution amplifiers. IEDs with dual IRIG-B inputs select the primary and fail over to the secondary automatically. For IEDs with single inputs, use an IRIG-B transfer switch (some distribution amplifiers include automatic failover logic).

How to detect: GPS receiver status alarm via SCADA. All IEDs simultaneously reporting unsynchronized status. SOE timestamps from multiple IEDs diverging from each other (each drifting at its own internal oscillator rate).

5. Ignoring TFOM and Clock Quality Monitoring

What goes wrong: The GPS receiver loses satellite lock — antenna degradation, constellation issue, or environmental interference. The receiver continues outputting IRIG-B from its internal oscillator, which drifts. The TFOM (Time Figure of Merit) increases to indicate degraded accuracy, but nobody is monitoring TFOM. The drift is undetectable until a fault investigation weeks later reveals that SOE timestamps are off by hundreds of milliseconds.

Why it happens: IRIG-B is “set and forget” infrastructure. The GPS receiver is installed, the green LED glows, and everyone moves on. No SCADA points are configured for GPS lock status, satellite count, or TFOM. The only indication of a problem is a front-panel LED that nobody looks at.

How to prevent: Configure SCADA alarming for GPS receiver health: satellite count (alarm below 4), TFOM (alarm above configurable threshold — typically TFOM 6 or higher), receiver lock status, and IRIG-B output status. Monitor IED sync status bits — most protection relays report their internal clock status via MMS or DNP3. Alarm on any IED reporting unsynchronized for more than 60 seconds.

How to detect: TFOM value in the IRIG-B control function bits (readable by IEDs that decode CF). GPS receiver front panel shows lock status. SCADA points for receiver health and IED sync status. Monthly check: pull SOE records from 3+ IEDs and verify timestamps agree within 1ms for the same event.

Field Tips & Tools

Oscilloscope Verification

The most important commissioning tool for IRIG-B is an oscilloscope. You cannot verify IRIG-B signal quality with a multimeter — you need to see the waveform.

Setup: Connect oscilloscope probe to the BNC output of the distribution amplifier (or at the IED input, which is where it matters). Set: DC coupling, 5V/div vertical, 2ms/div horizontal, trigger on rising edge.

What to look for:

Signal	Healthy	Problem
Pulse amplitude	3.3-5V (DCLS)	Under 2V: cable too long or bad termination
Pulse edges	Sharp transitions, under 1us rise/fall	Rounded edges: cable length limit exceeded
Pulse widths	Clean 2ms / 5ms / 8ms	Intermediate widths: noise or reflection
Baseline	Clean low level, no ringing	Ringing after transitions: unterminated cable
Noise	None visible on pulse or baseline	60 Hz hum: ground loop. Spikes: EMI from power cables

GPS Signal Strength Verification

Before trusting the IRIG-B output, verify the GPS input is healthy.

SEL-2488: Access via AcSELerator Quickset. Check: satellite count (should be 6+), PDOP (under 4.0), timing mode (GPS Locked), TFOM (should be 4 or lower for normal operation).

Arbiter 1094B: Front panel LCD displays satellite count and lock status. Press MENU to access signal quality details. The green LOCK LED must be solid (not flashing).

Microchip GridTime 3000: Web interface shows satellite tracking, constellation view, and timing quality metrics.

Commissioning Sequence

Validate each layer independently — antenna, receiver, distribution, IED reception — so upstream problems don’t get masked by downstream symptoms.

1. Verify GPS antenna installation — Confirm mounting location matches design drawings. Check antenna cable continuity (TDR or ohmmeter). Verify surge protector at building entry. Confirm antenna is connected to the correct GPS receiver.

2. Confirm GPS receiver lock — Power on receiver. Wait for acquisition (can take 15-30 minutes on cold start). Verify satellite count (6+ for good geometry), PDOP (under 4.0), and timing mode (GPS Locked). Record initial TFOM value.

3. Verify IRIG-B output signal quality — Connect oscilloscope to each distribution amplifier output. Check pulse amplitude (3.3-5V DCLS), pulse widths (2/5/8 ms), edge quality, and absence of noise. Compare against manufacturer specification.

4. Connect distribution amplifier — Verify all output ports active. Label each output with its destination IED. Record cable route and length for each run.

5. Connect each IED and verify sync — Connect IRIG-B cable to each IED one at a time. Verify IED reports synchronized status (front panel LED, configuration software, or SCADA point). Record sync acquisition time.

6. Cross-check SOE timestamps — With all IEDs connected and synchronized, inject a test event that triggers multiple devices simultaneously (e.g., a binary output pulse to multiple binary inputs). Pull SOE records from 3+ IEDs and verify timestamps agree within 1ms. If they do not agree, investigate the outlier (cable, termination, IED configuration).

7. Configure SCADA alarms — Verify GPS lock status, TFOM, satellite count, and IED sync status points are mapped in SCADA. Trigger a test alarm (disconnect antenna briefly) and confirm the alarm reaches the operator.

Troubleshooting Flowchart

  IED reports "UNSYNCHRONIZED"
          |
          v
  Is IRIG-B cable connected?
  ├─ No → Connect cable, check BNC connector seating
  └─ Yes ↓
  
  Check signal at IED input with oscilloscope
  ├─ No signal → Check distribution amplifier output
  │               ├─ No signal → Check GPS receiver IRIG-B output
  │               │               ├─ No signal → GPS receiver fault
  │               │               └─ Signal OK → Distribution amp fault
  │               └─ Signal OK → Cable fault (break, bad connector, wrong port)
  └─ Signal present ↓
  
  Is signal quality good? (clean edges, correct amplitude, correct widths)
  ├─ No → Check for:
  │       - Rounded edges → cable too long (DCLS limit ~40m)
  │       - Low amplitude → cable loss or missing termination
  │       - Ringing → missing or wrong termination impedance
  │       - 60 Hz noise → ground loop (shield grounded at both ends)
  │       - Spikes → EMI (re-route cable away from power conductors)
  └─ Yes ↓
  
  Is IED IRIG-B input configured correctly?
  ├─ Check: Format (B004/B005), modulation (DCLS vs AM), input enabled
  │  Check: Year decoding enabled, IEEE 1344 extensions enabled
  └─ If configuration is correct and signal is good:
     → IED firmware issue (check vendor release notes)
     → Try power cycling the IED
     → Contact vendor support with oscilloscope capture

Vendor-Specific IRIG-B Notes

• SEL relays — IEEE 1344 for year decoding. Without IEEE 1344 extensions enabled on both the receiver output and the relay input, event timestamps default to year 2000. Verify that the EIRIGYEAR setting (or equivalent per relay model) is explicitly configured — it is not always the default.
• SEL relays — format-B000 silent mismatch. Most SEL relays default to IRIG-B format B000 (unmodulated, no extensions). If the receiver outputs IRIG-B with IEEE 1344 year and quality bits but the relay’s format setting is B000, the relay may report “synchronized” while ignoring the extended data — losing TFOM awareness without any alarm. Verify that the relay’s IRIG-B format setting matches the receiver’s output format exactly.
• ABB relays (REF/RET) — format-select mismatch. These accept both AM and DCLS inputs but require explicit input format selection. A wrong selection produces no error — the relay reports valid time with incorrect values. Always verify the configured input type matches the actual signal format.
• GE and ABB relays — free-running fallback hides sync loss. Some relay families accept IRIG-B on a dedicated hardware input but fall back to an internal free-running clock if the signal is lost, without generating a communication alarm. The front panel may show “synchronized” based on the last successful sync, not the current state. Check the relay’s sync status bit via SCADA (DNP3 or MMS), not the front panel.
• Arbiter GPS receivers — TFOM threshold mismatch. The TFOM threshold may ship at a value that differs from what SEL relays expect for their time-quality assessment. A mismatch causes the relay to reject the IRIG-B source and free-run silently — no alarm on the receiver side. Verify TFOM threshold alignment between receiver output settings and relay input expectations during commissioning.
• Arbiter 1094B — permissive TFOM default. The factory TFOM threshold may be more permissive than your project’s accuracy requirement. If GPS degrades but stays below the default threshold, the receiver keeps outputting IRIG-B without flagging a quality issue — the IEDs sync to a degraded source with no alarm.

Deep Dive

IRIG Standard 200-16 Frame Structure

The IRIG-B time code transmits 100 bits per second in a continuous stream. Each 1-second frame encodes the complete date and time using Binary Coded Decimal (BCD) and binary formats.

Frame composition: 100 bit positions (numbered 0-99), organized into ten 10-bit groups. Each group begins with a Position Identifier (P0 through P9) — an 8ms reference pulse that serves as a group delimiter. The on-time reference point is the leading edge of P0, which coincides with the beginning of the second being encoded.

BCD encoding: Time-of-year fields use packed BCD. Each decimal digit occupies 4 bits: seconds (00-59), minutes (00-59), hours (00-23), day-of-year (001-366). The BCD values are transmitted LSB first within each group.

  Frame structure (100 bits, 1 second):

  Group 0 (bits 0-9):   P0 + Seconds ones (4 bits) + Seconds tens (3 bits) + 2 unused
  Group 1 (bits 10-19): P1 + Minutes ones (4 bits) + Minutes tens (3 bits) + 2 unused
  Group 2 (bits 20-29): P2 + Hours ones (4 bits) + Hours tens (2 bits) + 3 unused
  Group 3 (bits 30-39): P3 + Days ones (4 bits) + Days tens (4 bits) + 1 unused
  Group 4 (bits 40-49): P4 + Days hundreds (2 bits) + 7 unused/CF
  Group 5 (bits 50-58): P5 + Control Function 1 (9 bits: year ones, year tens)
  Group 6 (bits 59-68): P6 + Control Function 2 (9 bits: LSP, DST, UTC offset)
  Group 7 (bits 69-78): P7 + CF continued (TFOM, parity, reserved)
  Group 8 (bits 79-88): P8 + Straight Binary Seconds bits 0-8
  Group 9 (bits 89-99): P9 + Straight Binary Seconds bits 9-16 + P0(next frame)

Straight Binary Seconds (SBS): A 17-bit binary count of the number of seconds elapsed since midnight UTC. Provides an independent cross-check against the BCD time-of-year — if they disagree, the decoder flags the frame as invalid.

On-time marker: Two consecutive 8ms reference pulses (P9 of the current frame followed by P0 of the next frame) form the on-time reference. The leading edge of P0 marks the exact start of the encoded second. This double-reference pattern is unambiguous — it cannot be confused with any data pattern within the frame.

DC Level Shift vs AM Modulation

IRIG-B defines three modulation types, each trading accuracy for distance and compatibility:

DC Level Shift (DCLS) — Type 0 in the format code (B0xx): The signal is a baseband digital waveform switching between two voltage levels (typically 0V and 3.3-5V). Bit values are encoded by pulse width (2ms, 5ms, 8ms). Accuracy is nanosecond-class because the decoder only needs to detect clean digital edges — a comparator with a fixed threshold.

Advantages: Highest accuracy. Simple decoder (comparator + timer). No demodulation jitter. Well-suited to modern digital IEDs.

Limitations: Signal attenuation over distance. Reflections from impedance mismatches corrupt the waveform. Practical limit of approximately 40m on RG-58 coax without repeaters. Requires proper 50-ohm termination at every endpoint.

Amplitude Modulated (AM) — Type 1 (B1xx): A 1 kHz sinusoidal carrier is amplitude-modulated with the same pulse-width encoding. Full amplitude represents the “high” portion of each pulse, reduced amplitude (10-30%) represents the “low” portion. The decoder extracts the envelope to recover the pulse pattern.

Advantages: Longer cable runs (150m+ on coax, 300m on STP). Better noise immunity on long runs because the carrier frequency is well above typical power system interference (60 Hz). Can drive multiple high-impedance loads from a single output (broadcast topology).

Limitations: Accuracy limited to microsecond-class because envelope detection introduces jitter. The zero-crossing of the carrier must be identified, and demodulation filtering adds variable delay. Adequate for T1 class (+/-1ms) but not for T2 or better.

Manchester — Type 2 (B2xx): Added in IRIG 200-98. Uses bi-phase Manchester encoding (transition in the middle of each bit period). Rarely used in substations. Primarily for applications requiring self-clocking capability over fiber without a separate clock recovery circuit.

IEEE 1344 Extensions and the C37.118 Profile

The original IRIG-B frame (pre-1995) carried only time-of-year — no year, no leap second warning, no quality indicator. IEEE 1344-1995 defined how to use the 27 Control Function bits (positions 50-78) to carry additional information:

CF Bits	IEEE 1344 Content
50-53	Year ones (BCD)
55-58	Year tens (BCD)
59	Leap Second Pending (LSP)
60	Leap Second Polarity (0=add, 1=delete)
61	DST Pending
62-63	DST value (00=off, 01=on, 10=going on, 11=going off)
64-68	UTC offset magnitude (0-23 hours)
69	UTC offset sign (0=behind UTC, 1=ahead)
70-73	Time Figure of Merit (TFOM)
74	Parity (odd parity over bits 50-73)

TFOM values (0-15, where lower is better):

TFOM	Accuracy	Meaning
0-3	Under 100 ns	GPS locked, full accuracy
4-5	100 ns - 1 us	GPS lock intermittent, holdover
6	1-10 us	Extended holdover
7-8	10-100 us	Significant holdover drift
9	1 ms	Approaching T1 class limit
10+	Over 1 ms	Time unreliable for SOE
15	Unknown	Receiver has no lock, no estimate

The C37.118 sign-reversal issue: IEEE C37.118-2005 (which superseded IEEE 1344) reversed the meaning of the UTC offset sign bit. In IEEE 1344, sign=0 meant “local time is behind UTC” (negative offset). In C37.118, sign=0 means “local time is ahead of UTC” (positive offset). This caused documented interoperability problems between devices implementing different standard versions. IRIG 200-04 adopted the IEEE 1344 convention. Most modern equipment follows the IRIG 200-04 convention, but legacy C37.118 devices may interpret the offset incorrectly. Check IED documentation and verify UTC offset sign during commissioning.

Time Distribution Architecture

A well-designed IRIG-B distribution system follows a star topology from the GPS receiver to every IED. The key design decisions are receiver placement, distribution amplifier sizing, and cable routing.

Receiver placement: Central to minimize maximum cable length. In a data center, this is typically the main electrical room or the protection relay room. The antenna cable (from roof to receiver) has its own length budget — GPS antenna cables tolerate longer runs than IRIG-B DCLS, but signal loss increases with length (specify low-loss antenna cable like LMR-400 for runs over 30m).

Distribution amplifier sizing: One amplifier output per IED. Common models provide 8, 16, or 24 outputs. For a facility with 40 IEDs, use two 24-output distribution amplifiers (one per redundant receiver) with spare capacity for future expansion. Each output is electrically isolated — a fault on one cable does not affect others.

Cable routing: Star topology from distribution amplifier to each IED. No daisy-chaining, no splitters, no T-connections (these cause impedance mismatches and reflections). Each cable is a dedicated point-to-point link with proper termination at the IED end.

Redundant architecture:

  [GPS Rx A] ──→ [Dist Amp A] ──→ IED Input 1 (primary)
  [GPS Rx B] ──→ [Dist Amp B] ──→ IED Input 2 (secondary)

IEDs with dual IRIG-B inputs automatically select the primary and fail over to the secondary on signal quality degradation. For IEDs with single inputs, some distribution amplifiers provide automatic 1:1 failover (monitor primary, switch to secondary on failure).

PTP Convergence Path

The power industry is transitioning from IRIG-B to PTP (IEEE 1588v2) for time distribution, but this is a multi-decade migration, not a technology replacement.

What drives PTP adoption:

• Process bus (IEC 61850-9-2 Sampled Values) requires T3-T5 accuracy classes — achievable with PTP but impractical with IRIG-B AM over long cable runs
• Modern IEDs ship with PTP support as standard — no additional cabling needed
• PTP delivers bidirectional delay measurement, allowing compensation for network asymmetry — something IRIG-B cannot do
• IEEE 2030.101-2018 provides a comprehensive design guide for PTP in substations

What keeps IRIG-B:

• Installed base of IRIG-B-only IEDs will remain in service for 20-30 years
• IRIG-B is independent of the Ethernet network — this diversity is a reliability and security property
• PTP accuracy depends on switch behavior (Transparent Clock or Boundary Clock support) — misconfigured or non-PTP switches degrade accuracy silently
• IRIG-B is simple to verify (oscilloscope) — PTP requires specialized test equipment and network analysis

Hybrid architecture (IEEE 2030.101-2018 recommended approach): Maintain IRIG-B distribution for legacy IEDs and as a diverse backup path. Add PTP Grandmaster capability to the GPS receiver (most modern receivers support both). Configure PTP-capable IEDs to use PTP as primary time source with IRIG-B as secondary. This provides defense-in-depth — a PTP network failure does not affect time synchronization for devices with IRIG-B backup.

Security Considerations

IRIG-B has no native authentication, encryption, or integrity protection. The signal is a simple analog waveform on coaxial cable. An attacker with physical access to the distribution infrastructure can:

• Inject a false time signal by splicing into a cable or replacing the distribution amplifier output. Downstream IEDs accept the false time without any verification. All timestamps in the facility shift by the attacker’s chosen offset.
• Delay the signal by inserting a cable extension, shifting timestamps by a predictable amount (3-5 ns per meter of added cable).
• Jam the signal by injecting noise on the coax, causing IEDs to fall back to their internal oscillators and drift.

Why this matters: A subtle time shift (e.g., 50ms) does not affect protection operation — relays still trip when they should. But SOE records become unreliable. In a post-fault investigation, the shifted timestamps could make it appear that a relay operated too late or too early, potentially shifting liability. The attack is undetectable during normal operation and only discovered during forensic analysis.

Mitigations:

• Physical security: IRIG-B distribution infrastructure (receiver, amplifier, cables) should be in locked equipment rooms with access control
• Redundant diverse sources: if IRIG-B and PTP agree, both are likely correct. If they disagree, alarm and investigate
• Cross-validation: compare IRIG-B timestamps against PTP and NTP sources. Significant disagreement triggers an alarm
• IED-level monitoring: modern IEDs report sync source and quality via SCADA — monitor for unexpected source changes
• IEEE C37.240 (Cybersecurity Requirements for Substation Automation) addresses physical security of timing infrastructure

Military Origins and Standards Evolution

IRIG-B originates from the Inter-Range Instrumentation Group (IRIG), established in 1952 by the Range Commanders Council (RCC) of the U.S. Department of Defense. The RCC coordinates the White Sands Missile Range, Patrick Space Force Base, and Vandenberg Space Force Base — facilities that needed precise time synchronization for tracking missiles, rockets, and aircraft during test flights.

The Telecommunication Working Group within IRIG began standardizing time code formats in 1956. The first standard, Document 104-60, was published in 1960 and defined six time code formats (A through H) at different pulse rates. Format B (100 pps, 1-second frame) became the dominant format for instrumentation and eventually for power systems.

Standards timeline:

Year	Standard	Significance
1960	IRIG Document 104-60	Original 6 time code formats
1995	IRIG 200-95	First “200” series revision
1995	IEEE 1344	Added year, leap second, DST, TFOM to CF bits
1998	IRIG 200-98	Added Manchester modulation (Type 2)
2004	IRIG 200-04	Incorporated IEEE 1344 extensions natively
2005	IEEE C37.118	Superseded IEEE 1344 (with sign-reversal issue)
2011	IEEE C37.118.1	Added CTQ bits, Annex D specifies B004/B005 profile
2016	IRIG 200-16	Current version. Added year to formats A, E, G, H
2018	IEEE 2030.101	Design guide for substation time sync (IRIG-B + PTP + NTP)

The path from missile tracking to substation protection illustrates how robust timing standards propagate: a military requirement for precise event correlation (when did the missile separate from the booster?) translates directly to a power system requirement for precise event correlation (which relay tripped first?). The “clock that keeps the grid honest” started as the clock that kept the missile range honest.

Vendor Implementation Differences

Relay families differ in how they handle IRIG-B control function (CF) bit decoding. SEL relays decode CF bits to extract TFOM and year information when the IRIG-B format includes IEEE 1344 extensions — but only if the relay’s format setting is explicitly configured to expect them. GE UR-series relays decode CF bits automatically when present but may interpret TFOM thresholds differently than the GPS receiver’s intent. ABB relays in some firmware versions ignore CF bits entirely, treating all IRIG-B signals as basic timecodes without quality metadata. The practical consequence: in a multi-vendor facility, the same GPS receiver output can produce different sync quality assessments on different relay platforms. A relay reporting “synchronized” doesn’t guarantee it’s using high-quality time — verify by comparing SOE timestamps across vendors for the same physical event.