Tripping relay 94 (PTRC): what it is and how it works

What the 94 tripping relay does

The 94 does not detect faults: it is the link between the function that decides and the breaker that opens. Its job is to make the command arrive on time, at every coil that must receive it and with the necessary energy. If that link fails, not even the fastest protection clears the fault, and only breaker failure 50BF or remote backup remain.

It prevents three problems: a protection contact burning when it breaks the current of a coil, a single output not being enough for several breakers, and the breaker staying closed on a fault because the close command was held.

• ANSI/IEEE C37.2: 94, tripping or trip-free relay.
• IEC 61850: logical node PTRC (protection trip conditioning).
• IEC 60617: auxiliary relay; on drawings it is identified with the project designation.

Where it is applied

At low voltage the trip unit is inside the breaker and there is no separate 94.

Operating principle

The 94 does not measure electrical quantities: it receives logic states. It works like this:

1. Groups the commands of the functions: 21, 87, 50/51, 50BF and those the engineering defines.
2. Decides the type of trip: single-pole or three-pole on lines with single-pole reclosing, depending on the fault and the state of the recloser.
3. Holds the command for a minimum time, so that the coil receives enough energy even if the function resets immediately.
4. Multiplies the output to trip coil 1 and trip coil 2, to the 86 and to the 50BF initiate.
5. Resets when the command disappears, unlike the lockout relay 86.

In electromechanical equipment the 94 was a fast auxiliary relay with heavy-duty contacts. In numerical relays it is internal logic with high-capacity output contacts.

Single-pole and three-pole tripping. On HV and EHV lines with single-pole reclosing, the 94 logic decides which pole to open. A single-phase fault trips only the faulted phase and starts the reclosing; a phase-to-phase fault, a fault during the dead time or a fault with reclosing blocked trips all three phases. If the breaker has no independent pole operation, every trip is three-pole. That decision keeps the line transferring power over two phases while the arc extinguishes.

Redundancy. At high voltage it is common for each main protection to have its own trip path to the breaker coils, with separate DC supplies. That way a single failure in a trip circuit does not leave the line without clearing.

The characteristic

The 94 has no curve. What is characterized is its behavior over time:

• Operating time: from the arrival of the command to the closing of the contact. A fast auxiliary relay operates in a few milliseconds; manufacturers publish the value for their fast outputs.
• Minimum pulse duration: avoids commands so short that the coil cannot act.
• Reset: automatic when the command disappears, or sealed in until the breaker auxiliary contact confirms the opening.

Setting criteria

• Which functions trip which breakers: documented in a trip matrix that crosses functions against outputs. The matrix is part of the detailed engineering and is reviewed with every change of functions.
• Trip through both coils when the breaker has them, with separate circuits and supplies.
• Single-pole or three-pole: depends on the automatic reclosing 79 scheme.
• Minimum pulse duration and seal-in until opening, according to the breaker opening time.
• Contact capacity against the coil current, with margin.

Interaction with other functions

• 50BF: the same trip starts breaker failure. If current is still flowing after the set time, the 50BF trips the adjacent breakers.
• 86: functions that require the equipment to be checked (87T, 87B, mechanical protection) trip through the 86 as well as the 94.
• 79: the trip initiates reclosing, except when it comes from a function that blocks it.
• Trip circuit supervision 74: monitors coil continuity with the breaker closed and open. See trip circuit supervision.
• Trip coils: their burden and voltage define the contact; they are covered in trip and close coils.

How each manufacturer implements it

• Siemens SIPROTEC (DIGSI): the trip logic of the circuit breaker object, with a minimum duration setting and output assignment in the relay matrix.
• SEL (AcSELerator QuickSet): trip equations that group elements, and a seal-in and reset logic.
• Hitachi Energy and ABB Relion (PCM600): the trip logic block, with a name ending in PTRC, assigned to the outputs in the configuration.
• GE Vernova Multilin (EnerVista): trip outputs assigned with FlexLogic.
• Schneider MiCOM and Easergy: output matrix and trip logic with minimum duration.

IEC 61850 logical node

PTRC receives the operate signal (Op) of the protection nodes (PDIS, PDIF, PTOC, RBRF) and publishes its own Tr (trip) and Op. That Tr is what the engineering connects to a physical output or to a GOOSE message towards the breaker XCBR, in another IED or in a process interface unit. Testing that chain is described in protection testing with GOOSE.

How it is tested

In general, with secondary injection and with the actual breaker, the following are verified:

• That each function trips the outputs and breakers the matrix says, and no others.
• The operating time of the trip outputs and the pulse duration.
• Tripping through both coils, each one separately.
• Trip-free operation: with the trip command present, the breaker must not stay closed.
• The 50BF and 79 initiate, and the 86 operation where applicable.

Typical mistakes

• Incomplete matrix: a new function that was never assigned to an output.
• Only one coil wired on a breaker that has two.
• Pulse too short with slow breakers or high-burden coils.
• Single-pole trip assigned to a function that should trip all three phases.
• Testing the relay with the output blocked and not repeating the test with the breaker.
• Low-capacity contacts breaking the coil current, which end up welded.

Do you need to review the trip logic of your relay, or understand why a breaker did not open? At Tiza Group we review the events and settings remotely, worldwide. See our remote relay configuration service or contact us.

Related guides

• Lockout relay 86
• Breaker failure 50BF

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: numbers 94 and 86.
• IEC 60255-1, Measuring relays and protection equipment – Part 1: Common requirements: output contact requirements.
• IEC 61850-7-4, Compatible logical node classes and data object classes: PTRC node.
• IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines: single-pole and three-pole tripping.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions