Breaker failure protection 50BF (RBRF): how it works

What 50BF (breaker failure) protection is

In IEEE C37.2 the suffix BF means breaker failure, and the function is commonly identified as 50BF because its main detector is an instantaneous overcurrent element. In IEC 61850 it corresponds to the logical node RBRF. The reference guide is IEEE C37.119.

Every protection scheme assumes the breaker opens when told to. Sometimes it does not: the trip coil is open, the mechanism is stuck, gas or air pressure is low, or the contacts cannot interrupt the arc. If nothing acts, the fault stays fed until the remote backup protections operate, which are slower and less selective.

50BF is the local backup. It supervises every trip command and, if the fault is still present after a set time, opens every breaker that can feed it. It complements the main and backup protections of each zone.

Where it is applied

• HV and EHV substations: it is practically mandatory in breaker-and-a-half, double bus and ring bus arrangements.
• MV substations and industry: where the clearing time of remote backups is not acceptable.
• Generating and solar plants: on the grid interconnection breaker and on the step-up transformer breakers.
• Transformer, line, bus coupler and generator bays.

How it works

The basic logic has four parts:

1. Initiation (BFI): the trip command of any protection that must open the breaker initiates 50BF. It can be three-phase or per phase on lines with single-pole tripping.
2. Fault detector: a per-phase (and sometimes neutral) overcurrent element confirms that current is still flowing. It must reset very quickly when the breaker opens, even with the CT residual current. For trips without significant current, the breaker auxiliary contact position (52a) is used.
3. Timer: if initiation and detector are still active when the time expires, the function operates.
4. Backup trip: opens the adjacent breakers, usually through an 86 lockout relay or the corresponding zone of the bus differential 87B, and sends a transfer trip to the remote end of the line.

Many schemes add a retrip stage: an immediate or short-delay command to the same breaker through the second trip circuit. Some relays also include logic for the dead zone fault (between the breaker and the CT), for breaker open with fault and for pole discrepancy.

Setting criteria

Time. The timer must be longer than the sum of the breaker interrupting time, the reset time of the current detector and a safety margin. At the same time, the total clearing time (protection + timer + backup trip + opening of the adjacent breakers) must be shorter than the critical clearing time for system stability. That critical time comes from the stability study; the margin from the utility’s practice and IEEE C37.119.

Current detector. Below the minimum fault current it must detect, so that no sensitivity is lost. In many schemes it can be set below load, because the function only acts when initiated; that is where it differs from ordinary overcurrent.

Initiations. Which protections initiate 50BF, which use the current criterion and which the auxiliary contact criterion.

Trip destinations. Which breakers open depending on the topology: in breaker-and-a-half and double bus the destination changes with the disconnect positions.

Values come from the coordination and stability study, the measured breaker times and the relay manual.

How each manufacturer implements it

• Siemens SIPROTEC offers it as breaker failure protection with retrip and backup stages; it is set in DIGSI, and the topology is modeled with the circuit breaker function groups.
• SEL includes it in line, transformer and bus relays and in dedicated breaker relays; it is set in AcSELerator QuickSet and the initiation and destination logic is built with SELogic.
• Relion (Hitachi Energy 670/650; ABB 615/620) presents it as one function block per breaker in PCM600, with initiations and outputs connected in the configuration.
• GE Vernova (Multilin) offers it as a breaker failure element per breaker; it is set in EnerVista and combined with FlexLogic.
• Schneider MiCOM / Easergy includes it with timed stages and configurable reset criteria; it is set in MiCOM S1 or Easergy Studio.

The most important differences are in how the detector resets, in the conditions for the contact criterion and in how initiations and backup trips are wired or sent over IEC 61850.

Why it trips or blocks

1. A breaker that really failed: open trip coil, low pressure, stuck mechanism. This is correct operation.
2. Initiation that does not reset: a stuck trip contact or a latched 86 that keeps initiating 50BF.
3. Testing with live outputs: the classic cause of full bus trips. An injection in a bay in service initiates 50BF and trips the neighbors.
4. Detector slow to reset because of residual current from a saturated CT, with a tight timer.
5. Auxiliary contacts poorly adjusted or wired wrong in the position criterion.
6. Wrong destinations after topology changes or GOOSE messages subscribed incorrectly.
7. Dead zone fault not considered in the logic.

What to check first: the sequence of events (who initiated, when the breaker opened, when the detector reset), the actual breaker opening time and the initiation logic.

How it is tested

50BF is tested by secondary injection with a source such as the Omicron CMC 356, simulating the initiation and keeping or removing the current to represent a breaker that fails to open or that opens.

The test verifies:

• The current detector pickup and its reset time.
• The retrip and backup trip times.
• The auxiliary contact criterion.
• Each backup trip destination and the transfer trip, with the outputs isolated or in a window coordinated with operations.

The check of destinations and transfer trip is completed with end-to-end tests. The actual breaker times are measured with circuit breaker tests.

Did your relay trip on 50BF?

Did your relay trip or block on this function and it is not clear why? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us with the brand and model of the equipment.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.119-2025, Guide for Power System Circuit Breaker Failure Protection.
• IEC 61850-7-4, logical nodes (RBRF).
• IEC 60255 (series), measuring relays and protection equipment.
• J. Lewis Blackburn and Thomas J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions