Breaker Failure Protection Testing: Current, Time and Dead-Zone Logic

Breaker failure relay testing proves that a 50BF scheme trips the upstream breakers when fault current persists after the initial trip command. It verifies the current threshold, supervision timer, retrip pulse, dead-zone current, and the seal-in logic. The result is a scheme that trips the upstream breakers only when the breaker has actually failed.

Key takeaways

• Three inputs matter. The scheme needs an initiation signal, a current measurement, and a trip output. Test all three, not only the current threshold.
• The timer starts at initiation. Measure from the initiation contact to the upstream trip, not from the current step.
• Current below threshold must not trip. Drop the current before the timer expires and confirm that the scheme resets.
• Dead zone is a separate path. A fault between the breaker CT and the line CT may not be seen by the breaker-failure current element; test the dead-zone input if the scheme has one.
• Use a state sequence. A static current and a manual contact are not enough to prove the timing and the retrip pulse.

What does a breaker failure test prove?

A breaker failure scheme monitors the current through a breaker after a trip command. If the current remains above the threshold for the supervision time, the scheme assumes that the breaker has failed and trips the upstream breakers. The scheme may also retrip the failed breaker and, in some designs, initiate a dead-zone trip.

The test proves that the current threshold is correct, the supervision timer is correct, the retrip pulse is present, the upstream trip is issued, and the scheme resets when the current falls below the threshold.

How do you test a 50BF scheme?

1. Isolate the relay and the trip circuits. Confirm that the upstream breaker trip coils are isolated or that the test is performed with the primary equipment out of service.
2. Wire the currents and contacts. Connect the test set currents to the breaker-failure current inputs. Connect a digital output from the test set to the initiation input. Connect the breaker-failure trip output to a digital input on the test set.
3. Set the scheme. Record the current threshold, supervision time, retrip pulse, and dead-zone settings.
4. Apply current above the threshold. Start the current, then send the initiation pulse. The test set should measure the time from the initiation to the trip output.
5. Verify the retrip. Check that the retrip output pulses for the set time and that the upstream trip output follows.
6. Drop the current before the timer. Repeat the sequence, but reduce the current below the threshold before the supervision time expires. The scheme must not trip.
7. Test the dead zone. If the scheme has a separate dead-zone input, inject the dead-zone current and verify the trip. If it does not, record that the dead-zone logic was not tested.
8. Check the reset. After a trip, reset the scheme and confirm that it is ready for the next operation.

What settings and test points should you use?

How do you wire the VAE-430 to the relay?

Use the 0005 State Sequence module to control the current step, the initiation pulse, and the timing measurement in one sequence. A manual contact and a stopwatch cannot prove a 150 ms timer with the required repeatability.

What is the pass criterion?

• Current threshold: within 5 percent of the setting.
• Supervision time: within 5 percent or 20 ms, whichever is larger.
• Retrip pulse: within 10 ms of the setting.
• Upstream trip: present and correctly timed.
• No trip below threshold: the scheme must reset when the current drops before the timer expires.
• Dead zone: correct operation if the scheme has a separate dead-zone input; otherwise record it as not tested.

Why does the breaker failure scheme not trip?

No initiation. The relay may not receive the initiation contact, or the contact may be too short for the relay’s input filter. Increase the pulse width and check the relay event log.

Current below threshold. The test set may be injecting less current than the relay measures because of lead burden or a wrong ratio. Check the relay metering, not the test set display.

Timer longer than the test. The supervision time may be 150 ms, but the upstream trip may have an additional 200 ms delay. Measure each stage separately.

Dead-zone path not tested. A dead-zone fault may use a separate current input or a separate logic path. If the scheme has one, test it explicitly.

Frequently asked questions

What is the difference between 50BF and 50/51?

50/51 protects the line or equipment against overcurrent. 50BF supervises the breaker after a trip command and trips the upstream breakers if the current does not stop. The current threshold is usually lower and the timer is short.

Can I test breaker failure with a static current?

You can prove the pickup with a static current, but you cannot prove the timer, retrip pulse, or reset. Use a state sequence with a current step and a contact initiation.

What is a dead zone in breaker failure protection?

A dead zone is the current path between the breaker CT and the line CT. A fault in that zone may not be seen by the breaker-failure current element, so some schemes use a separate dead-zone current input or a direct trip from the protection relay.

Why does the scheme trip when the current is below the threshold?

The threshold may be set lower than the test value, the current may be measured on a different ratio, or the scheme may use a separate sensitive element. Check the relay metering and the settings, not only the test set.

Can I use the VAE-430 for a breaker failure test?

Yes. The VAE-430 has three currents, four voltages, eight digital inputs and four digital outputs. The 0005 State Sequence module can control the current step and the initiation contact while measuring the trip time.

Where VA-TEK test sets fit

The VAE-430 provides the current source, the initiation contact, and the timing input for a 50BF test. For busbar schemes with more current channels, the VAE-660 provides six currents and six voltages. Start with the relay protection testing hub and the relay testing whitepaper.

Related reading: busbar protection testing for the overall 87B scheme, and overcurrent relay testing for the line protection that initiates the scheme.

Standards and references

• IEEE C37.91-2021 — guide for protective relay applications to power transformers (breaker-failure coordination).
• IEEE C37.93-2004 — guide for protective relay applications to power system buses.
• IEC 60255-151:2009 — functional requirements for over/under current protection.
• Relay manufacturer’s technical manual — breaker-failure logic and timing.