MV Breaker Failure 50BF: Build the Timer and Intertrip Budget

1. Define the failed interruption

Breaker-failure protection asks whether the selected breaker has removed its current contribution after a legitimate trip command. It is not just an extra overcurrent delay. On an MV bus supplied by two incomers and a closed coupler, opening only the upstream breaker of the failed feeder may leave another source feeding the fault. Define the isolation set from the single-line diagram for every permitted topology. The SEL breaker-failure paper discusses initiation, reset and backup tripping. Use the feeder-setting guide for ordinary grading; this guide concerns failed interruption and its escalation.

2. Obtain the timing boundaries

Record the initiating protection functions, trip-output delay, interposing relays, breaker maximum interrupting time under the specified duty, current-detector reset, auxiliary-contact timing and timer tolerance. Obtain minimum fault current through the failed breaker for each source configuration and CT performance at maximum current. Specify where the timer starts: an internal protection bit and an external trip contact do not have the same delay. Use the actual relay manual, not a setting name from another model. The SEL-311C sheet illustrates that detector timing is a device-specific specification. Do not apply that timing automatically to a different IED.

3. Illustrative local timing budget

Assume the timer starts when the protection trip command asserts. Allow 5 ms for the output/interface, 60 ms for breaker interruption, 15 ms for detector reset and 20 ms engineering margin. The resulting illustrative timer is 100 ms.

If failure persists, suppose backup logic and communication take 10 ms and the backup breaker interrupts in 70 ms. Clearing after initiation is then 180 ms.

Fault detection before initiation is excluded and must be added for total fault duration. These assumed values are not recommended settings. Check thermal duty, arc-energy assessment, process consequences and any stability constraint against the complete timeline, including worst-case tolerances.

4. Match the logic to the initiating trip

The SEL-352 data sheet identifies different schemes for fault and other conditions. Auxiliary contacts provide useful position evidence but are not interchangeable with measured interruption. A pole can fail to interrupt even when a mechanism contact changes state. Conversely, retained contact status can falsely suggest failure after current has ceased.

5. Coordinate every source and restore selectively

Define which incomer, coupler, generator or remote breaker receives backup tripping with each bus configuration. Include reverse power flow and embedded generation; the normal power-flow arrow is not the fault-source map. Prevent automatic reclose or transfer from immediately re-energizing the failed isolation zone. Document lockout, reset authority and the conditions for restoration. A retrip can improve clearing for some control-path faults, but sending it down the same failed coil cannot cure that coil's failure. The final timer must allow successful primary interruption while meeting the project backup-clearing requirement.

6. Commission the failure rather than only the timer

With an approved isolated test setup, demonstrate successful opening resets the scheme before escalation. Simulate persistent current and verify retrip, backup destinations and lockout. Separately test missing initiation, stuck auxiliary status, low-current initiating trips and communication loss. The SEL paper explains why CT subsidence current can delay a detector's reset after primary interruption; include the manufacturer's relevant test method. Capture initiation, current detector, timer, outputs and backup response together. For Ethernet intertrip, apply the GOOSE failure-supervision workflow. Retain as-built logic and topology cases alongside the setting file.

7. Setting-review questions

Can a fixed timer serve every MV breaker?

Only if project evidence supports all included breakers and operating cases. A familiar value is not that evidence.

Is an open auxiliary contact enough to declare success?

Not for every failure mode. Use the designed combination of current and position evidence.

Should 50BF start on every trip command?

Define the initiating set deliberately. Maintenance commands, nonfault trips and protection trips may require different supervision and authorization logic.

8. Primary references

• SEL-352
• SEL-311C
• SEL — Breaker-Failure