Introduction - Suppose you've got a power distribution system with some loads fed from a source. But then, OH NO!! A fault takes place on your downstream system near one of your loads? In a well-designed system, the breaker nearest the fault should open up and stop the fault with limited interruption to the rest of the system. What about when this doesn't happen?
Breaker failure is the situation that takes place when a circuit breaker, usually at medium voltage, fails to open when sent a command from a relay. This means that although a trip command was sent to the breaker, it didn't open up in the time frame it should have.
Do you need to plan for breaker failure in your designs? That's a tough question. In a well-coordinated system like the one shown in Figure 1, the upstream breaker should open up after some time even if the downstream breaker fails. If the system is designed for all components to withstand that downstream fault magnitude for the duration of time it takes the upstream breaker to trip, you are probably okay. However, this may not be the case. You may have conductors and/or equipment that are designed only to withstand faults with a clearing time from their local upstream overcurrent protection device (like the breaker feeding LOAD2 in Figure 1). In that case, you'll need to make use of breaker failure protection.
Figure 1: Breaker Failure Example
How does it work? - Breaker failure protection can be implemented in a number of ways, but the simplest is a 2-step check:
Has a trip command been issued by the controlling relay/trip unit?
After some time delay from that trip (for the breaker to pick up the command and operate), is the breaker status still closed?
If the answer to both questions is yes, then the controlling relay/trip unit detects that the breaker has failed. This detection usually corresponds to an auxiliary contact on the relay changing states from open to closed. That contact can be hardwired into other relays or controllers, or sent over a network communication like Modbus TCP/IP back to a control system.
Hardwired signals are often used to trip upstream breakers quickly when breaker failure is detected. This lets time-current curves remain well-coordinated with delay gaps, but ensures a single point of failure doesn't take out the entire system. A network communication isn't as useful for protecting a system because of processing delays, but it does let an operator know what took place and why the upstream breaker was opened instead of the downstream. Otherwise, investigations into the fault may not start in the right place; operators could be confused by the upstream breaker tripping and assume a different location for the fault than reality.
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