Diesel Generator Breaker

[FaradayFF]

Hi Folks,

What's the function of diesel generator circuit breaker? I understand that it is used to protect the generator from internal faults. However, once a generator relay detects internal fault, shouldn't it send a command to stop the engine? How would tripping the breaker aid in "suppressing" the fault, apart from preventing contribution from the system(large induction motors?).

I guess I'm asking a stupid questions, but wanted to hear an explanation.

Thank you,
EE

 

[zbang]

What's the function of diesel generator circuit breaker? I understand that it is used to protect the generator from internal faults. However, once a generator relay detects internal fault, shouldn't it send a command to stop the engine? How would tripping the breaker aid in "suppressing" the fault, apart from preventing contribution from the system(large induction motors?).

a) overcurrent protection and as a disconnect
b) and external wiring faults
c) "generator relay"?? how big is this generator?? I've never seen one under maybe 400KVA that has a "protective relay", but they do have a large CB.
d) If the generator controls (not a classic protective relay) detect an internal problem, they might kill the engine, but that means more sophisticated controls than many gensets have.
e) I'm not sure what that means.


How big a generator are you asking about? How modern is it? What's the application?

 

[wbdvt]

Unless it is an extremely large genset, the function of the generator breaker is really no different than any other breaker which is to protect the conductor and possibly overloading of the generator. If there is an internal fault in the generator, the shutdown will be because of other protective devices such as voltage regulator.

Think of it this way. In order for the generator breaker to trip on a fault, it needs to see fault current in excess of it's trip curve. Typically a genset is the only source of power, so where is the external current flowing into the generator fault coming from that would pass thru the generator breaker?

 

[ron]

In larger generators, the controller has protection functions for the alternator, but not "listed" as a protective relay, so the code required breaker protects the feeder and downstream

 

[Birken Vogt]

Standard breaker curves do not really protect the generator windings properly. They are mostly there to provide a handy disconnect and so that the 115% rule does not have to be complied with. In the olden days it was more common to have generators with no immediate main breaker.