CB Fail Philosophy wrt to current check

[dfdt]

There are two school of thought regarding the philosophy for setting current check/fault detector associated with CBF or BF scheme. One is to set current check at a low threshold level of say 0.2 pu to confirm if the breaker has opened and is used to reset the CBF scheme.
Other is to set current check above the load current to make it more secure against inadverent operation of the CBF scheme while being tested.My question is which of the two philosophy would you endorse/recommend and why?

 

[davidbeach]

If the BF function is in the relay that does the tripping there isn't much reason to set the current detector above minimum; any BF initiation means that there truly has been a trip sent to the breaker. On the other hand, if the BF function is initiated through an input contact on the BF relay you may want to consider a threshold above load to avoid a BF trip for a spurious input.

A significant advantage of every relay doing its own breaker failure protection is the elimination (reduction) of those inputs that can cause breaker failure initiation on spurious assertion of the input.

 

[slavag]

0.2p.u is old style, with long time delay, with long time trip reset, but still used today.
It is historical issue.

Setting of BFP according to fault current or load current, with additional criterion, is better, IMHO.

 

[dfdt]

Thanks guys for your views which were very helpful and FYI w[highlight ][highlight #3465A4][/highlight][/highlight]e set current check level around 50% on min fault level

 

[piterpol]

Attached you will find the paper "Breaker Failure Philosophies"

 

[dfdt]

Piterpol - thanks for the link it has some good info