Feeders parallel work
Feeders parallel work
(OP)
Is it permissible to work momentarily (time – 50 ÷100 ms) on parallel switchgear feeders (level voltage 10kV) when during this work the initial short circuit current is 35% higher then the initial short circuit current of switchgear. Time given above is necessary to change over between feeders circuit breaker and coupling circuit breaker.






RE: Feeders parallel work
When you do a supply changeover as you propose, you parallel incomers for a very short time only and you are doing this at a time when there is no fault on your downstream system (i.e. downstream of any feeder circuit breakers on your switchboard). For things to go bad, you would need a downstream fault to develop a few mS after you paralleled your incomers. Whilst possible, I think that realistically it is not going to happen. Also, especially with an automated transfer, one of your incomers is going to be in the process of opening anyway.
You could possibly have a look at introducing a small delay to all "instantaneous" trips on feeder circuit breakers. This would ensure that the feeder would be tripping at about the same time as the incomer and would ensure that it would never be attempting to break the higher fault current available when the incomers were paralleled.
I expect there will be some differing views.
RE: Feeders parallel work
Another thought is to provide automatic controls on the coupling breaker(s) to prevent sustained parallel operation in the event of manual operation or primary control failure. You should look at all sources of fault current to evaluate each breaker and bus bracing. Consider significant motors and generators.
RE: Feeders parallel work
To apply the make before break requires that both supplies are synchronized.
RE: Feeders parallel work
http://www.bluesea.com/Instruction/9775.pdf
for: Make Before Break Ignition System
http://www.solidstatecontrolsinc.com/applications/nps.h...
for: Make Before Break Transfer Switch
http://www.tuban.net/CutlerHammer.html
for: Make Before Break Transfer Switch and its advantages
etc. for more info
RE: Feeders parallel work
If the higher short circuit current exceeds the switchgear rating, then this is an NEC violation, based on strict interpretation. I don't think there is any exception for this. If a fault occurred as you closed the tie breaker, it could cause a catastrophic failure.
However, what you describe is commonly practiced, NEC violation or not.
We have worked on projects where the owner negotiated a variance with the local inspector to allow this practice. This generally requires automatic tripping after a fixed time delay to eliminate parallel condition and remote operation of the breakers, so that the operator is not required to be standing directly in front of the equipment.
RE: Feeders parallel work
RE: Feeders parallel work
Bung
Life is non-linear...
RE: Feeders parallel work