Spot Network AICs 2

[mparenteau]

Folks,

Question: We have a number of spot networks on our campus. Most configurations are as follows: 13.2kV to a 480V or 208V system where we have two matched step down transformers, each feeding a NP to a common bus.

Here's my question: From an interrupting perspective, I understand that under NORMAL operating conditions, each NP will only have to interrupt/withstand 1/2 of the total available fault current. What about in case of NP failure, e.g. internal failure of the cartridge (circuit breaker)? Wouldn't the NP see the full available fault current in this case, and wouldn't you rate the NP for this condition? Or, is the likelihood remote that you don't consider that scenario? I know utilities have used them for years...I just don't know how the systems are designed.

Thank you for your comments,

Mike

 

[davidbeach]

DO NOT count on the NP to be a fault interrupting device for the purposes of arc flash considerations or low-side fault clearing. Unless you know, as a fact, that it will interrupt in any reasonable amount of time, assume it will be very slow. What network protectors are good for is tripping for reverse, or insufficient forward, power flows. The typical network protector will wait to allow something downstream to clear first. After all, you don't want to drop the whole network for a fault on a circuit coming off the collector bus. A typical network installation is more likely to rely on cable limiters to clear collector bus faults than it is is to rely on the network protector. For 208V area networks, many systems simply allow cable faults to burn themselves out. Can't do that at 480V, but take care.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[mparenteau]

I maybe should ask: from a withstand perspective, would you include all the available fault current or just 1/2 as described?

Thanks,
Mike

 

[davidbeach]

The maximum current through one NP is definitely less than the total fault current available if all are closed. One without the other could get to more than 50% but not to any where near 100%. Run the fault studies.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[mparenteau]

Fault studies are complete, which precipitated the question. My inference is that only normal operation is considered, and failure modes are not.

Mike

 

[bacon4life]

I suppose there are different interpretations of "complete". Personally I would only consider a fault study complete once it includes a reasonable analysis of potential failure modes and likely alternate system configurations. Having a NP out of service is definitely a case I would want included in a fault study.

 

[FacEngrPE]

Easypower indicates Network Protectors can be part of a bus definition.

http://help.easypower.com/ezp/9.7/Content/13_Database_Technical_Ref/dtr_Bus_Data.htm

I agree with bacon4life, expected, and even undesirable system configurations are a necessary component of fault and coordination studies.