main/tie/main

[jim dungar]

Let me try to explain this better. Power comes out of a 400a breaker at a outdoor sub. 13.8kv underground to a existing building entering a load interupter switch[ a main without a key] ,busses to a fused interupter switch witch feeds into the building sub. , outdoors after the fused switch there is a switch [tie without the key] than another fused switch going in the building to another sub. than after that switch there is aload interupter switch [main without a key] than out to the next building underground. Picture this with 3 buildings. As long a 1 switch is open somewhere in this senerieo all is good. You would have 2 breakers feeding 2 systems. If that switch gets closed you would complete a circle and that would not be good. There must be a violation here somewhere. You would also be feeding some of these load interupter switches on the load side so if you open the switch to open the door it would still be hot on the load side. This system is serviced by only experienced people.

It now sounds like you describing a loop feed arrangement and not a parallel one. How many sources (substation transformers or utility feeds) are there?

It is common to have one source feed (2) breakers, one feeding the loop clockwise, the other counter clockwise. In these cases the fault current,on the switchgear bus, is not doubled like it is with a parallel feed.

 

[zog]

Zog,

Why the breaker will not able to interupt the fault if the interlock is removed??

Well Laz is right I was making an assumption of the way the OP's system is laid out. But in a typical M-T-M configuration like I post the drawing of in an earlier post the AIC rating of the breakers is typically based off the fault current from 1 transformer. If you remove the interlock and close all 3 breakers you have parraled the 2 transformers and doubled your fault current, which usually means it exceeds the AIC rating of the equipment. The key interlocks are installed to prevent this from happening.

An alternitive is to use breakers with higher AIC's, which usually means fused breakers like an AKRU (GE), KDON (ABB), and DSL (Sq-D)

 

[mayanees]

An AHJ can always approve a time limited automatic closed transition as the likeliness of a fault while transferring is infinitesimally small. (IEC recognizes this as a valid transitional mode without needing to double the SC withstand.)

Laszlo,

I've always stayed away from any overlap, because I've convinced myself that regardless of the odds, the destruction associated with a fault while transitioning can be catastrophic. And the point of switching probably has a much higher inicidence of problem-related activity than under normal operation.
If I were to ever approve a limited-time overdutied bus during a closed-transition automatic transfer, I would make stipulations that the area be clear of personnel that could be affected if it faulted.
I think today's systems are much easier to fully rate than much of what's out there. But I also realize that there are allot of systems out there that need to be dealt with.. safely...

John M

 

[Mike01]

Originally Posted by weressl
An AHJ can always approve a time limited automatic closed transition as the likeliness of a fault while transferring is infinitesimally small. (IEC recognizes this as a valid transitional mode without needing to double the SC withstand.)

Laszlo,
This is intresting I have never seen this do you know where in the IEC I can find this reference. thanks.:grin:

 

[john d]

this is not a parallel feed there is 1 power source, with 2 breakers the power starts at a 400a breaker going clockwise and a 400a breaker right next to the other 400a breaker going counter clockwise.

 

[weressl]

Laszlo,

I've always stayed away from any overlap, because I've convinced myself that regardless of the odds, the destruction associated with a fault while transitioning can be catastrophic. And the point of switching probably has a much higher inicidence of problem-related activity than under normal operation.
If I were to ever approve a limited-time overdutied bus during a closed-transition automatic transfer, I would make stipulations that the area be clear of personnel that could be affected if it faulted.
I think today's systems are much easier to fully rate than much of what's out there. But I also realize that there are allot of systems out there that need to be dealt with.. safely...

John M

John,

I appreciate your POV.

Risk assesment and evaluation is at the core of any system that addresses safety.

I wonder what your opinion of the Division 2 area classification approach is, in contrast with your position above.

Just to be on the same page; the Division 2 presumes that ignitable mixtures are only present infrequently, not part of normal operation and as the result of process equipment failure, or the like. It is stipulated that electrical equipment that does not present a source of ignition - either by arcing or heat - under normal operating condition is acceptable in Division 2 location, and equipemnt listed for those locations are tested for that criteria.

Mind you that there are magnitudes more Division 2 equipment are out there compared to M-T-M and the explosion in a Classified area are generally far more devastating than the failure of an electrical switchgear.

I think I start a poll