NEC Emergency System Selective Coordination Requirement

[VTer]

2011 NEC 700.27 states that emergency system OCPDs shall be selectively coordinated with all supply side OCPDs.

My question is regarding the wording of "all supply side OCPDs" Does this include the normal source of the automatic transfer equipment?

My interpretation is that coordination requirement applies to devices on the load side of ATS and alternate source side (emergency side). This seems to agree with definition in 700.2 which indicates that the intent of the emergency system is to automatically supply power to selected loads in the event of failure of the normal source. To me this indicates that the normal source supply upstream of the ATS is not part of the emergency system.

NFPA 110 also defines Emergency Power System Supply as a complete functioning Emergency Power Supply (EPS) system coupled to a system of conductors,disconnecting means and overcurrent protective devices, transfer switches, and all control supervisory, and support devices up to and including the load terminals of the transfer equipment needed for the system to operate as a safe and reliable source of electric power. Even here there is no mention of the normal power source upstream of the ATS.

In addition, practically speaking, in the event of a fault downstream of device on the load side of the ATS and mis-coordination with the device upstream on the normal source side, the ATS will simply sense loss of normal power and start the transfer sequence to emergency source. Once transferred to emergency source, I can see the importance for total coordination requirement. I also believe that achieving total coordination on the normal side is ideal but sometimes it is completely impractical in the instantaneous region due to the magnitude of available fault levels. I do not understand how this can be required in the NEC?

I was just curious what are your experiences, opinions, facts... regarding this requirement and dealings with the AHJ in the NEC world?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[neher]

It required on every project i have ever done.

When an elephant is in trouble even a frog will kick him.

 

[VTer]

neher, can you explain further your meaning that is was always required? Did you have scenarios on the normal source where the fault levels were above the tested combinations levels? What was your solution to breakers where instantaneous trip curves overlap? I do not see how this is even practical on stiff system with molded circuit breakers and branch circuits where instantaneous curves overlap? Also, as I noted above, I do not think it's reasonable to conclude that the normal system is part of the emergency system by its very definition in the NEC and NFPA 110.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[VTer]

The inspector is indicating that the normal system upstream of the ATS has to totally coordinate as well all the way to the main. I do not understand how the NEC can be interpreted that way. Does anyone have any experience with similar issues?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla