Dedicated ATS for emergency loads, Why?

[sceepe]

Ok, heres the deal: New School, owner wants emerg lights / exits, coolers, freezers, fire alarm, security system, UPS, and fire pump, on generator.

It is pretty clear that 700.6(d) requires the fire pump and emerg lights / exits on a separate transfer switch from the coolers, and freezers. I am not questioning if you have to do it but why.

If the generator is sized to run all the loads simultaneously, what does the second transfer switch get you? Is it that it insures separation of loads?? Same Generator to fail or run out of gas, or have a dead battery. Whats most likely to fail the generator or the transfer switch? Separate ATS are not redundant so no benefit there. I suppose a short in the feed to a non-emergency load could open the upstream breaker for the emergency loads if the breakers are not well coordinated.

There is a lots of extra expense for two generator backed distribution systems throughout a large facility. It seems that the code is really pushing design toward battery packs (ATS for fire pump is normally integral to controller and thus separate). Leaving the generator suppling only optional loads.

The owner's maintenance staff ends up with a school full of integral batteries or wall warts that have to be maintained. Nice.

 

[caj1962]

Re: Dedicated ATS for emergency loads, Why?

My take and IMO the reason why is that the stuff you mention first, fire pump egress lighting, ETC. Is the LEGALLY required stuff, that has to be on no matter what. The other stuff you mention is optional loads, nice to have but don't have to. Just my 0.02 worth.

 

[W6SJK]

Re: Dedicated ATS for emergency loads, Why?

Originally posted by sceepe:
There is a lots of extra expense for two generator backed distribution systems throughout a large facility. It seems that the code is really pushing design toward battery packs (ATS for fire pump is normally integral to controller and thus separate). Leaving the generator suppling only optional loads.

Before the need for two transfer switches was added/clarified (in 1996), you still needed to keep the emergency circuits wired separate from the optional STANDBY circuits and general circuits, so a failure in the general or standby circuits would not take out an em circuit.

But you can run the optional circuits with the general wiring. So the amount of wiring changes very little.

So the added expense caused by the 1996 code "change" is just for the 2nd transfer switch. The code was changed to clarify that the em loads had to be kept separate from the general/legal/optional loads everywhere, including the transfer switch(es).

Again, the intent was to prevent faults in the general wiring and legal/optional standby circuits from taking out the emergency circuits/transfer switch. Having a second switch also allows you to delay the amount of load transfered to the genset at one time, by delaying the transfer of the optional circuits beyond the 10sec requirement for Em. circuits.

 

[charlie b]

Re: Dedicated ATS for emergency loads, Why?

I see it as a matter of reliability, and specifically of conditional probabilities. First, what is the conditional probability that a generator will fail, given that the utility power has been lost? Well, those two events should be independent. Therefore, the answer is the same as asking for the probability that the generator will fail, regardless of what else is happening around the world. If you keep up the maintenance and testing required by code, the generator should be adequately reliable, and should be there when you need it.

But what about the conditional probability that a fault in the ?optional standby? equipment will occur, given that the generator is running? Are those two events independent? Can anyone think of a scenario in which (1) The utility power is not available, and (2) You need the generator specifically because you need the fire pump and emergency exit lights, and (3) A failure in an optional standby load occurs? I can. A fire could get you there, specifically a fire in an area that impacts the utility service components and that engulfs a nearby room with who-knows-what wires inside. I think that is why the emergency circuits must be separated to the maximum extend possible.

But how does the extra ATS add to the reliability, and specifically to the separation of emergency from optional? By placing the point of common connection between the two systems one step further from the potential fault point (i.e., in the optional equipment). Putting it another way, it reduces to one, and only one, the number of components that serve the optional equipment, and whose failure could prevent the operation of the emergency equipment. That one is the generator itself. If you don?t add the second ATS, then there would be two components (the generator and the ATS) that serve the optional equipment, and whose failure could prevent the operation of the emergency equipment.