Transfer switch downstream fault

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John Wacker

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We have an automatic transfer switch with a main lug panel connected to the output. The overcurrent protection is at the source panels to the switch. The result is that if there is a downstream fault, the transfer switch will transfer and the fault will be hit a second time.

Does the code speak to this issue?

Should we install a downstream breaker?
 
The code normally wouldn't address a design issue like this. The code would only cares that the feeders are protected, it wouldn't care about poor reliability except in certain situations such as OCPD selectivity in life safety ckts.
If the MLO panel is the only load, then whether the two upstream breakers trip or the main of the downstream panel trips, you still drop the load.
 
Thanks for reply. The issues is that there is no donwstream breaker, only the upstream breakers. So the fault will get hit twice. The first upstream breaker will see the fault and trip. Then the transfer switch will transfer closing into the fault. The second upstream breaker will then see the fault and trip.
 
Say it is a 400 Amp ATS fed by two 400A breakers. If you put a 400A breaker on the ATS output, you have no coordination between the breakers and for most faults, both the normal power source breaker and the downstream breaker will trip, or maybe just one of them will. You could still have the smae problem.

If you are really worried about it, add bell alarm contacts to the breakers and wire the contacts into the transfer switch to prevent transfer if the breaker trips. Depending on the breaker size and type, there may be separate contacts for overload and short circuit trips.
 
This isn't a code issue but a design issue.

What is downstream of the ATS? What I mean is if there is just one load and it has shorted on you, then the fact the ATS is going to switch sources and then fail again is immaterial, as the load is still busted and will never work until you fix it. Though Bob is correct; you dont really want to close a contactor onto a short; it wont like that, but a single downstream breaker coordinated with the upstream breakers will fix that problem.

But it introduces a second problem; ifyou get a transient short on the load then one upstream source will open, the ATS will flip, and then everything continues as it should. The dual source breakers and no downstream breaker improves the chances the load will come back on strong. It doesn't solve the problem of why the short occured in the first place, of course. Overall though, this may be a better outcome, when balanced against reducing the service life of the ATS contactor.

However... if there are multiple downstream loads then the downstream panelboard breakers should (be set to) discriminate correctly so that the one faulty load is isolated, leaving everything else running.
 
arc flash

arc flash

Thanks to everyone for all of the comments.

When I made the original post I was thinking more of safety issues, especially arc flash. I was not too worried about equipment damage, but was thinking that it would not be good to have the transfer switch transfer and reclose into the fault because a second arc flash exposure would result.

In reality, in my case the exposure is really very small. The transfer switch will be setting setting next to the panel, which will either be a main lug panel or a main breaker panel. The only difference in exposure is for a fault on the bus of the main panel itself - really very little exposure.
 
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