What happens in a separately derived system when the neutral fails to switch.

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Piet de Pad

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Location
Queretaro
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Solar system Developer and Installer
Hi experts,
I Install solar panels and solar and Gas based backup systems. I use ATS switches to implement non separately derived system and separately derived systems. I have however, one worry about the separately derived solutions.

In a split phase utility with a backup connected in separately derived mode theoretical there is a possibility that the neutral fails to switch. When that happens, there is the possibility to have way too high voltages because there is an electric path from the phase through the devices to the other phase.
Is my worry too theoretical or is this a real possibility, specially when the ATS gets older and the contacts get worse.
PS: i try to add a picture to this thread, but I don't see how to upload images other than HTTP links to other existing images.
 
tom baker said:
I note your business is in Mexico, our answers are based on the NEC…
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Thanks, yes, I am aware of that.
I must be missing something, I see in other posts that images are added are these uploaded or only HTTP links? I made a similar post on and other forum, so I can make a link now.

seperate-derived-system-with-neutral-failure-jpg.18033
 
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hillbilly1 said:
It’s rare the neutral pole fails because it’s usually lightly loaded, but is a possibility. It would create the same voltage imbalance as an open neutral.
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Correct, I agree it's the less loaded contact. If this can happen, is that something to take seriously? The backups I install are mostly residential and if that happens most non 240V equipment will fail forever when they are not hardware switched and hang on the house grid.
 
It is a real worry. You can fry loads if the neutral is open.

My understanding is that an ATS that switches the neutral contains special features to make sure the neutral connection is 'break last, make first' when switching sources. Hopefully such features will stop the switch from moving completely if the neutral is failing to switch over, i.e. 'fail safe. But I don't know. A good reason to do due diligence on any ATS you're using in such an application. Don't try to use one that isn't designed for it.
 
It's possible... but its also possible that it won't transfer at all or the generator won't start or someone left the generator in manual.
Anything can happen.
I wouldn't worry about the neutral not switching anymore than all the other things that can go wrong.
 
I’ve replaced a lot of transferswitches that would “hang” up between positions (display would actually say “Hang”) but usually all blades would be in the same position.
 
I don't believe I have ever wired an SDS system; the neutrals have always been unswitched.
 
jaggedben said:
It is a real worry. You can fry loads if the neutral is open.

My understanding is that an ATS that switches the neutral contains special features to make sure the neutral connection is 'break last, make first' when switching sources. Hopefully such features will stop the switch from moving completely if the neutral is failing to switch over, i.e. 'fail safe. But I don't know. A good reason to do due diligence on any ATS you're using in such an application. Don't try to use one that isn't designed for it.
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That a good point a "break last, make first" I'll remember that
 
ActionDave said:
Unless there is ground fault detection there's no reason to.
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You'd be surprised at how many customers specify and show on their drawings a switched neutral.
Some are utility to generator and some are utility to utility.
 
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