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A "split" transfer switch


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  #1  
Old 01-29-2016, 05:17:24 PM
Dave Edmonds Dave Edmonds is offline
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Smile A "split" transfer switch

Question for the Resident Smart Guys who regularly work on automatic transfer switches (I don't have one - mine's manual):
A post by Power (post #34) in a recent thread "Onan 12.5 JC, What have I destroyed" got me to thinking - -

Many, if not all, automatic transfer switches usually have two motor-driven contactors in them, one for the utility power and one for the generator power ( I think).

My question is - are these two contactors mechanically interlocked so that both of them cannot be closed at the same time, or are they independent, and depend upon the electronics to operate only one at a time?

My thought (and this is just a thought - I don't plan to do this) is that, if they are not interlocked, and your generator is in an outbuilding connected to a subpanel (like the thread listed above), could you "separate" the two contactors and move the Utility one from the outbuilding to the main panel, and control it with small-gauge wiring back to the automatic transfer switch located in the outbuilding? The Utility one would have to be put in a weathertight cabinet and connected between the meter and the main breaker, but that wouldn't be an insurmountable problem. Since most automatic transfer switches are not weatherproof, having it in the outbuilding would solve that problem as well.

Thanks for any thoughts from the Resident Smart Guys.

Dave Edmonds
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Old 01-29-2016, 05:28:09 PM
len k len k is offline
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Default Re: A "split" transfer switch

In theory it could work. But in practice the reliability of a mechanically independent switch (relay) really comes into question (sticking, welding of contact, ect). I have not seen any 2 part transfer switch device like this sold or UL rated for this purpose.

I think the grave risk of death from a failure, is why you don't see mechanically independent transfer switches. Even the ones in a transfer switch boxes have the 2 switches mechanically tied together, not just 2 independent relays.

Also since the 2 ends of your "transfer switch" design are likely not to be within sight of each other, there is the possibility some one may just cut wires and "power on" the relay if the sending end fails. Not knowing it's half of a transfer switch.

Last edited by len k; 01-29-2016 at 06:47:30 PM.
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Old 01-29-2016, 07:40:07 PM
Ted_Cool Ted_Cool is offline
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Default Re: A "split" transfer switch

Just for the sake of completeness, I will mention here that all the designs I have seen are both mechanically and electrically interlocked. Double safe, like belt and suspenders...

The basic setup with contactors and switches already wired correctly is used by the millions for electical motor reversing controls. You can find dozens of them ready to go on eBay at any given time.
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Old 01-29-2016, 07:50:58 PM
Tracy T Tracy T is offline
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Default Re: A "split" transfer switch

Mine is mechanically tied together where one HAS to break before the other can make and operated by one linear motor.
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Old 01-30-2016, 09:07:23 AM
Kyle K Kyle K is offline
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Default Re: A "split" transfer switch

Actually a lot of the larger gear in commercial/industrial applications are set up similar to your idea. Main on my end and generator on other end with motor driven circuit breakers with no mechanical interlock. From a practical point you would need to run wires to generator so why not just run the power wiring at this point. Also remember that most transfer switches have a service entrance rating on them where what you would build would not. Also on a side note I am seeing more and more automatic and manual transfer switches come through as 2 independent switches not a true double pole double throw we where all used to. Your idea has merit but in reality would probably cost more than just doing it right which to be honest with you is the norm in the electrical business! Also as a public service announcement I think Interlok Kit is the greatest thing made as it eliminates the happy homeowners dryer plug back feed but we have found some inspectors do not accept it do to a UL rating issue. Typical egomaniac inspectors lets add more hazard just because you want to prove your authority but anyhow just wanted to throw that out there.
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Old 01-30-2016, 01:09:44 PM
Gunny Gunny is offline
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Default Re: A "split" transfer switch

Almost all automatic transfer switches have a mechanical interlock of some sore to prevent being connected to both sources at the same time.

A nice home made set up I saw a while back was (2) breakers mounted side by side. One was up side down. The were connected in parallel on the bottom, which was the load side. On top, one side was utility, the other was generator power. Handles were linked, both breakers moved the same time. One was always on, the other was always off.

Else, there are a lot of different ways to do it. Just think it through and think of what might/could happen.
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Old 01-30-2016, 02:07:39 PM
len k len k is offline
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Default Re: A "split" transfer switch

I was thinking about motor driven switches to improve reliability. And you could add limit switches to confirm that one source was off before turning on the other switch. But by the time you design in all that stuff and get it UL listed and present it for sale , it's likely cheaper to just run a wire from gen to main breaker and use interlock or standard transfer switch. Plus by using standard techniques you likely won't run into problems with picky inspectors not familiar with this new equipment for residential uses.
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Old 01-30-2016, 02:09:16 PM
Kyle K Kyle K is offline
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Default Re: A "split" transfer switch

Let me clarify something. The mechanical interlock still exists but it seems they build it around componets of a standard switch.
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Old 02-02-2016, 05:57:11 PM
Dave Edmonds Dave Edmonds is offline
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Smile Re: A "split" transfer switch

Thanks for the replies.

I can definitely see benefits in putting some type of limit switches on the contactors to make sure one is in its open position before the other one can be closed. I have met a few stuck contactors on occasion; having both contactors closed at the same time would let a LOT of magic smoke out of something. I would also think that some well-placed fuses ought to be part of the equation, to provide something that would give up before any total disaster happens.

I'm not actually planning on doing this; my home unit has a 200-amp manual transfer switch, and it works fine. My idea was, since most automatic switches are not weatherproof and, if the only practical place to put the utility disconnect contactor is outside, the generator and the generator "half" of the transfer switch (including controls) could be located remotely (in a remote building or similar enclosure), and conductors rated only for the generator current capability (in my case, 60 amps) plus some small-gauge control and limit switch conductors would need to extend from the remote location to the service entrance. The utility disconnect contactor could be located in a much smaller weatherproof enclosure next to the meter base.

Dave Edmonds
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Old 02-02-2016, 11:14:11 PM
pegasuspinto pegasuspinto is online now
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Default Re: A "split" transfer switch

Standard ATS units are usually both mechanically and electrically interlocked. They do make more specialized switches that can do a 'closed transition' (both sources parallel briefly to prevent a power interruption) or can parallel the generator and utility long term. Both kinds of units require extra controls to keep the electrons in their place....
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