In recent musings about generators, UPS systems etc. I have been thinking about transfer switches.
Is there much difference between a "transfer switch" such as this:
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a suitably rated contactor such as this:
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(rating would need to be sized according to electrical load)
Obviously one comes in a nice box, with accessories - but as long as the contacts break before they make, surely these will both do the same job?
I'm also unsure what extra complexity an "automatic" transfer switch has, beyond perhaps a short time delay circuit that will activate the contactor once the secondary supply starts up..
I've no plans to implement anything just yet, but am definitely curious as to how much could indeed be d-i-y-able, or which parts would still need to be purchased off the shelf.. Previous discussion threads regarding generators, earthing etc. would of course still apply.
Well as I read the description of that it is a simple 3 pole on/off switch with an extra aux on/off pole. It has no "change over" function and as all contacts are NO you can't do "something clever" with the wiring.
If the contactor you linked to was at least double pole change over break before make yes but as I see it it isn't... There may also be regulations about contact separation within the switch, a contactor might not have enough.
The B&S transfer switch does have everything in the one box, the RCD is essential IMHO.
What are you backing up with a genset, ordinary domestic stuff or work/income related? TBH unless you *really* need 100% up time an autostart genset, UPS to bridge the gap, and all the associated control and switch gear is not worth it. If you have to drag the genny out of the garage, run a cable etc, is operating a switch that much more?
Don't forget what happens if the genset fails. Do you want to drop back onto the failed, over/under voltage mains supply? Give much more thought to failure modes and what happens as a result.
We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Jonathan saying something like:
The Briggs changeover switch is all in one and being manual, is relatively foolproof.
The contactor needs to be a changeover type - much pricier and you need detection, timing and activation relays or circuitry to make it work reliably and, more importantly, safely. It's not so much your safety, it's the life of the bloke working on the power line that's important. By the time you've built a safe changeover panel you will probably equal or exceed the cost of the B&S switch and if your diy panel kills someone, your arse will be in a sling.
I used to make these things for a living - trust me on this, it's not worth making one from scratch unless you can fully test it and guarantee it will fail safe. If I cocked it up, I was covered by my employer's insurance - you could lose your house.
There are plenty of old changeover panels knocking around on the used market as gensets are ripped out or upgraded, you could pick one up cheaply enough, but since you're here asking, I must assume you don't have the knowledge to fault find and rectify on a used one - and they do develop faults.
Seriously - if you really want one, bite the bullet and go for the fully manual B&S - it's much safer.
On Sun, 31 Aug 2008 15:31:35 -0700 (PDT) someone who may be Jonathan wrote this:-
To add to what others have said, some things to consider in your musings. There are probably more things to consider, these are off the top of my head:
What do you want to do?
Automatic changeover on failure of external supply?
Automatic change back when external supply healthy? What equipment are you dealing with?
What happens while the generator is starting up?
How are you going to do it?
What constitutes a failure, how do you detect it?
How do you design and install the equipment?
How do you test it?
Some thoughts on these:
Automatic changeover on failure of external supply? This is "easy" from the operational point of view. You don't have to worry your pretty little head about it, a box of tricks does the work. However, such complication may not be necessary.
Automatic change back when external supply healthy? This is a little more problematic. Does the box of tricks switch back to the external supply immediately, shutting down the generator just as the external supply fails again, or does it wait? For how long? How does the box of tricks monitor the health of the external supply? Is automatic change back necessary, even if automatic changeover is considered necessary? What equipment are you dealing with? If this is in support of a large computer installation then that is a different matter to a house or office.
What happens while the generator is starting up? Is it acceptable to have a total failure of electricity in the building while the generator starts up? Does the building have a few battery powered lights so there is not total darkness? Are you going to use this time to shed loads (few generators are rated for the full load of a building)? Is there any equipment which must keep going, via a UPS. How many attempts at starting up is the generator to be allowed and can the UPS run for this long?
What constitutes a failure, how do you detect it? Not as obvious an answer as it seems. While in many cases a voltage operated device will do the job in some circumstances a frequency operated device may be a better one to use.
How do you design and install the equipment? The equipment must detect the failure of the external supply, initiate the starting sequence of the generator, shed loads as necessary, disconnect the external supply, change over the earthing arrangements without the possibility of leaving the building un-earthed, detect that the generator has run up and is ready to take the load and connect the generator to the building. It must prove these steps have been taken before it goes to the next step. That is just automatic changeover.
How do you test it? Not just the box of tricks. The rule of generators is that no matter how often they are tested by simulating the failure of the external supply, at least some of the dammed things will fail to start properly when needed for real.
In my opinion for a generator at a house there is no need for all this rigmarole. Have a torch by the bed, with charged cells. Have a few emergency luminaires on the stairs. Have a few candles and matches in the house. Excavate the generator from the garage and connect it up.
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