My experience with sourcing transfer switches:
1. Cheapest and best. Order a transfer switch with the set. It doesn't have to match the set.
I have ordered 100 Amp ATSs for a set that normally came standard with a 40 Amp ATS. Forget the gen-set, I had to switch 100 Amp mains.
I found that my suppliers would supply an over-rated ATS at the same markup level as the standard ATS. (Mark-up, nil, they want the sale of the set.)
2. For one reason or another there was no ATS available with the gen-set. Roll your own. A pair of interlocked reversing contactors and a phase loss, voltage monitoring relay. It's been a while but I was able to find octal socket relays that were economically priced and would check for phase loss and phase rotation. Add a few timers if you want. Some seconds delay on re-transfer to the utility is a good idea. Consider also a cool down timer for the engine if it has a turbo.
3. Most expensive, Buy a new ATS "off the shelf".
I have several times seen all the electronic devices wiped out in a home when the neutral went open for some reason or other.
Anecdote alert: One EX customer wanted some work done in his home. I told him when I could be there and he said that that was ok.
Then he didn't wait. He got someone from the shallow end of the gene pool to do the work.
The guy crossed a hot with the neutral on a sub panel and burned up his new flat screen TV among other things.
We discussed the work still to be done and when I could be there. All OK.
Then he hired another technically challenged individual who opened the neutral with the power on. This time he lost his micro-wave oven and the rest of his home electronics.
Then he got mad at me. Over the years he was one of several customers that I "fired". grin
The point is that I will do what it takes to avoid switching a neutral.
Sometimes I have had to "Try Harder", but I have always found a way.
That said, I have been fortunate to never have encountered a situation where there was no alternative to switching the neutral.
If you are switching the neutral with a manual switch, it must have an early-make, late break neutral contact.
Contactors are available with an early-make, late-break pole.
A case that I investigated for the utility where a customer claimed that the utility was responsible for the damage to his electronics had a three pole manual transfer switch switching the neutral.
The code allows switching the neutral if all poles make and break simultaneously.
All three poles were equal but the neutral was a few thousandths of an inch less equal.
I explained the issue to the owner and solidly connected the neutral and made him happy. He quit claiming damages.
For a manual switch you may be able to shim the neutral pole or trim 1/8" or 1/16" from the hot blades to ensure the early-make, late-break.
For a contactor you may bend or shim the neutral contacts or put a dab of silver solder on the neutral contacts.
Both the line side and the standby side.
You see why I have always "Tried Harder".
OFG, with the greatest respect; I have never seen a residential set or a commercial set that had an intentional warm up time.
A couple of sets had a few seconds delay (less than 5 seconds) to allow the AVR to stabilize.
A standby set has a tough life. Most of the sets that I have installed and serviced went from cold to full speed and then had about a 300% block-load dropped on them. all within less than 10 seconds. Then they settled down with about 10% to 25% of rated load.
A standby set has a rough life.
Oh, and don't forget, a 110 KVA standby set will be rated at only 100 KVA as a prime power set.
I did have one industrial installation in a remote sawmill where we did need a warm-up time.
We needed two sets in parallel to start the big hammer hog motor.
One set had a hydraulic governor and one set had an electronic governor.
When the big motor when on-line the electronic governor would open faster and hog the load and trip off on overload.
If we started the unit with the hydraulic governor first and let it take the load for about 15 or 20 minutes and then start and paralleled the electronic unit, the oil would be thin enough in the governor that the motor could be started without tripping out.
A thought Keith.
An invertor may not be as forgiving as a generator winding in regards to back EMF of spinning motors.
70 ms is only about two revolutions. you may want to arrange an intentional delay of 100 ms or 200 ms or even allow a few seconds if there are reclosers on the distribution circuit. You don't want to subject your invertor to an intimate relationship with a recloser banging in and out about 3 or 5 times in a few seconds.
I have lots of experience applying ATSs to generators and none with invertors.
I did one a couple of invertor jobs but they were manual transfer switches.
Keep us in the loop.
Bill
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"Why not the best?"
Jimmy Carter