# Automatic Transfer Switch Help



## LightSparks (Dec 26, 2017)

This is not for a generator, sorry if this is the wrong forum.

Okay so I have both AC and DC equiptment that I would like to keep on 247. Currently the plan is to have everything powered from the street power until power loss, then it is run on batteries, and when street power is restored, it will revert back.

The design uses 2 different DPDT relays, 1 for 120vac, and the other for 12vdc so that the street power can also continuously power the inverter, instead of needlessly draining the batteries.

My question is if having both relays on with no delay, similiar to a bypass switch on a generator relay timer delay, will there be an issue since both switches have to operate correctly when an outage occurs, more specifically the dc relay needs to work a few milliseconds faster that the ac relay.

Is it as simple as extending the cord length to the ac relay to increase the time the electricity takes to reach it, or is there a simpler means of achieving this, because since the inverter is also on a relay, a standard 10 second generator power trasfer delay timer will cause a power outage.


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## motormonkey (Jul 10, 2016)

Why would the DC relay need to work before the AC relay? Why make it more complex than it has to be?

All you are doing is creating a UPS (Uninterruptible Power Supply).
When utility power is present, the UPS charges the batteries and allows utility power to pass through from utility power to the loads.
When utility power is not present, the UPS energizes the inverter and switches the loads to the inverter. (Not necessarily in that order.)
The only difference will be that you are using the UPS batteries to power your DC loads. No switching is necessary there.

If the DPDT switch you use for switching AC is a "break before make" type, (The utility is disconnected from the load before the inverter is connected to the load during switching.) it won't matter whether the inverter is energized or the switch is switched first. This is probably what you have.

If the DPDT switch you use for switching AC is a "make before break" type, (The inverter is connected to the load before the utility is disconnected from the load during switching. The utility and the inverter are momentarily interconnected during switching.) your inverter must be synchronized with the utility power prior to switching. That's a big can of worms to open.

BTW, cord length is probably not going to create the delay you want anyway.


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## thehandyman1957 (Feb 11, 2017)

I agree that your set up is a bit over complex. Why do you need a 12 relay set up anyway?
Is your 12 load higher than the battery charger can provide? If so, your talking big money for 
a 12 volt relay that could handle those types of loads.

Normally you would only have one relay for the 120 shore line, via inverter or shore power to your home.
Have your battery charger hooked to the relay so that it energizes only when on shore power but is 
off when running on the inverter. 

You can do this by giving a shore line only output from the transfer switch that allows the 
charger to only run when shore power is present and is not switched when power is gone.

And as Motormonkey said, normal transfer switches are "break before make" and that is what you would 
need to use. Using a "make before break" type could not only potentially back feed inverter power to the grid, it will instantly 
fry your inverter as it cannot handle the back feed into it just to name a few of the issues with that set up.


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## KRE (Nov 29, 2011)

Instead of trying to reinvent the wheel, just pick up a Trace inverter an be done with it. A Trace inverter is not cheap but has built in ATS an float an equalize battery charger. They can be had in most all configurations. What your trying to do can be done with SCR's an gated diodes but, to be on the safe side will require redundancy which you have not allowed for. Never power your DC gear from a power supply if you have battery's on site. If your trying to cancel RF hash there are many ways to do that.


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