Automatic transfer switch issues

[RelayDesignMan]

I'd like some feedback on an issue I'm having. I want to upgrade the automatic transfer switch we're using for the substation station service AC feeds. The station service transformers from each bus need to feed into an ATS which should feed into an AC load center panel. In the past, we have let contractors who put in the AC load center panel provide a couple of contactors with AC coils and throw those into a junction box with the two AC feeds wired through them so only one contactor at a time is feeding the load center. It's cheap and dirty; but I'd like to move to an actual ATS switch like you would buy from ASCO or LakeShore.

Can anyone help me with some arguing points against wiring the loose contactors in a junction box and moving toward a real ATS? I don't believe what I'm seeing out there now is the right way to go.

Thx

 

[waross]

Your big arguing point is the possibility of both contactors closing at the same time. I have installed many contactor based transfer switches, but they always have a mechanical interlock such as is used with a reversing starter.
I would consider at least a reversing starter with the mechanical interlock as well as the electrical interlock but without the overload relay. If you buy it in an enclosure the saving in contractors labor charges may cover the extra cost.
Other points;
A latching type is not wasting coil power.
A latching type won't drop out momentarily on a power dip.
Beware, if you choose a very fast open transfer type, both feeds should be on the same bus. This type does an open transition so fast that it looks for synchronism before it switches. However it may switch fast enough that motors may stay running and computers may not re-boot.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RelayDesignMan]

Thanks, those are great points!

 

[rcw retired EE]

Auto transfer switches can have more intelligence to prevent mulitple transfers. Example: The normal power source voltage dips due to a bad connection in the feed to the contactor ATS, so the ATS transfers to Alternate power. With no current through the bad connection the sensing voltage jumps back to normal, so the contactor transfers back. Teh load current dropps the sensing voltage, causing transfer and so on. An ATS can be programmed with time delays or a retransfer lockout.

 

[Ceast]

Well, a ASCO 300 amp ATS will cost you about $3500. Sounds like your labor/parts cost will be close that with a make shift unit.

 

[slavag]

Hi.
I think today, is simple make ATS by smart relay like to Siemens LOGO or Shneider ZELIO with few undervoltage relays.
Simple, cheaper and possible build logic according to your
needed.
Regards.
Slava