Tuesday, March 8, 2011

Automatic Transfer Switch (ATS)

The Automatic Transfer Switch (ATS) the ATS is a relay used to switch power automatically to either shore power or generator power. It's most important function is to prevent generator power from being back through the shore power line, a potentially deadly situation for a linesman working on a line that he thinks is dead. The ATS is a small metal box mounted on the back of your converter/charger.


No power from one of the two sources

The transfer switch uses electromagnetics of the current flowing to make the switch "switch". It might be possible that yours is failing on the genset side. Generally replacement of the ATS is the only option.

If the relay contacts are bad, the relay needs replacement. The contacts do not have enough material for refinishing and a new relay will cost just about as much as the whole box.

There isn't much else inside the ATS besides the relay. Since getting to the ATS is so hard, it's best to go ahead and replace it.


Parallax Power Supply (800-443-4859) makes a replacement ATS. Parallax does not sell directly to the public; Parallax suggested Mastertechs (800-848-0558). They also sell refurbished units. These are mostly customer returns, but are checked and warranted. Pricing was significantly better. The refurb ATS was $42 vs. $65.

The job took 2 1/2 hours, as I very carefully removed and replaced the wires one at a time.

Caution: Be sure all power sources are disconnected.

The directions that came with the unit were OK, but I learned a few things. Of course, I had made sure there was no 120 volt power on, but I almost forgot to disconnect the 12 volt. Also, after struggling with the wiring inside the ATS for a long time, I realized that the top is removable. Once I did that, I had much more room to work, and things went faster. Interestly, the wiring in the old ATS was aluminmum, while the new one is copper.

Contributors: Michelle Cook, Larry Wade, George Kaplan

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