What kind of wall switch did I get??


Old 09-25-08, 01:11 PM
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Question What kind of wall switch did I get??

A few weeks ago I installed a 20A. DP switch to interrupt the 220V to my well pump. This is at my vacation cabin & is for the purpose of shutting off the well when I leave after the weekend. The switch was pretty pricey at the local ma & pa hardware but it looked like a high-quality switch. It was the only one they had in the "20A 2-pole" spot on the shelf & was in an opened box. Some of you are already nodding knowingly ;-) There were no "Line" & "Load" markings by the 4 screw terminals so I verified them with an ohmmeter.

Last weekend I needed to go down in the pit to work on the well pipe so I switched off the pump & removed the little door...and the pump was running and light was on! I got out my meter and found I have 220 across the line AND the load with the switch On or Off. I removed the wires from the well side & there was no power on them. The switch continuity was Open when Off--normal for a DPST switch. But with the wires attached I again had juice on the load side at all times.

This has me completely stumped. What kind of switch is Open to an ohmmeter but conducts current at 220VAC (lots of it to a 3/4HP motor). I'm not a noob but I sure don't know what I brought home from that hardware store. Either that or it's something obvious and I'm having a "senior moment".
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Old 09-25-08, 02:44 PM
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A four-way switch maybe. Here is an animation of a 4-way switch. http://www.wfu.edu/~matthews/courses...itchesTut.html

Notice it doesn't ever open the circuit. It just changes which poles are connected to each other. An ohm meter on A and B would show continuity in one position but open if you flipped the switch because continuity crosses to A & C. Motor wouldn't care though it would still see power on both poles.
Old 09-25-08, 02:51 PM
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Did you by any chance buy a four-way switch instead of a DPST switch? If you have the right type of switch, be sure it has an adequate horsepower rating for the pump motor. An inductive load, such as a motor, briefly draws a very large inrush current when it starts and the switch must be able to handle this current. It's possible the switch contacts could weld together if they can't handle the load.
Old 09-25-08, 07:48 PM
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That's gotta be it! Thanks guys. The animation really helped me to see what was going on. I've installed 4-ways before (deliberately!) but never thought about HOW it worked. This also explains why there's no "Line" "load" markings.

Holy crap if this is just an ordinary light switch I'm amazed it hasn't fried yet from the motor inrush current. Thanks bunches to the idiot that opened the box & swapped the switches
Old 09-26-08, 09:26 AM
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What are the model numbers of your enclosure and the switch that's inside?

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