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Old 10-25-2012, 08:35 AM   #1
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SPST Switch Strange Functions


I have a SPST (single pole, single throw) switch that controls two lights, but in a strange way. Light #1 is on and light #2 is off when switch is off. When you turn the switch on then light #1 turns off and light #2 turns on.

How is this possible with a SPST switch? My first thought it was switching the neutral, then I thought there must be a relay built into the system.

This is an older house, no grounds involved. Any help to understand this would be appreciated.

Dan

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Old 10-25-2012, 09:32 AM   #2
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SPST Switch Strange Functions


If you had a 3way switch and each light was wired off a different traveler terminal it would work as you describe.

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Old 10-25-2012, 10:24 AM   #3
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SPST Switch Strange Functions


How many wires in the switch box?
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Old 10-25-2012, 01:02 PM   #4
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SPST Switch Strange Functions


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If you had a 3way switch and each light was wired off a different traveler terminal it would work as you describe.
It's definitely not a three way switch. Only two terminals.
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Old 10-25-2012, 01:08 PM   #5
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SPST Switch Strange Functions


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How many wires in the switch box?
Four wires in the switch box, all black (K&T wiring). Two wires connected, the other two wires terminate at the switch terminals.

Last edited by DanS26; 10-25-2012 at 01:49 PM.
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Old 10-25-2012, 01:15 PM   #6
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SPST Switch Strange Functions


Are the light bulbs the same wattage?
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Old 10-25-2012, 01:20 PM   #7
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SPST Switch Strange Functions


I'm thinking that some how the are run in series, but at the same time I can't see how with the SP switch interrupting power.
Just typing while thinking.
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Old 10-25-2012, 01:25 PM   #8
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SPST Switch Strange Functions


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Are the light bulbs the same wattage?
Both bulbs same wattage.

I'm still thinking there has to be a relay buried somewhere in the system.
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Old 10-25-2012, 04:19 PM   #9
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SPST Switch Strange Functions


There has to be a relay, thats the only way this is possible that I can see. The bigger question is why in the world anyone would ever wire something that way, where one light HAS to be on all the time?
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Old 10-25-2012, 06:12 PM   #10
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SPST Switch Strange Functions


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There has to be a relay, thats the only way this is possible that I can see. The bigger question is why in the world anyone would ever wire something that way, where one light HAS to be on all the time?
Light #1 is in a hallway and light #2 is in the closet. The switch is in the closet and when turned off the hallway light comes on and the closet light goes off. Yeah, it is strange!!
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Old 10-25-2012, 06:17 PM   #11
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SPST Switch Strange Functions


Has this ever worked properly? Sounds like someone may have tapped a new light off a switch loop.
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Old 10-25-2012, 06:56 PM   #12
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SPST Switch Strange Functions


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Has this ever worked properly? Sounds like someone may have tapped a new light off a switch loop.

Circuit has always been this way since the house was purchased several years ago.

Could you diagram a "tapped ..new light off a switch loop" that would act like this? I don't see how that is possible, but a wire diagram would help.

Thanks,

Dan
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Old 10-25-2012, 07:22 PM   #13
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SPST Switch Strange Functions


Is there anything else controlled by this switch, for example a small heater?

Is the lamp (either lamp) that is off completely off versus glowing slightly?

Try exchanging one lamp with a lamp of a different wattage. Does the switch behave differently?
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Old 10-25-2012, 07:41 PM   #14
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SPST Switch Strange Functions


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Is there anything else controlled by this switch, for example a small heater? No, as far as I can tell the switch controls only these two lights.

Is the lamp (either lamp) that is off completely off versus glowing slightly? Lamps are completely off when off.

Try exchanging one lamp with a lamp of a different wattage. Does the switch behave differently? Different wattage lamps make no difference.
The old K&T wiring in this house is going to be replaced next month, maybe the electricians can shed some light on this.

Dan
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Old 10-25-2012, 08:46 PM   #15
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SPST Switch Strange Functions


When the switch is closed, is the "off" light really off ? If you put a meter on it, is it zero? Could the second light be connected from hot to a poor nuetral or ground, and when the switch is closed, current travels through the normal path?

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