SPST Switch Strange Functions

DanS26

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

Jim Port

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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J. V.

How many wires in the switch box?

DanS26

It's definitely not a three way switch. Only two terminals.

DanS26

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

Gac66610

Are the light bulbs the same wattage?

Gac66610

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.

DanS26

Both bulbs same wattage.

I'm still thinking there has to be a relay buried somewhere in the system.

patented

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?

DanS26

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!!

joed

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

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DanS26

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

AllanJ

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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DanS26

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

SPS-1

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?