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My pole dusk to dawn light goes out when my heat pump starts up. The lights in the house will flicker and the outside pole light goes out, then about 5 minutes later it come back on. When the heat pump starts up it goes out again. Off and on all night long.

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welcome to Stack Exchange –  shirlock homes Jan 11 '11 at 20:52
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2 Answers 2

I am pretty sure what you have is classic case of voltage drop. Ballasts for vapor lights and some cheap light sensors for incandescent par bulbs will be tricked or drop output, so light goes out and has to restart when voltage is restored. The reason for the sudden drop is really the concern. Since the heat pump employs a fairly large motor, it draws a lot of current at start up. This is called inductive kick. It takes a few seconds for the coils of the motor to saturate and start he armature spinning in the magnetic field. Since the coils are a dead short until saturation is achieved, they draw large current. E = I/R, ohms law explains this. So what is the solution? You may need to have the heat pump motor checked out or add a motor starting capacitor across the windings. I cannot tell you what size capacitor you will need as it can vary widely depending on motor size. This is a very common problem in residential services that are close to current capacity with an unbalanced motor load. Even though you may have a 100 or 200 amp electrical service, it can be overtaxed temporarily by an unbalanced motor start up

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BTW, this sudden and often low voltage situation can be harmful to electronic equipment as well, especially power supplies. Make sure you use a decent UPS on your expensive computers and electronics. I simple surge protector will not help in this situation. –  shirlock homes Jan 11 '11 at 20:22
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When the heat pump first starts up, it draws a larger current than it does while it's running. This large current is causing a temporary dip in the mains supply voltage at your house.

The dusk-to-dawn light fixture has a sensor that measures the ambient light level and compares it to reference levels for "dusk" and "dawn"; those references are derived from the mains supply voltage. When that voltage dips, it changes the reference levels enough that it believes the ambient light level is "dawn", so it turns the light out.

To fix the problem, the light fixture might have a sensitivity setting that you can adjust (or you could replace it with one that does). Failing that, a much more expensive fix would be to upgrade your electrical service so that the heat pump's start-up current draw doesn't pull down the voltage as much.

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looks like we are on the same page Niall. lol –  shirlock homes Jan 11 '11 at 20:22
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