400 watt HPS lamp flips the breaker when it is turned off or unplugged??

Hey,
I have a 400 watt HPS light that feeds some plants in the basement during the winter. When I unplugged the lamp in the morning (I reversed the day/night cycle so it also keeps them warmer during the cooler nights of winter), the circuit breaker would flip.
I also tried putting the lamp on a timer - which turned the lamp on fine, but when it turned the lamp off - the breaker would flip.
Oddly enough, I did find that if I pulled (more like quickly snatched) the plug from the wall - and I do mean quickly - the breaker would not flip - and I was able to reproduce this consistently.
So, unless you know of a timer that snatches plugs from the wall to turn them off - I was wondering if any of you knew what the problem would be? What's the deal with the breaker flipping when the load on the circuit drops - i.e. why isn't the breaker flipping when the light turns on but flips when it turns off?? I'd like to solve the problem while I have plenty of time before this winter...
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snipped-for-privacy@gmail.com wrote:

I have two total guesses.
A problem with the neutral (floating) or a weak breaker. I would try switching the breaker.
--
Joseph Meehan

Dia duit
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Hey,
Is it ground-fault-protected?

Hmmm.
The ballast may be making a small arc in the switch when it turns off. An old motor run cap might help. If the ballast lists power P and power factor PF, you might correct it to 1 with Xc = V^2PF/(Psqrt(1-PF^2)) and C = 1/(377Xc). For instance, a 120V 400 W ballast with a 0.8 power factor might have Xc = 120^2x0.8/(400sqrt(1-0.8)) = 48 ohms reactive and C = 1/(377x48) = 55 microfarads, or less, since 55 would make the resonant frequency 60 Hz, which might be a bad idea. A 1 uF cap might fix the breaker problem.
Our local YMCA seems has the opposite problem: after a short power failure, the ballasts draw 3X their normal start current, which blows the breaker. This doesn't happen if the lights are off for 20 minutes or so.
Nick
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I am surprised HPS is any good for plants. Years ago, when I used to have an indoor vegetable garden, the challenge was to simulate daylight; which HPS couldn't be any further from.
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Well, I was advised it was the cheapest solution if I also wanted to make any of the tropical plants flower/bloom/fruit. I had to purchase a metal halide conversion bulb for regular daylight and use a standard HPS bulb when I was ready to change their "season."
This kept me from having to purchase an MH lamp and an HPS lamp separately.
Also, there are multiple circuits in the basement, but the outlet I have been using is the only thing drawing on that circuit - and it happens to be the "test" breaker in my panel as well. However, I tried it on another outlet and the same thing happened (it flipped).
Are arc fault breakers something I can add to the current panel, or will the whole panel have to be changed out?
Toller wrote:

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Andrew wrote:

If the breaker has a "test" button it is probably a GFCI. Try it on a "normal" circuit.

Most panel have AFCIs available. Toller is guessing the light may be on an AFCI circuit and the AFCI sees the arc across the opening contacts as a problem and trips (an AFCI causes the problem.)
You could also try sci.engr.lighting . They may have ideas on odd ballast behavior (and could tell you more than you want to know about the lighting spectrum of the lamps.)
bud--
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