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[APD] Re: incandescent-replacement type fixtures -- or - living with an ON/ON switch

The right value capacitor across the switch terminals can
prevent momentary arcing that's due to the switch contacts
almost touching during part of the switching cycle. They
highrer the voltage the wider the gap the arc can reach
across. What value capacitor will prevent the arcing on the
output, high voltage side, depends on the ballast and bulb
operating characteristics. Maybe one of the EEs can help
you there.

OK, this is getting scary... You should *NOT* be switching the OUTPUT (lamp) side of the ballast! On an instant-start ballast you can get pulses of many hundreds, maybe even upwards of a thousand volts in some cases during the "start" pulse. You would need a kilovolt level pulse-rated switch for this which is neither a common nor a cheap item. A regular lamp switch will normally be rated for 300 volts (the minimum UL and the NEC allows), or 600 volts for the better ones -- and prolonged arcing will quickly destroy them.

A capacitor will help to reduce the RFI (Radio Frequency Interference) emitted by the arc but will not necessarily prevent an arc from occurring. Typically a ceramic disc capacitor of about 0.01 microfarads is sufficient to keep the RFI down, and these capacitors are cheap and durable. They aren't particularly necessary unless you expect the switch to change state slowly (think thermostat in a heater that doesn't "snap" over but takes a little while to change from "off" to "on" and vice versa) and thus arc for a little while during each state change.

Whether the Ballasts are remote mounted or not shouldn't
affect whether the on/off switch arcs *if* the switch is on
the input side of the ballast.  I'd rather have my switches
on the input wires and run the wires to where I want a
switch than put the swtich on the wires that are closest --
I'd rather not fiddle with the high voltage if I don't have
to and I only have to intall the wires once.

In no case should you switch the output side of the ballasts. If you use a remote-mounted ballast, either mount the switch on the ballast enclosure or run an extra pair of wires to the light hood so that the switch can be mounted on the hood but still switch the power on the *input* side of the ballasts. Some ballasts will fail if left open-circuit for a long time (with either a bad bulb or "turned off" on the output side). If I remember correctly, some of my MH ballasts actually state that they draw *maximum current* when the output is open-circuit!

BTW, you could solve this whole problem by just using a timer to control the lights and then not need a "switch" at all. Might be worth considering...


***************************** Waveform Technology UNIX Systems Administrator

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