Lighting around water

AFSQ

Member
Hello all!

I've got a film shoot coming up that's going to involve a shallow pool of water. I'm looking for best practice solutions for keeping things safe. My big question is where and how to use GFCIs. I've got some in line GFCIs that I'd like to put on the lights, but my understanding is that they will not work correctly on a dimmed circuit. What happens if I put a shoebox dimmer through a GFCI? I'm sure the dimmer will function fine, but will the GFCI be doing any good in that case with the dimmer pack between it and the fixture?

Thanks much,
AFSQ
 
It won’t.

just make sure the crew you hired for lighting know what they are doing and you will be fine.
 
I'll second Amiers. And for a film shoot, don't bother with the dimmers. Flood your lamps out, use screens or back them off. Dimmers will screw with color temp.
 
First off, this is something where you really need to know (or find out) what the code requirements are and follow them, not merely ad-hoc something together that seems like it should be safe-ish. I don't know what said code requirements would be. I'd very much hope that a good lighting crew would be well aware of such things and do what is safe and correct; after all, that's a good part of why you hired them in the first place. I'd also agree, based on my very limited knowledge, that using dimmers for a film shoot is not at all common practice.

However, a GFCI should work just fine (and provide useful safety protection for) a connected dimmer and light. For a standard SCR or TRIAC based dimmer, a ground fault should still be detected and still cause the GFCI to trip, since it would still cause an imbalance in the hot and neutral currents that the GFCI is monitoring. Furthermore, the ground/neutral fault detection mode (a separate and lesser-know function of GFCIs from the tripping due to a current imbalance) would be unimpeded. The GFCI obviously must be rated for the current consumption of the lights that are connected, which is a basic requirement for any wiring. It would not surprise me greatly if the dimmer packs that have two source plugs, for connecting to two different circuits for greater power handling, would cause the GFCI(s) to trip if so connected due to internal connections of the neutrals, although if properly designed there oughtn't be any such interconnection.

If GFCIs were ineffective for lights on a dimmer, they'd be equally as ineffective for such devices as variable-speed corded electric drills, which generally use much the same basic circuit design for their speed control.

A GFCI between the dimmer and the light, on the other hand, will generally not work properly. The line side of the GFCI needs to be connected to a normal power line circuit, not to some chopped-up lower voltage version of it. The load can be pretty much as non-linear as it needs to be.
 
Since there are no longer dimmer-rated GFCI's available NEC section 520.9 exempt outdoor circuits in article 520 venues from the standard outdoor GFCI requirements. The OP did not state whether this was a outdoor application or not. However, there are some possible approaches:

1. Don't use a dimmed circuit, and use a standard GFCI.
2. Use a ground-fault protection device like the "Shock-Block", generally available from film rental houses. These do not qualify as Class A GFCI devices because they have adjustable trip currents above 6mA. However, they can provide a good level of protection, and the adjustment can be used to eliminate nuisance tripping caused by the non-sinusoidal current waveform of phase-control dimmers.

For more guidance, see
ANSI E1.19 - 2015
Recommended Practice for the use of Class A Ground-Fault Circuit Interrupters (GFCIs) intended for personnel protection in the Entertainment Industry

The standard is available free here: https://tsp.esta.org/tsp/documents/published_docs.php

ST
 
If GFCIs were ineffective for lights on a dimmer, they'd be equally as ineffective for such devices as variable-speed corded electric drills, which generally use much the same basic circuit design for their speed control.

Steve Terry addressed most of your misconceptions in his posts, but I want to hit this aspect in particular since it sounds very reasonable at first glance. The main difference is that all of the parts after the "dimmer" in an electric drill are enclosed within the body of the drill and are unlikely to present a shock hazard separately from the parts on the line side of the "dimmer". If you drop a drill into a puddle of water, the whole thing gets wet--not just the motor. When you're talking about actual dimmers you have the entire light fixture(s) and probably some cable connected to the output of the dimmer that can all present separate hazards from everything on the input side. As Steve explained, the GFCI connected to the input of the dimmer won't necessarily be able to detect ground faults with anything on the output of the dimmer due to the non-sinusoidal nature of SCR dimming.
 
First off, this is something where you really need to know (or find out) what the code requirements are and follow them, not merely ad-hoc something together that seems like it should be safe-ish. I don't know what said code requirements would be. I'd very much hope that a good lighting crew would be well aware of such things and do what is safe and correct; after all, that's a good part of why you hired them in the first place. I'd also agree, based on my very limited knowledge, that using dimmers for a film shoot is not at all common practice.

However, a GFCI should work just fine (and provide useful safety protection for) a connected dimmer and light. For a standard SCR or TRIAC based dimmer, a ground fault should still be detected and still cause the GFCI to trip, since it would still cause an imbalance in the hot and neutral currents that the GFCI is monitoring. Furthermore, the ground/neutral fault detection mode (a separate and lesser-know function of GFCIs from the tripping due to a current imbalance) would be unimpeded. The GFCI obviously must be rated for the current consumption of the lights that are connected, which is a basic requirement for any wiring. It would not surprise me greatly if the dimmer packs that have two source plugs, for connecting to two different circuits for greater power handling, would cause the GFCI(s) to trip if so connected due to internal connections of the neutrals, although if properly designed there oughtn't be any such interconnection.

If GFCIs were ineffective for lights on a dimmer, they'd be equally as ineffective for such devices as variable-speed corded electric drills, which generally use much the same basic circuit design for their speed control.

A GFCI between the dimmer and the light, on the other hand, will generally not work properly. The line side of the GFCI needs to be connected to a normal power line circuit, not to some chopped-up lower voltage version of it. The load can be pretty much as non-linear as it needs to be.
(never mind..)
 

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