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Tripping GFI
Robert Piascik:
Over July 4 we did an outdoor show at a local university. They supplied us with a breaker panel with different circuits all with GFI outlets. When the band began playing we tripped four of the outlets (not simultaneously) at various times but never any breakers. Panel was located 35' from where we had our rack. Sound system was a modest outdoor set up, we used four circuits: one Crown iTech8k amp powering four Danley TH118 subs, another iTech8k amp powering two EAW LA325 tops, one circuit for the rest of the rack (mixer, etc.), one circuit for the stage. At first all I did was reset the GFI, when it happened again I moved the cable to another circuit. It happened to all of the circuits (except the mixer) at various times. Eventually I just brought levels down and it stopped but I'm not sure that what I did is what solved it. If so, why would the GFI trip and not the breaker? One other factor: we were battling rain and wet conditions but nothing was laying in puddles or anything like that.
Any ideas what was going on here?
Thanks for any insight
Kevin Graf:
In effect, GFCIs are designed to trip when they notice a very small amount of current leaking into the Safety Ground, Planet Earth or another circuit. So it could be a power conditioner dumping noise current into the safety ground or a stinger capacitor or stray current from one circuit using your interconnect shields to return on another AC circuit.
Mike Sokol:
--- Quote from: Kevin Graf on July 07, 2016, 10:01:06 AM ---In effect, GFCIs are designed to trip when they notice a very small amount of current leaking into the Safety Ground, Planet Earth or another circuit. So it could be a power conditioner dumping noise current into the safety ground or a stinger capacitor or stray current from one circuit using your interconnect shields to return on another AC circuit.
--- End quote ---
Yes, power conditioner strips with MOV's often leak a few mA current to the EGC (Safety Ground), so a couple of them onstage can easily add up to the 6 mA trip point on a GFCI. Same for a stinger cap on a backline guitar amplifier. However, there shouldn't be be a path for shield ground loop currents through the GFCI's sensing transformer. That suggests that ground loop currents through the shields shouldn't be able to trip an upstream GFCI unless there was a double-bond of the neutral and EGC Ground in your power distro somewhere. But that mis-wiring condition would make it very easy to trip the GFCI so just turning down the volume wouldn't fix it. More to think about on this problem.
Kevin Graf:
If one of the circuits has a Neutral/Safety Ground swap, then the chassis's will be a different potentials and there will be current flow through the interconnect shields. But even without the swap component's AC supply sections do leak current.
Mike Sokol:
--- Quote from: Kevin Graf on July 07, 2016, 11:28:36 AM ---If one of the circuits has a Neutral/Safety Ground swap, then the chassis's will be a different potentials and there will be current flow through the interconnect shields. But even without the swap component's AC supply sections do leak current.
--- End quote ---
Yes, everything leaks a little. And it's all additive. So a single piece of gear on a GFCI may not trip it. But put 2 or 3 things on a GFCI and the leakage currents can easily add it to the 6 mA tripping threshold. The fact that in this instance the GFCI's would trip under heavy current draw, but not under more moderate current draw suggests some intermingling of the Ground and Neutral. First thing I would do is check your own gear to make sure there's no accidental G-N bond or G-N swap in any of your equipment or extension cords. That's the only thing I can think of that would create a load-dependent GFCI random trip.
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