Incoming AC power caps shorting my GFCI

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sonicmojo
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Incoming AC power caps shorting my GFCI

Post by sonicmojo »

I've run across an issue with certain amps I am servicing, but only in my garage where I mostly work. When I have an amp that has a capacitor to ground on BOTH sides of the power cord supply (hot and neutral), it will sometimes trip the GFCI and be unusable on my garage outlets until I remove them. In other cases where I have an old style two prong amp with a capacitor on one side, it will sometimes trip when I switch the ground switch to one of the sides. Do you think I have a grounding issue or other problem with my GFCI? It doesn't appear to happen in outlets in the rest of my house.

Also, what is the main purpose of these caps? I've seen it this way in several amps and I assume it has to do with noise filtering but it is counterintuitive to me to put caps to ground on both sides of the incoming power.
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Re: Incoming AC power caps shorting my GFCI

Post by pompeiisneaks »

That sounds to me like what is called the 'death cap' It's a capacitor that used to be common and is now forbidden unless done with an X2 capacitor that is guaranteed to fail open instead of short. Basically they start grounding all the incoming AC and it trips the GFCI like it should. They should always be removed and a 3 prong cord installed.

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sonicmojo
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Re: Incoming AC power caps shorting my GFCI

Post by sonicmojo »

Yes, that cap for a vintage two pronger. But even when switched, it doesn't trip the circuit normally but properly puts the neutral side to ground and reduces buzz typically along with make it safer. I always clip it when adding a 3 prong cord which I always recommend but some people just won't have their vintage cords changed.

But some amps I've seen, all custom, have been built with a cap hard wired on each side without any ground switch. I assume it fixes noise somehow but I don't know what you call these caps placed there so I haven't found the info about them yet. I don't like these caps for the danger potential you mention.

But I sort of solved my problem. I think it has to do with another amp that was plugged into the same power strip but not on. I'll have to figure out which one......
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martin manning
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Re: Incoming AC power caps shorting my GFCI

Post by martin manning »

GFCI's are tripped by a difference in the current flowing in on the hot lead and the current flowing out on the neutral, any of which is an indication that there is an alternate path to ground, possibly through a human. The typical line filter "death cap" is 0.047u, which looks like a 56k resistor at 60Hz. That means if you have the cap switched to the hot lead you will have 2.1 mA flowing from hot to the ground... provided that the cap is not leaky. The condition required to trip a GFCI is 4 to 6 mA, so even a perfect cap has already used up half of the quota. If you plug two amps with line filters into the same strip that doubles the non-neutral current, and that could be enough that the GCFI sees a ground fault. There is nothing wrong with having the cap there, using either a 2- or 3-wire cord, as long as it is a type X-Y. These caps are a noise filter, and they are supposed to shunt incoming RF to ground as well as prevent the equipment from putting RF out on the mains circuit.
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Re: Incoming AC power caps shorting my GFCI

Post by Stevem »

A lot of old amps with these caps only had a 400 volt rating and that's too close to the peak AC voltage seen with some 120 volts RMS!
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Re: Incoming AC power caps shorting my GFCI

Post by sonicmojo »

Line Filter, line bypass, okay now I see some articles on it. Thanks Martin! And your explanation of having more than one amp in the same circuit creating the overload makes sense. More reading to do about X and Y caps now....but the '98 Holland amp I'm working on has 715P .047 Orange drops on the incoming line supply plus another on the standby switch for good measure. Yikes, perhaps? I've seen these short out on Mesa Boogies. I'll talk to the customer about changing this up to something safer.
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sonicmojo
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Re: Incoming AC power caps shorting my GFCI

Post by sonicmojo »

Stevem wrote: Thu Nov 09, 2017 12:27 pm A lot of old amps with these caps only had a 400 volt rating and that's too close to the peak AC voltage seen with some 120 volts RMS!
Hmm, all the "safety" caps I am seeing have rated voltages in the range of 250-300AC, but I'm seeing notes about much higher voltages tested for 1 minute mentioned in some specs so I think they should be good.

Such as:

http://www.justradios.com/X1Y2capacitors.html
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Re: Incoming AC power caps shorting my GFCI

Post by martin manning »

Mouser stocks the same parts: https://www.mouser.com/ProductDetail/KE ... pU76C1I%3d
The largest value in the series is 0.01u, but that will limit the ground current to 500uA and prevent GFCI tripping. Most amps are quiet enough with the cap removed, so perhaps 0.01u is large enough, and it will provide a local AC ground for the PT primary circuit.
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