4 ms·

> Specifically if your GFCI in the bathroom (or anywhere) keeps tripping it’s because something is wrong with your wiring or something plugged into the outlet.

> Specifically if your GFCI in the bathroom (or anywhere) keeps tripping it’s because something is wrong with your wiring or something plugged into the outlet. Unfortunately this is not quite true. For two-prong plugs, yes, it is 100% accurate. However larger devices with grounded plugs, especially heavier machinery, will typically contain a line filter with Y capacitors running line to ground. This shows up as a tiny leakage current, which some GFCIs will detect (because it really is leaking) even though this is a nuisance trip (because it is completely safe and normal operation). That's part of where the 30mA trip specification for certain types of non-normal-residential-circuit comes from.

pugworthy 2y ago
Fair enough but it’s not the best example to use to introduce GFCI. Something like this? “Though you rarely will see your bathroom GFCI trip (unless you’re dropping the hair dryer into the bath water), some kinds of larger, more complicated machinery can cause nuisance trips.”
- exmadscientist 2y ago
  Sure, I wouldn't normally go there, but the subject of this discussion is exactly the "larger, more complicated machinery"!
mike_d 2y ago
You are talking about industrial applications. Devices running in residential or commercial environments should be better designed. Proper shielding and grounding is a much better solution than a Y capacitor.
- exmadscientist 2y ago
  Well, they aren't. Line filters are everywhere. They're generally required to pass conducted emissions requirements. They aren't going away soon, and our fault protection devices need to work with them, not blindly ignore how we've been meeting EMC requirements for decades.
  - timewizard 2y ago
    I thought it's the other way around. There are standards for line filters for precisely this case. The device is either non conforming or damaged or the power in this home is out of tolerance in one way or another.
    - ethbr1 2y ago
      The standard is the actual deployed base, not the book it's written in.
amluto 2y ago
A pair of matched Y capacitors on a 240V appliance from a single phase supply will cancel each other out. This doesn’t help people with 208V who are using two out of three phases, though. edit: 120V is not immune. I once worked on a project involving powering equipment in a Faraday case at 120V. The mandatory filter (to prevent the wire from conducting emissions out of the cage) instantly tripped GFCI. The right solution IMO would have been one of: a) A better filter that doesn’t pass 50-60Hz. This is doable in theory, but I’ve never seen one. b) A transformer or other isolated supply outside the cage, with a GFCI inside the cage (to protect the people who may enter to touch the equipment). c) A transformer outside, not necessarily right next to the cage, supplying the inside with two opposite-phase lines at 60Vrms each. No net leakage! This would require that everything inside accept such a supply, which rules out anything that wants a real neutral. d) I bet someone could design a transformer with a shield between the windings that would integrate with the Faraday cage. Power would couple in inductively through the shield. Common-mode RFI would have a very hard time escaping, and different mode RFI could be filtered and probably matters quite a bit less anyway. Of course, the actual solution chosen by the site’s electrical people was rather less classy.
- exmadscientist 2y ago
  An ideal transformer will give you perfect galvanic isolation. A GFI therefore becomes useless on transformer secondaries, because there is no way to complete a circuit through earth back to the transformer. If you have earth grounding of stuff inside the cage (not just the cage itself)... then you have a very strange test going on. Of course, real transformers are not ideal. This: > I bet someone could design a transformer with a shield between the windings that would integrate with the Faraday cage. Power would couple in inductively through the shield. Common-mode RFI would have a very hard time escaping, and different mode RFI could be filtered and probably matters quite a bit less anyway. exists and it is called an "electrostatic shield" inside a transformer. Magnetics people do not like to make these, but they are a standard enough request that they'll do it for you and the only complaints you'll get will be from the people on your side who see the invoice. Electrostatic shields are also not perfect but they help tremendously. The shield connection usually comes out of the potted assembly on another wire so you can hook it up as required. Protip: if you are ever trying to replace or redesign equipment that uses an electrostatically shielded transformer, do ask why they went to that effort. Don't just try to use a normal transformer and then wonder why you can't get it working right... despite man-months of effort....

pugworthy 2y ago

Fair enough but it’s not the best example to use to introduce GFCI. Something like this? “Though you rarely will see your bathroom GFCI trip (unless you’re dropping the hair dryer into the bath water), some kinds of larger, more complicated machinery can cause nuisance trips.”

exmadscientist 2y ago
Sure, I wouldn't normally go there, but the subject of this discussion is exactly the "larger, more complicated machinery"!

mike_d 2y ago

You are talking about industrial applications. Devices running in residential or commercial environments should be better designed. Proper shielding and grounding is a much better solution than a Y capacitor.

exmadscientist 2y ago
Well, they aren't. Line filters are everywhere. They're generally required to pass conducted emissions requirements. They aren't going away soon, and our fault protection devices need to work with them, not blindly ignore how we've been meeting EMC requirements for decades.
- timewizard 2y ago
  I thought it's the other way around. There are standards for line filters for precisely this case. The device is either non conforming or damaged or the power in this home is out of tolerance in one way or another.
  - ethbr1 2y ago
    The standard is the actual deployed base, not the book it's written in.

amluto 2y ago

A pair of matched Y capacitors on a 240V appliance from a single phase supply will cancel each other out. This doesn’t help people with 208V who are using two out of three phases, though. edit: 120V is not immune. I once worked on a project involving powering equipment in a Faraday case at 120V. The mandatory filter (to prevent the wire from conducting emissions out of the cage) instantly tripped GFCI. The right solution IMO would have been one of: a) A better filter that doesn’t pass 50-60Hz. This is doable in theory, but I’ve never seen one. b) A transformer or other isolated supply outside the cage, with a GFCI inside the cage (to protect the people who may enter to touch the equipment). c) A transformer outside, not necessarily right next to the cage, supplying the inside with two opposite-phase lines at 60Vrms each. No net leakage! This would require that everything inside accept such a supply, which rules out anything that wants a real neutral. d) I bet someone could design a transformer with a shield between the windings that would integrate with the Faraday cage. Power would couple in inductively through the shield. Common-mode RFI would have a very hard time escaping, and different mode RFI could be filtered and probably matters quite a bit less anyway. Of course, the actual solution chosen by the site’s electrical people was rather less classy.

exmadscientist 2y ago
An ideal transformer will give you perfect galvanic isolation. A GFI therefore becomes useless on transformer secondaries, because there is no way to complete a circuit through earth back to the transformer. If you have earth grounding of stuff inside the cage (not just the cage itself)... then you have a very strange test going on. Of course, real transformers are not ideal. This: > I bet someone could design a transformer with a shield between the windings that would integrate with the Faraday cage. Power would couple in inductively through the shield. Common-mode RFI would have a very hard time escaping, and different mode RFI could be filtered and probably matters quite a bit less anyway. exists and it is called an "electrostatic shield" inside a transformer. Magnetics people do not like to make these, but they are a standard enough request that they'll do it for you and the only complaints you'll get will be from the people on your side who see the invoice. Electrostatic shields are also not perfect but they help tremendously. The shield connection usually comes out of the potted assembly on another wire so you can hook it up as required. Protip: if you are ever trying to replace or redesign equipment that uses an electrostatically shielded transformer, do ask why they went to that effort. Don't just try to use a normal transformer and then wonder why you can't get it working right... despite man-months of effort....