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I'm getting a fake short on my fuse tripping GFCI

Home Improvement Asked by Greg on October 3, 2020

I recently went to change an outlet to a GFCI (It was the first in the line)

After I installed it (as I have done dozens of times) and It kept tripping. I replaced the outlet thinking it was faulty, but the outlet kept tripping.

I got my volt meter out and tested continuity between the hot and neutral and found they were indeed touching. Keep in mind the old outlet worked fine.

This outlet is the first in the line for the whole upstairs (which is a terrible 1950’s design). So I unplugged everything in every outlet upstairs. I found that a set of computer speakers tested positive for a short between the hot and neutral.

This struck me odd because it did not trip the Fuse, or short any other device. Since I couldn’t add the GFCI I just replaced the old outlet with a normal duplex. And everything is working fine as it has for years.

Part 1: So why are the speakers tripping the GFCI and causing a "short" but not tripping the actual Fuse or causing irregularities with anything else that is plugged in?

Part 2: Even after I unplugged the Speakers the GFCI kept tripping with nothing in any outlet and the hot and neutral stopped testing positive for a short. Any thoughts? (The GFCI was tested to be fully operational)

I’m an electronic fan boy, so I appreciate in depth responses down to the electrical components on the PCB.

Thank you for your help,

3 Answers

I found that a set of computer speakers tested positive for a short between the hot and neutral.

The rest of this (GFCI, etc.) has been well explained by others.

But the speakers "positive for a short"? I got burned by a variant of that myself recently. I was replacing a kitchen exhaust fan/light and decided to double check ground to neutral (good) to make sure that I connected my ground correctly and check hot to ground to make sure I hadn't messed up. I spent a good bit of time taking apart plenty of other parts of the circuit (it was an adventure) until I gave up for the night and posted here. I quickly got back the (correct) answer that if there is anything beyond a pure resistance load elsewhere in the circuit then you will get what appears to be a dead short with a normal multimeter. That was indeed the case - parts of at least 3 other rooms (lights & receptacles) are all on that same circuit, and I had not bothered to disconnect everything. Sure enough, after I disconnected all the easy loads (didn't bother with lighting), the short magically disappeared.

The big reason this happens is transformers. A simple little transformer, which is the first stage of plenty of electronic power supplies, is a transformer only to alternating current. But your garden-variety multimeter is using direct current. A transformer is just a spool of wire, with very low resistance, to direct current.

That doesn't answer the GFCI trip, but it does explain the apparent short with a meter that never tripped any breakers.

Answered by manassehkatz-Moving 2 Codidact on October 3, 2020

As an electronics "fan boy", you have rich opportunities to expand your knowledge in the areas of what circuit breakers do, and what GFCI devices do. I'll try to help.

Plain Circuit breakers

Plain circuit breakers are straight overcurrent devices, but with some complexity. Their job is to prevent wiring in the walls from overheating, and to protect devices from exploding. Because of this, they have two completely different actions inside them.

  • Thermal trip -- this is a bi-metal strip current passes through. It is designed to heat about the same speed as the wiring in the walls. It's a race: whether the wire in the walls gets hot enough to start a fire, OR the bi-metal strip gets hot enough to bend forcefully enough to trip the breaker. Obviously, the deck is stacked so the bi-metal strip wins everytime.

But this only works if you stick to the Electrical Codes. Nothing keeps you from putting the wrong breaker on the wrong wires and the bi-metal strip loses the race.

Working properly, a breaker might tolerate a 120% overload for 30 minutes. A 200% load for far less time, etc.

  • Magnetic trip - this is a couple of loops around a magnetic core, that acts like a solenoid to trip the breaker. This action happens at extreme overloads like 5-10x breaker rating. This is for "bolted faults" or dead shorts between the wires, allowing hundreds of amps to flow. This action operates in milliseconds.

It is important to note that circuit breakers do not work on the neutral wire. They are not connected to neutral, and have no idea what current is on the neutral. What keeps them from overloading? Only correct wiring, where the neutral carries ONLY current from its partner hot, or hots in the case of a multi-wire branch circuit (MWBC) or 120/240V load. If neutral is poached or crossed with other circuits, it becomes possible for neutral to carry 2 circuits' worth of current, and overheat. That is also possible if a MWBC is mis-wired. Because of this, crossed neutrals are very serious.

GFCI devices (aka RCD)

As you know, current always flows in loops. The intended path is from hot through the appliance and back on neutral. (or back on the other hot in the case of 240V loads). All the current that goes out the hot should come back on the neutral. Remember the part where neutral is only supposed to serve its partner hot(s)?

People often get killed by current leakage from damaged devices. What is happening is that due to insulation failure or misuse, current has found a third path - some current is returning via the human instead of all on neutral. That is normal. Current takes ALL paths available to it, in proportion to their conductance. (conductance is 1/resistance and the unit is siemens: 0.1 siemens = 10 ohms. Now suddenly, the paralleled resistor formula makes more sense!)

So, how can we detect current leakage? By comparing current on hot(s) to current on neutral. They should be equal. If we're dealing with 3 or more wires, and we account for direction as negative current, the currents in all working conductors should sum to zero. Make sense?

If it doesn't sum to zero, that means current is leaking somewhere, e.g. through a human.

This would be hard to detect in the DC world... but in AC, it is easy because AC throws a considerable magnetic field (that's how transformers work in AC but not DC). When there are two equal but opposite currents, the magnetic fields cancel out. Big deal: very big deal. This is the key to it all.

A GFCI is a mini-transformer that has hot(s) and neutral wound so they normally cancel each other out. If any leakage current occurs, then energy appears in the transformer secondary. This snaps the GFCI device.

The detection level is 5 milliamps for human-protection GFCIs in the US, and 30 milliamps for arc-fault detecting AFCIs in the US and RCDs in Europe. We won't get into arc faults today.

Practical use of GFCIs

In The Matrix, it was explained to Neo that "everyone falls the first time".

With GFCIs, what everyone does the first time is blame the GFCI. Which is like shooting the messenger. The GFCI is a detection device. It's doing its job. It didn't alarm yesterday because you hadn't installed it yesterday. The problem was here the whole time, only now you're just detecting it.

So now you can settle into the task of isolating the problem.

Part 1: So why are the speakers tripping the GFCI and causing a "short" but not tripping the actual Fuse or causing irregularities with anything else that is plugged in?

Because the speakers are leaking more than 5 milliamps, but less than 150 amps (the sure magnetic trip on a 15A breaker).

Part 2: Even after I unplugged the Speakers the GFCI kept tripping with nothing in any outlet and the hot and neutral stopped testing positive for a short. Any thoughts? (The GFCI was tested to be fully operational)

Well, then, it's not the speaker, is it? It sounds to me like the speaker was a red herring owing to poor testing hygiene, and the real problem is current leakage in the wiring.

Chasing that down is something that Ed Beal has covered quite well.

However be on the lookout for crossed neutrals, as GFCIs won't tolerate those.

Also, if your circuit leaves the service panel in /3 cable (black red white), that is an MWBC which is a case of planned, safe sharing of neutral. GFCIs on those are complicated.

Answered by Harper - Reinstate Monica on October 3, 2020

Gfci’s trip at aprox 5 ma your fuse is 15 or 20 amp so this small leakage would not be enough to blow a fuse or trip a breaker and the issue could have been there for years. 1950 wiring is usually cloth wrapped and the insulation can be in bad shape. Measuring resistance on a plug in device will not tell you if it is the problem many devices that have motors or transformers will show close to a short across the blades that plug i. this is a dc value, ac values are many times this reading.

The leakage also has to be to ground or not running through the hot and neutral. Once the imbalance is ~5ma the GFCI trips and is referred to as a ground fault because some place in the circuit current is leaking or shorting to ground.

How to trouble shoot a ground fault with nothing plugged in, first you could remove the wires off the “load” terminals this will verify the GFCI is working when you press test and reset. If that works re connect the load to the GFCI.

the next step count how many outlets are fed by the GFCI (any outside lights?) Cut the number in half and with the power off inspect the wiring at that receptacle. If you don’t find any problems some times I will pull the hot and cap it then see if the circuit will hold. If the circuit trips we start working back towards the GFCI if it holds we work towards the end of the run each time cutting the total in half and repeating the inspection until the fault is found. If no fault is found again verify outside lighting or receptacles, a little water in a outside receptacle won’t trip a fuse or breaker but will trip a GFCI. I usually narrow it down to the segment of wire or receptacle in 4 cycles by using this 1/2 cut rule and working to the fault. After verifying that the GFCI holds without a load.

Remember anything on the circuit can cause the fault your lights could be fed for the room or outside as I mentioned. If you see more wires in a box than just the feed from the last device and the cable going to the next run that could indicate a branch in your circuit to another location and outside fixtures have been the culprit many times for me.

Answered by Ed Beal on October 3, 2020

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