Does a GFI actually trip on over current?

[davefullmer]

I noticed in the Generic Benroy Plans that Mike says to put a GFI recepticale as the first recepticle (SP) in a 110 V circuit and it will act as a fuse.

Does the GFI actually trip on over current and not just when it sees a ground fault? If so, I have learned something new (which I do quite often).

I know you can buy GFI's with either a 15A or 20A rating, but I thought they were just that, a rating and had nothing to do with the trip point.

Even though I am a licensed electrician in the State of Tenn, I gained that license through the "grandfather clause" when they changed the rules and required all new licensees to take a test a few years ago. Prior to that I had a "limited" registration as an electrical installer. which allowed me to wire buildings, charge for my services but I could not contract for any job that went over $30,000 (as I remember). So there are lots of things I may not know about.

If the GFI actually trips at the rated current, that is a good thing. If it only trips when it sees a ground fault, then it is not because the wire could be subjected to meltdown current before there is a ground fault.

Anybody know for sure?

Cheers and HAPPY Holidays. Remember the reason for the season.


Dave

[prohandyman]

Does the GFI actually trip on over current and not just when it sees a ground fault?

Dave
It is my understanding that a "ground fault" is "an immediate current overload", the accidental connection to a ground being the path of discharge, no matter if it is provided by equipment malfunction, or human error. So, any amount of overload will trip the interupter, wether it is a outlet or breaker.

[jeep_bluetj]

A GFI will ONLY trip for a ground fault. Not for overcurrent. (untill it blows up... but that's not really a fuse, is it?)

So what's a ground fault? And why is a GFI such a wicked cool safety device?

A ground fault is a current flowing to ground. Current should NEVER flow to ground unless there's a 'fault'.

MoInfo: Assume current comes from the 'hot' leg (black in US residential wiring, the skinny side of the outlet).
That current flows through the load and back into the 'neutral' leg (white, the wide side of the outlet)

With everything working right, the amount of current going 'out' is exactly equal to the current coming 'back'. If your load is pulling 10A, there's 10A on hot, and 10A on neutral.

So what's happening if y'all got 10A on hot, and only 8A on neutral? It means 2A is flowing somewhere else (ground), and that's a bad thing. Cause that shouldn't happen. It happens in situations like when you touch an extention cord when you're wet and standing in a puddle. Since 2A flowing through YOU is a very bad thing, the GFI detects the mismatch in current and trips.

If you pull 50A through a 20A GFI, and if 50A is flowing back in, it will not trip. It'll likely burst into flames. That is also a very bad thing.

Use a breaker AND a GFI. For everything wet or outdoor.

[davefullmer]

Jeff,

That's exactly my understanding of a ground fault indicator.

But I didn't want to come on and sound like a know it all, especially since I could have been wrong. There could be any number of situations where a ground was detected, but if you depended on the ground detecting an over current situation, the wire has already melted from the hot to ground and fire and shorting potential has already happened.

I didn't think that you could use a GFI and consider it a fuse. I will be putting in a low cost 2 breaker box like you use for hot water heaters and AC units in mine. Just remember to swap out the disconnects that are in them with 20 amp breakers if you are using 12 ga wire. They are small in size and at Lowes or Home Depot they only cost about $15.

Also, I don't mean to nit pic Mikes design. It just hit me when I read it that people might be depending on it for a safety device that wasn't really protecting for what they needed.

I haven't looked at the paper work in the box yet either. Monday, I will surely have to go to our supply house to get something for work and when I do, I will pick up a GFI and see if there is any thing in the packaging that clears up my question.

I hope Steve F or someone else that is a master electrician sees this and comments on it. Master electrician I am not. Just an machine automation technician.

By the way, always use a GFI because working at the galley is standing on the ground and a GFI is a real safety feature (for the potential ground faults through your body).

Dave

[davefullmer]

Dan wrote:

It is my understanding that a "ground fault" is "an immediate current overload", the accidental connection to a ground being the path of discharge, no matter if it is provided by equipment malfunction, or human error. So, any amount of overload will trip the interupter, wether it is a outlet or breaker.

Dan,

Sorry but that is not strictly the case. A ground fault can be in the order of milli amps (.001 amp) and will trip the breaker. But you can have a load on the circuit that will draw more than the wire is rated for and still not have a ground fault.

I.E. If you have the TD wired with 12 ga wire and your shore line is plugged into a 30 amp circuit and then you overload your TD with say a heater, a crock pot and an electric fry pan at the same time, you could easily go above the 20 amps that the TD is wired for. In that case, you would have no ground fault but you would have overload of the safety point of 20 amps.

Strictly speaking, a GFI tripping on a ground fault is not overloaded, it just saw the ground fault and said, "this is not good, I am going to trip".

Cheers and remember the reason for the season,



Dave

[prohandyman]

Then is there a potential problem with not having the ground bar bussed to the neutral bar, and a ground rod installed when camping, as is the case with most residential wiring?

[davefullmer]

Then is there a potential problem with not having the ground bar bussed to the neutral bar, and a ground rod installed when camping, as is the case with most residential wiring?

Dan,

I don't think that's what Jeff and I are saying. Technically, you do have the ground and neutral connected back at the shoreline connection. But the GFI separates the ground from the neutral and then detects when there is current flowing through the ground wire. That is the best way I can explain how a GFI works.

After re reading the replies to my question, I think there may be some confusion on what a ground circuit truly is. Granted a ground rod ties the grounding buss to the neutral wire at the incoming power source. But the reason for a 3 wire circuit is to make sure there is a neutral path in case the neutral (white) wire is opened by burning in two, loose connector, etc. A GFI will detect such a condition and also detect a current path through the ground wire that goes back to the grounding buss bar. ANY current through a ground wire is a fault.

In a safe non faulted circuit, whether a GFI or a normal recepticle, all the current flows through the hot and neutral white wire. But as Jeff pointed out, if part of it is flowing through you to ground then a ground fault exists whether you have a ground rod installed or not. Even in a properly installed residential wiring. That's why all kitchen and bathroom circuits are required to be on GFI circuits.

In a TD, you are right, there is no ground rod installed (except back at the source) and if you are using a GFI, the GFI will detect ground faults (and also if you have a failure of the grounding wire in the shore line cord).

Think of your TD as an extended room connected to the power source. Just like a bedroom at the opposite end from the breaker box of that 150 foot long house.

So, it's not a question of the potential problem being greater because of no grounding rod at the TD, it's a question of overload protection of the circuit wire. The two are a separate problem.

Overload is excess current in a circuit and ground fault is current in the ground wire circuit where there should never be any.

Maybe someone can jump in and explain it better.

Dave

[Kevin A]

Then is there a potential problem with not having the ground bar bussed to the neutral bar, and a ground rod installed when camping, as is the case with most residential wiring?

Dan,

I don't think that's what Jeff and I are saying. Technically, you do have the ground and neutral connected back at the shoreline connection. But the GFI separates the ground from the neutral and then detects when there is current flowing through the ground wire. That is the best way I can explain how a GFI works.

In a safe non faulted circuit, whether a GFI or a normal recpticle, all the current flows through the hot and neutral white wire. But as Jeff pointed out, if part of it is flowing through you to ground then a ground fault exists whether you have a ground rod installed or not. Even in a properly installed residential wiring. That's why all kitchen and bathroom circuits are required to be on GFI circuits.

In a TD, you are right, there is no ground rod installed (except back at the source) and if you are using a GFI, the GFI will detect ground faults (and also if you have a failure of the grounding wire in the shore line cord).

Think of your TD as an extended room connected to the power source. Just like a bedroom at the opposite end from the breaker box of that 150 foot long house.

So, it's not a question of the potential problem being greater because of no grounding rod at the TD, it's a question of overload protection of the circuit wire. The two are a separate problem.

Overload is excess current in a circuit and ground fault is current in the ground wire circuit where there should never be any.

Maybe someone can jump in and explain it better.

Dave

If I'm understanding the GFI correctly, it's simply a device that detects a difference in current flow between the hot and neutral lines due to a direct short to ground. The current flow is then cut off by an internal circuit breaker. So, to answer the original question: It doesn't appear that a GFI is going to trip due to over or excessive current, that is the job of a fuse/circuit breaker.

[davefullmer]

Yep Kevin,

That is what I thought when I read the statement on page 40 of the Generic Benroy Plans. I just felt that someone might get led astray and think they had their TD protected when in reality, it wasn't.

Dave

[bdosborn]

Dave,

This has been discussed before:

http://tnttt.com/viewto ... 749#144749


Kevin is on the right track.

Bruce

[Alphacarina]

If I'm understanding the GFI correctly, it's simply a device that detects a difference in current flow between the hot and neutral lines due to a direct short to ground. The current flow is then cut off by an internal circuit breaker. So, to answer the original question: It doesn't appear that a GFI is going to trip due to over or excessive current, that is the job of a fuse/circuit breaker.

Actually, a GFCI doesn't know where the 'missing' current is going . . . . just that it's missing - It's seldom a 'direct short to ground'

If there is 2.125 amps flowing on the hot wire but only 2.124 amps coming back on the neutral wire, then it trips - The missing milliampere could be going through your body into the puddle you're standing in . . . . which is what GFCI's guard against - No 'direct short to ground' necessary for it to work as designed. It trips simply because there is more current on one wire than there is on the other

Don

[davefullmer]

Thanks everyone for your input. I didn't think I was off base in my thinking. We are in agreement that a GFI recept will not act as a fuse and protect the wiring of an TD for over current.

I am satisfied and willing to let this subject drop. I just hope that someone who doesn't know won't get hurt by using a GFI as an over current protector.

Dave

[Kevin A]

If I'm understanding the GFI correctly, it's simply a device that detects a difference in current flow between the hot and neutral lines due to a direct short to ground. The current flow is then cut off by an internal circuit breaker. So, to answer the original question: It doesn't appear that a GFI is going to trip due to over or excessive current, that is the job of a fuse/circuit breaker.

Actually, a GFCI doesn't know where the 'missing' current is going . . . . just that it's missing - It's seldom a 'direct short to ground'

If there is 2.125 amps flowing on the hot wire but only 2.124 amps coming back on the neutral wire, then it trips - The missing milliampere could be going through your body into the puddle you're standing in . . . . which is what GFCI's guard against - No 'direct short to ground' necessary for it to work as designed. It trips simply because there is more current on one wire than there is on the other

Don

For the sake of a good argument, I suppose we need to define the term direct short to ground. In my mind, if you've grabbed hold of some faulty electrical device and current is flowing through you to that puddle of water, you've created a direct short to ground. If that GFCI wasn't in the circuit or it has somehow failed to operate properly, in all likely hood you're gonna have quite a shocking experience, no? The sensitivity of that GFCI is there to prevent you from killing yourself in that type situation.

[angib]

Actually, a GFCI doesn't know where the 'missing' current is going . . . . just that it's missing - It's seldom a 'direct short to ground'

Yup, often it's a direct short to arm or leg.....

Andrew

[prohandyman]

Thanks everyone. I was one of those that thought that a GFI would trip in the case of an overload. You set me straight.