I've mentioned some of the trouble with the "whole-house" main RCD/GFI employed in many homes here, so I thought I'd give you an example to think about from a nearby house. May be a long story, so get comfortable.....

This is a typical U.K. installation for a rural area: Neutral & ground separate at service entrance, grounding to a local rod only, and a 100mA main GFI.

The main panel fuses are 30A range, two 30A rings for general recepts., 15A water heater, & two 5A circuits for lights.

First, they found the GFI tripped every time they turned on the electric kettle. Nothing surprising there, and obviously led me to suspect a faulty element leaking to ground. However, when I tried it there was no problem. I put my megger on the kettle (500V DC is our standard test voltage) and got over 200 megohms.

I started asking if anything else had been on when the breaker kept tripping. After some head-scratching, we realized the water heater had been on before, but wasn't now.

I flicked it on, and nothing tripped. Undeterred, I disconnected the heat-resistant cord to the element & did an insulation test on the heating element anyway. It was an old element, so I was expecting quite a bit of leakage. Result was just over 400K. Certainly not low enough to trip a 100mA GFI, although these elements have a nasty trick of shorting out once hot, so I left it on to warm up while we all went to get a hot drink.

As the kettle was switched on again, off went the GFI.

To cut a long story short (whaddya mean "Too late"?!), I found that with either the water heater or the kettle on alone, no problem. Put both on together, and out went the breaker immediately.

By the way: the water heater is 3kW, the kettle about 2.4kW, and on different circuits remember.

Further experimentation revealed that the GFI also tripped with the kettle and a couple of rings on the stove on. With the kettle unplugged it also tripped when trying to run the stove and the water heater together.

At this point I realized that the kitchen had just been remodeled (yeah, I should've noticed earlier, but it must have been a bad day!) and headed for the main panel armed with screwdriver & megger.

I'll let you all puzzle over the strange foreign wiring a while before I finish the story: What was going on?

Wow... Sounds like some of my troubleshooting jobs...

The kettle would be a hot plate? I'm lost in "culture shock" again... Or like a crock pot?

Guess it really doesn't matter since everything is 240V there...

Hmmm...

I'm lost...

Paul,

I think you've got our interest here!
I wish We had some schematics of your distribution system and this main GFI.

Bill

Virgil:

Sorry, another late-night post for me & I wasn't thinking that you might not know what I was talking about. Darn common language of ours again....

The traditional British electric kettle is a stainless-steel container with handle & spout, looking very much like a kettle you might put on a stove to boil water, except that it has a coiled element inside. Modern "trendy" types are heat-proof molded plastic and more like an upright jug ("pitcher" to you), but the principle is the same.

Just about EVERY kitchen in the country has one, plus every workshop, office, etc. Most commonly used for boiling water to make tea. (There's a surprise...) With all the wonderful gadgets in the average Ameican kitchen, this is the one thing I find it odd to not have!

Bill:
With the set-up I'm using here at the moment, I can't even get some CAD line-drawings to post. If I get the chance though, I'll sketch out the different supply arrangements & mail them to you.

Paul,

Your situation seems like there is an accumilation of leakage current existing on the remaining circuits, which becomes excessive [over 100 ma] after the Kettle and Stove Elements / Water Heater together are connected.
What I am saying is that there's may be a leakage of 80 ma on the remaining circuits in the house, then when the other loads are turned on [the Kettle and Water Heater, or Stove Elements], their leakages add possibly another 20 to 30 ma leakage, resulting in the RCD/GFI tripping.
This is just a guess, but might be possible [???]. It's a common complaint I have heard when asked to troubleshoot GFCI circuits [when two loads run together, GFCI trips. When run separate, no trip]. The biggest difference between our power system's configuration and yours, there is no "Balanced Neutral", or Multiwire Circuits to deal with in the house. The way your situation is reacting, makes it sound like a Multiwire circuit and GFCI circuit mixup, or some other mixup on the LOAD side of a GFCI device, or an over leakage situation [most likely situation].

Hope that didn't make this message any more corn-fusing than it already is It's been a very, very long day [and night!] for me...time for bed!

Anyhow, maybe the problem is just too much leakage current, or just a real sensitive device.
Still, those values measured from the Megger are really low! The 200M would result in 0.00012 amps @ 240 volts. If the value was 2M, that would be 0.012 amps @ 240 volts.
The 400K results in 0.006 amps. If the value was 40K, that would push 0.06 amps @ 240 volts. If the lower Resistance values were the actual ones, that would be 0.072 amps - which combined with an existing leakage of 0.030 amps, would be above the maximum imbalanced &/or charging current level of 0.1 amps - causing a trip.

Man, I can sure talk some bull, huh??? Almost goes in circles!

Lastly, If you want to send me the CAD drawn stuff mentioned in the most recent message, maybe I can convert and post for you. Let me know what you think.

Scott SET

Scott:

What I meant to say was that I don't have any CAD software here at the moment, and I don't have a scanner either, so a hand sketch by fax or snail-mail is about all I could manage right now.

The possibility of the combined leakage current of different circuits occurred to me as well, which is why I decided to go to the main panel and megger the whole lot to get an overall reading with various appliances on and off.

The 200M+ and 400K were the correct readings. Admittedly that's a 500V DC test and the actual leakage at 240V AC could have been higher, but this is of no consequence to the final outcome. By the way, I don't want to sound critcal, but I think your long hard day has taken its toll....<g>. You'd better check your math again!

The idea of an accumulation of current is along the right lines, but it's not earth leakage through any of the appliances mentioned.

Any more thoughts before I reveal all?

The plot thickens!

Pauluk, OK a kettle is something new to me...
Thanks for the explanation...

perhaps the 'remodel' included affecting some wiring ( ie..nail, screw..etc..) , brought out by a load on the circuit?

Well, I suppose the best place to make tea would be at the bottom of Boston Harbor..... And although a kettle has water IN it, it might not work too well completely immersed.... Hehe...

Sparky:

You're moving in the right direction now. It's real late here, so I'll leave you to ponder a while longer and finish the story tomorrow (er.... make that later today!).

The parallel path and a ground fault...

The leak exceeds 100MA when two devices are on...

Possible path to ground ..Screw or nail, or staple...

Man... It's going to make so much sense when you tell us...

Still no guesses yet for me...

Well, you're so nearly there now that I'll give you another chance to get it. Keep thinking "parallel paths," and don't forget that the GFI in question feeds the whole house.
P.S. Don't let the fact that the kettle was plugged into a ring circuit worry you too much. The fact that the outlets were wired on a ring was of little or no consequence to the fault.


[This message has been edited by pauluk (edited 09-22-2001).]

My next guess would be a crossed or somehow paralelled return or neutral. did some one combine the neutrals of more than one circuit?

You're very close with the parallel neutral idea, but the neutrals of different circuits hadn't been mixed up.

This wouldn't have tripped the main GFI anyway, as it's on the main intake ahead of the fuses and main neutral busbar.

Another hint:

The kitchen re-model hadn't involved the moving of outlets or the rerouting of cables in any way.

Continue to think "parallel paths," and go back to the screw/nail idea.

Is it possible to have nailed a nuetral and have whatever it's nailed to create a parrallel path, achieving the ma needed to trip the GFI under load ??

Did some one hit two dirrerent neutrals when installing a conductive cabinet or mounting an appliance to the wall?

Do you have a neutral to ground fault were the flow to the fault only breaks the 100 milliamp barrier when the neutral is heavily loaded and the voltage drop creates enough of a voltage difference to drive the leakage current beyond the trip point?
--
Tom

Well, between you all, you guys have just about cracked it, so at long last here's the conclusion to tonight's story....

I took the cover off the main panel and shut off the main GFI. Like all our main switches, this is double-pole, so it isolates the neutral from the incoming line. With my meter on the neutral and ground busbars (separate here remember!) I read almost a dead short. Pulling neutrals off the busbar one at time narrowed the short down to the ring circuit feeding the kitchen sockets.

My first thought then was a nail through a cable in one of the kitchen walls, espevcially as this hous belongs to a builder! I left an audible continuity tester connected at the panel and started pulling recepts off the wall, intending to disconnect the neutrals at strategic points to narrow down the location of the short.

At about the 2nd or 3rd outlet I unscrewed, however, the short disappeared as soon as I pulled it forward from the box (these are one piece outlet and faceplate combined - not separate).

Further examination revealed a cut in the insulation of the neutral where the wire was trapped between the side of the metal back box, the screw lug and one of the recept. mounting screws.

It turned out the sockets had been unscrewed and pulled forward to allow for decorating the walls, so it must have happened when someone was rather ham-fisted in screwing them back to their boxes.

Now if this were a PME system with the ground busbar bonded to the neutral, that parallel path would've tripped the GFI as soon as just a 60W light was turned on.

BUT.... The only ground return path here was via the local earth rod, which obviously had a much higher impedance than the solid neutral return.

So with a 3kW load, the parallel ground path was pulling less than 100mA. Any combination of appliances of around 5kW or so total resulted in more than 100mA flowing to the ground rod.

Paul,
great thread !
you had us thinking , mainly because those numbers would be different here..

I could tell you were all giving the old gray matter a good workout.....

I guess it takes some getting used to the different arrangement, especially having a ground system not bonded to neutral and the GFI feeding the whole house.

I haven't caught onto the advantage of the high impedance ground. I think this is another case for having the EGC and GC bonded somewhere -- even at the service drop.

There's really no advantage to the ground being by way of the local rod only. In my opinion, the high loop impedance that results is a liability.

That's probably why the utilities here have been converting their ditribution sytems to PME (Protective Multiple Earthing) so that customers then have the option of bonding the house ground to the neutral, American-style.