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#132852 - 09/20/01 07:15 PM GFI tripping problem  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
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?


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#132853 - 09/20/01 11:19 PM Re: GFI tripping problem  
sparky66wv  Offline
Member
Joined: Nov 2000
Posts: 2,236
West Virginia
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...

[Linked Image]


-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

#132854 - 09/21/01 07:52 AM Re: GFI tripping problem  
Bill Addiss  Offline
Member
Joined: Oct 2000
Posts: 3,878
NY, USA
Paul,

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

Bill


#132855 - 09/21/01 03:12 PM Re: GFI tripping problem  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
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.


#132856 - 09/21/01 04:23 PM Re: GFI tripping problem  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
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 [Linked Image] 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.

[Linked Image] 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 " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#132857 - 09/21/01 05:41 PM Re: GFI tripping problem  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
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?


#132858 - 09/21/01 07:56 PM Re: GFI tripping problem  
sparky66wv  Offline
Member
Joined: Nov 2000
Posts: 2,236
West Virginia
The plot thickens!

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


-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

#132859 - 09/21/01 08:38 PM Re: GFI tripping problem  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,307
perhaps the 'remodel' included affecting some wiring ( ie..nail, screw..etc..) , brought out by a load on the circuit?
[Linked Image]


#132860 - 09/21/01 09:16 PM Re: GFI tripping problem  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
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!).


#132861 - 09/21/01 10:37 PM Re: GFI tripping problem  
sparky66wv  Offline
Member
Joined: Nov 2000
Posts: 2,236
West Virginia
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...

[Linked Image]


-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI

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