Odd fault in new (ish) electric oven

My son installed a new Stoves electric oven a couple of months back. Although the oven light and fan came on, the clock display didn't and neither would the elements. On taking the back off I found a wire (with spade type socket) had come adrift from the neutral of mains input connector block. It was a relatively thin wire going to the control/clock unit. Tightened up the connector a bit, put it all back together and the oven worked fine.
Yesterday got a call saying the oven had stopped working, same symptoms as before. He took the back off and checked the wire, still in place. Mentioned that earlier in the day he'd got a "tingle" off the oven. The oven has it's own mcb, but no rcd protection. Told him to isolate it and wait for dad!
Called in today, isolator switch on the wall was off, so before going further I turned it on and used a multimeter on ac volts range to check between the oven casing and the adjacent sink tap (plumbed in copper, bonded to earth). 230 volts!! (he went and put some shoes on at that point!). He said he'd loosened the isolator from the wall while doing some grouting, so flicked off the mcb and unscrewed the isolator switch. Earth conductor to oven floating loose. Put it all back, switched everything on, oven clock/timer now working and no voltage on the casing. Oven now works ok, but I'm still wondering why a disconnected earth would result in 230v on the casing. In retrospect, maybe I should have put on some sort of "load" when doing the voltage check, I suspect that the voltage is coming from the "neutral side" of a highish impedance load, maybe the electronic clock/timer, but this still seems wrong to me.
Any thoughts?
David.
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It sounds like the floating wire should be connected to neutral, but has been connected to earth.
This will be connected to the metal casing, so a disconnected earth in the oven switch would mean the clock module will fail to work, and you will get a "tingle" from the case, equal to the mA consumption of the clock (and a meter will register the mains voltage).
The MCB will NOT trip in this case (an RCD would if the clock tried to pull more then 30mA), and the oven will function with the earth connected through (as neutral and earth are (should) be at the same potential.
So, double check where this thin wire is connected!
The other possible problem is, the cable to the oven has been wired wrongly, and neutral/earth has been reversed.
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You could also try this test....
Turn off the oven isolator
Disconnect the earth again
Turn the isolator back on, but DO NOT TOUCH THE OVEN
The clock should fail to work again
TURN OFF THE ISOLATOR
Now connect your red meter probe to the current position (mA) and select the AC current mode of the meter (mA if it has this)
Now one of you hold the black probe on something earthed (or stick it in the earth terminal of a nearby socket) Then, again, WITHOUT touching the oven casing with your hands/feet etc hold the other probe on the oven and turn the main switch on does the clock work with the meter as an earth bridge? what current does the meter show?
Please note, doing this is dangerous, as you are breaking the safety earth connection, so if you are not sure, don't try it!
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On Fri, 28 Dec 2007 17:39:01 GMT DavidM wrote :

Perhaps it was touching the screw of the live terminal before you loosened it? Could have been very nasty from what you say.
--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk


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I doubt if there is a fault other than the disconnected earth. The symptoms you describe, the slight tingle, and the full indicated mains voltage on a high impedance instrument like the digital voltmeter that you used are to be expected on this kind of appliance with no earth connected. What concerns me is why loosening the switch to grout behind it caused the earth to become disconnected, seems the connection wasn't tight enough in the first place, and also cooking in bare feet is not a good idea for several reasons, the risk of electrocution only being one of them
--
Graham

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Ignore my response and listen to the others. I re-read your post and realised my reply was inappropriate given the full circumstances sorry.
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Graham

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DavidM wrote:

Suppressors mainly. Its normal to have a capacitor between live neutral and earth - all three on a goodish setup.
Or maybe no suppressors and simply leakage from live to case. Ring elements are favorites for that.

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Neither of which would explain the clock failing to work with an earth connection though!
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You made the same mistake as I did, and didn't read the OP properly. It looks to me that when he "repaired" the original fault he connected the return wire to the clock circuit to earth instead of neutral. The circuit isn't protected with an RCB. By some strange quirk of fate the second fault revealed the first by robbing the clock of a return path. Quite fortunate really, it might have gone unnoticed for years otherwise.
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Graham

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Sorry Sparks, that was of course directed at TNP, I'm not having a good day am I?
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Graham.
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Thanks for your thoughts (in particular Sparks). I'll recheck the connections inside the oven next time I'm at his house. btw, I had checked the wiring up to the oven, LN and E all connected correctly (except loose earth wire). Happy New Year. David.
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