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.
Reply to
DavidM
Loading thread data ...
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.
Reply to
Sparks
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.
Reply to
Tony Bryer
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
Reply to
Graham.
Ignore my response and listen to the others. I re-read your post and realised my reply was inappropriate given the full circumstances sorry.
Reply to
Graham.
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!
Reply to
Sparks
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.
Reply to
The Natural Philosopher
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.
Reply to
Graham.
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.
Reply to
DavidM

Site Timeline Threads

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.