EXPECTED A FAIL BUT IT PASSED

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EXPECTED A FAIL BUT IT PASSED

Postby nevets » Wed Jan 18, 2012 1:33 pm

Hi cutting a long story short.

An electric hob was blowing the main MCB. Out of interest i rigged it up to my europa plus and ran it through the tests. All hobs on, etc. It passed every test with flying colours and showing it was pulling around 3.4kw.
Why would this happen i would have expected a fail either on IR and/or earth.

Just to be certain i wired it back to the junction box and the MCB blew even with the hobs all in off position.

Cable from MCB to cooker switch ok and from switch to cooker outlet ok. Oven is attached to same cooker outlet and its running fine.

Have i got a problem with my tester, i dont think so as its picking up faults on other equipment, or is there some other explanation
nevets
 
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Re: EXPECTED A FAIL BUT IT PASSED

Postby Jim Wallace » Wed Jan 18, 2012 8:53 pm

Hi Nevets

The MCB can trip because of over-current or earth leakage. The hob alone is consuming 3.4kW which does not sound excessive, certainly not enough to cause an MCB to trip due to excess current. If there was an earth leakage fault this should show up as either low insulation resistance (as you point out) or high earth leakage current. The fact that the MCB trip when the hobs are off suggests a problem either in the wiring or before the switches in the hob. At what point does the MCB trip? If the hob is off what are you connecting or switching on that causes the MCB to trip?
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Re: EXPECTED A FAIL BUT IT PASSED

Postby nevets » Thu Jan 19, 2012 12:06 pm

Hi Jim,

Thanks for the reply. The wiring to and from the switch is all ok, and as i said the oven which is wired to the same point is working fine.

Your thought process appears to be the same as mine and the only conclusions i can reach is a) the tester was not working correctly. But i think it unlikely as it was measuring the current. or B) The flex from the hob to the junction box is loose inside the hob and in a certain position shorts to something. This would support the fact that the oven still works at the same junction and the MCB blew when the hob was reconnected to the junction even with nothing switched on.
nevets
 
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Re: EXPECTED A FAIL BUT IT PASSED

Postby Jim Wallace » Mon Jan 23, 2012 10:14 pm

Hi Nevets

If the MCB trips when the hob is connected but switched off it does sound like a wiring problem.
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Re: EXPECTED A FAIL BUT IT PASSED

Postby Alan Freeman » Mon Jan 23, 2012 11:19 pm

When connected to your Europa, do you use the cable from the hob to the junction (twin & Earth) - or a 13A to bare end cable? If the latter - try the cable fitted to the junction with a 13A plug fitted to that.

Do you have a multimeter??

Assuming this is connected up as standard to a cooker switch and junction box, I would then suggest, with the hob connected up and failing, try a resistance reading - with the MCB and switch off - between Live and Neutral and again between Live and Earth of the cooker switch.

I would expect one of these to read short.

As an alternative, I would suggest re-wiring the hob end or even totally replacing the twin and earth cable.
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