Induction hob spark fault

What actually sparked?

Z.

Faulty appliance, possibly IMHO.

Incorrect polarity, most unlikely, there are still supplies in Europe with BOTH sides live. And polarity is a bit random in other places, standard appliances are used in such places. And of course in the USA 208 volt or 240 volt circuits have both sides live.

Incorrect voltage, most unlikely as such appliances are sold internationally including places where the voltage varies varies a lot more than it does here.

In student housing do not ignore the possibility of misuse or rough use. Try to find out WHAT EXACTLY arced. Any arcing outside the appliance suggests a faulty appliance AND lack of earthing/bonding. 

Could be almost anything from static electricity at the harmless end , up to the horrific possibility that the house is off-earth, and the whole lot floats relative to true earth. Where was the spark, and did anyone get a shock off something and if so what ?

Eliminate the worst possibilities by bringing a wander lead in from true earth (screw driver in the lawn style, and verify that the cases of the units are really at earth voltage not just connected together) Then check  between the MET  and all the things that should be earthed .

Mike

I've never heard of this. The cooker connection unit / plate Zs was 0.28 ohms. Then there was 240V between line and the metal casing of the hob and oven showing it is connected to earth / cpc. Doesn't the Zs reading prove the earth fault loop path...hold on I think I get you...as in the earth connection at the distributors transformer could be broken? So you test from true earth to make sure? It could just go down the boding though to gas and water? You disconnect these?

They just said the hob sparked when they were using it and it tripped the RCD. Why would no earth connection cause a spark anyway out of interest?

Just had an email from the company - “Half of the hob isn’t working following a visit from the electrician and when we try to turn the stove off at the switch there is a sparking noise”

A Zs test verifies a path from the live back to something - but it may just be neutral at the company cut out on a TNC-s.  I have heard (but only heard of, never seen)  where the supply to the house was L-N reversed after some DNPO works , so that all the house metal work was alive. No shocks except off the outside tap. More common is a high impedance earth cable, or a faulty PEN.  I;d not bother to disconnect bonds at this stage. Just verify that the cooker really is earthed before starting, a screwdriver in the lawn is a 10 minute check of sanity to eliminate the worst possible case, that very rarely happens. But if it ever does, stop and think.

But you have a good Zs from the cooker body ? Good.

But actually sparking sounds from the switch and some rings not working sounds like it may need a new switch, and something nasty has happened in the hob or its controls.
But even before that just check it is not one of those universal 3 phase /split phase/ single phase hobs with far too many terminals and some sort of link kit for single phase for  the UK model and someone has only put live to phase one, but not but power to the other phase or phases or forgotten to tighten one or something.

(if in doubt take a photo of the terminals and post it - we may well have seen it before.)

Mike.

Thanks for the help. I'll go back and do the true earth test and check the hob terminals. Then I can suggest changing the cooker switch and the hob. Seems strange that the same thing happened before with the previous hob and oven like it's a problem caused by the installation, not the appliances. 

yes - that is why I wonder about a 3 phase type cooker and a 1 phase supply, and a 'our shop's skilled installer' not realizing there should be a link kit, or verifying L and N the switch on the wall.

Is that true earth test just on low ohms setting? Aka a dead test

It is better done with no dismantling as a 'live'  AC volts test . Just a metal peg, screwdriver or garden fork, and a length of almost any old wire with a croc clip to get it indoors... It is only a quick 'is it safe to proceed?' thing.
Usually followed by reading a very low voltage (no more than half a dozen volts if all is well) and a sigh  of relief. But if it isn't then stop.

A normal ohm-meter is DC, being battery driven, and tends to be fooled by  electrolytic effects as the mud and the metal electrodes form a half-hearted battery of their own..

A proper earth rod test meter of the kind you use to test electrodes when no supply is available in effect reverses the DC many times a second, and is really a  meter that applies an alternating test voltage if you do it as resistance

M.

Sorry if I'm being dumb here but can I do this test with a multimeter (basic Megger)? That's all I have currently. I've got some copper tube and a long lead. So drive that tube in the ground, connect the long lead to it, then will testing the earth terminal at the cooker work on DC low ohms reading with the circuit dead? Or a live Zs test between line terminal on the cooker to the copper rod in the garden? Both to see if the current finds that path back through the ground to the transformer true earth connection? Kind of like a TT system (which I've never worked on). I might expect to get under 200 ohms, but any reading would suggest the transformer sub station true earth connection is sound?