Shock off hob

[Stonesman]

Chaps,

Got a call to check a solid plate hob in a local council youth club... staff have had 2 belts off it.

Above counter metal clad 45A switch is fed from a surface conduit direct from CU containing a 6mm T&E. Fed from a B32 from non-RCD side of wylex CU.

R2 checks out from hob to CU... 0.01 Ohm
Insulation L-E is >250 MOhm
Insulation N-E 30 MOhm
Zs at appliance is as near as damit Ze at CU

insulation test carried out with tails from hob disconnected from 45A switch. I'd expect the same result from L-E and N-E in a stand-alone appliance - is it just opposite ends of an element?

Want to make sure I've not missed anything before I give a clean bill... maybe I'm just being paranoid!

Stones.

 

[Marvo]

If someone gets a shock from an appliance then you should be looking for at least 2 faults. If the shock report sounds genuine then you need to find the problems before you even consider giving it a clean bill of health. It's not going to look good if someone gets hurt or killed shortly after you've said it's safe as houses.


Firstly there has to be an insulation fault causing current leakage to the chassis of the appliance and through the unfortunate victim. Measure the IR with the stove cold and again hot. Also the elements may be 2-pole switched so you may need to test the elements as separate components and not as part of a global reading.

Secondly there has to be a poor earth on the entire appliance or on a part of the appliance that's allowing the touch voltage to rise to a dangerous level. To test for touch voltages use an analogue voltmeter not a digital one. Visually checking the earth connections is equally as important as using a tester to find the earth impedance, make sure all the internal earth fly leads are present and correct.

Finally it's alway worth remembering you can get a shock in two directions, if there's another appliance within reach that has a high touch voltage then the chassis of the stove or hob may be the earth path not the source of the live so don't be afraid to haul out the trailing lead and test between appliances.

If I was you and I couldn't find a fault then I'd consider putting the circuit supplying the appliance on an RCBO if it's practical, at least until a time you're certain the fault isn't going to reoccur.

 

[Stonesman]

Thanks for the great reply.

The metal chassis of the hob is heavily enamel painted - so much so I has a real job to scratch enough off to get an earth result.

The actual 4 solid plate elements aren't earthed - does anyone know if this is normal? all internal fly leads etc are fine.

Same women got the shock both times - apparently off the white chassis (the hot plates were 'hot')

I may suggest I move the supply MCB to RCD side of CU.

 

[coldyron]

To test for touch voltages use an analogue voltmeter not a digital one.

This might be a silly question as it's very early in the morning, but why only use an analogue meter? I've not seen one used for years and don't own one so it's interesting to know.

 

[telectrix]

because analogue meters aren't fooled by phantom voltages.

 

[Marvo]

As Tel says digital testers are over sensitive and prone to giving meaningless readings in certain circumstances.

 

[Marvo]

I may suggest I move the supply MCB to RCD side of CU.

I would only do this as a safety precaution if you can't put your finger on a definite fault or there's even a remote chance it might reoccur. There's a good chance it's going to cause intermittent tripping which will be an annoyance and secondly it's very bad practice to treat the symptoms of a fault and not the root cause. There's no free lunch I'm afraid you need to actually find the problem and fix it.

 

[snowhead]

I'd expect the same result from L-E and N-E in a stand-alone appliance - is it just opposite ends of an element?

Yes when the Hob control switches are on.

 

[itselectric]

I suppose if tests are showing good, static could be the answer.

 

[ChrisElectrical88]

Is the gas bonded?

 

[Rockingit]

I was leaning towards static, especially if they've got nylon chairs, nylon carpets...

 

[telectrix]

or it could also be a fault on something else and the hob metal is providing the earth path.

 

[FAA Greenie]

As above, get the long lead out and confirm for definite that all relevant parts of the appliance are earthed to the MET.

If it is, then the shock is either static discharging from them from the cheap nylon youth club carpet, or they are touching something else with a fault at the same time.

Have you asked the ladies who got the shock about it? Whether it was a quick shock with a small 'tick' sound or a longer one with a tingling? Arm ache afterward? All this info helps.

I would go with the static option first!!

 

[telectrix]

so you advise they replace their frilly nylon knickers with cotton ones? no shocks there, then.

 

[Silly Sausage]

Get them to fit one of these
http://www.amazon.co.uk/Aluminium-I...1382188065&sr=8-4&keywords=metal+door+curtain
in the kitchen doorway.
Get it Earthed, or is that Bonded, then that will discharge any static they may be in possession of.