Problem with hob or supply earthing ?

[937carrera]

I'm reasonably competent in most aspects of DIY, but have an electrical problem which has me a little confused and prompted me to register rather than lurk in the background, hopefully you can help.

I have a Philips solid ring hob, I noticed on Saturday a slight tingle when I touched the handle of a metal pan.

The hob is wired onto a separate circuit which only feeds the oven and hob, so can be easily isolated using the wall switch which only feeds the hob. I grabbed my multimeter and measured for AC between the metal plate of the hob and earth on an adjacent socket. Results as follows:

Wall Switch off - Less than 1 volt
Wall Switch on - around 18 volts
Wall Switch on , any hotplate on - around 80 volts

Clearly not good.

I disconnected the hob and identified / suspected that a couple of the plates had become porous as a continuity test showed they had around 3 k ohm resistence between the supply feeds and earth. Others plates had zero continuity (or at least 20 M ohm as that's the max on my meter)

My original plan was to buy replacement plates and reinstall, but as luck would have it I managed to pick up another identical hob (removed only due to kitchen upgrade) to use as a source for spares / replacement unit.

I therefore checked this replacement hob out electrically and made sure all plates had zero continuty to earth and there was no continuity between L / E, L / N, N / E on the hob with switches in the off position before reinstalling. Each of the switches was also checked to confirm that the resistance varied correctly according to the switch position.

I then reinstalled the hob and carried out the same tests

Wall Switch off - Less than 1 volt
Wall Switch on - around 18 volts
Wall Switch on , any hotplate on - around 45 volts

I am now wondering if there is some earthing problem on the supply to the hob as two separate appliances are showing the same problem.

I would be grateful for any further thoughts or advice on the likely cause of the problem or my fault diagnosis approach.

I don't want to decide to buy a new hob if there is a problem on the supply side and I don't have access to an IR tester to disgnose myself

 

[Adam_151]

I would suspect as you suspose that there is a lack of earth continuity to the hob.

Test for continuity between the earth from the socket top the screws on the isolator for the hob to establish whether whether its before the isolator or not, if you get continuity there, try to the metalwork of the hob.

If you get continuity between earth from the socket and the isolator for the hob, but not the hob itself then check all connections between the isolator and the hob, if you have no continuity to the isolator then you'll have to trace the cable back to the fuseboard for any joints after checking both ends are terminated correctly.

 

[937carrera]

Thanks, that was really useful.

Having pulled the oven & microwave out of the wall unit I got access to the cooker connection point. From here I now believe there is a mini-ring main, which provides power to a fused switch that is used for the washing machine, and the wall switch for the cooker connection point which has an integral 3 pin 13A socket.

I also pulled out my el-cheapo chinese multimeter which allegedly goes to 2000 M ohm

From the chassis of the hob to the screw on the cooker connection point I get 0.8 ohm resistance, which is the same value as when connecting the probes together (as usual the connectors are rubbish). For me that's continuity.

I then went from the screws on the cooker connection point to the screws on the wall switch and this showed around 5.5 M ohm

From the chassis of the hob to the screws on the wall switch the result was around 25 M ohm

Now the maths doesn't add up because there's another 20 M ohm resistance somewhere, but this does support the idea that there is an earth fault. Perhaps it's just the result of 25 years corrosion due to being near steaming pans ??

I'll isolate the circuit tomorrow and see if I can identify any poor connections on the cooker circuit.

Having said that, I am still not sure why the chassis is seeing any voltage at all. It doesn't seem right that the chassis should carry any voltage, even if it can go earth with a good connection. Is it possible that the earth is somehow getting a feed through the wiring or wall switch ?

 

[DaftPunk]

The values you state sound typical for a circuit tested without an effective earth tested with a high impedence volt meter.

The voltage will probably be a result of either inductance, capacitive coupling or a slight leak to earth on the appliance.

It's a normal occourance but as I'm sure you're aware, it needs sorting ASAP.

Testing for continuity between the appliance and a known good earth until you locate the break is the way to approach this.

Well done on having a multimeter and knowing how to use it! Far too many people come here without even the most basic of understanding of what they're doing or the tools to work safely and find out what's wrong.

 

[937carrera]

So, I came home from work early and set about removing the fuse from the consumer unit and having checked it was really dead started removing the cover plates.
The cooker switch had a poor earth connection to the earth on the cooker supply. None of the terminal connections used both retaining screws, only the outer one had been used.

On the cooker feed plate the earth simply came free when I removed the faceplate and one of the live connections also came free without removing any retaining screw indicating that was probably not secured correctly.

I cleaned everything up and checked that there was continuity across all 3 earth connections and zero continuity between L / E, N / E and L / N before reconnecting the supply.

Upon testing for AC voltage to earth there was zero volts whether power was on or off, and the hob ring was on or off. Result

Thanks have been given to you both for your help, much appreciated.

Now I'm wondering if I have incorrectly diagnosed minor shorts on the other hob / rings. Does anyone know what sort of resistance values I should be seeing for a solid hob ring, when new / after use / when it needs to be replaced. If I have spares I'll keep them, otherwise they're for the bin.

They rings themselves are manufactured by EGO, part numbers 13.14453.30 and 13.18463.30, 1000W / 2000W and seem to be widely used across many makes

 

[EFLImpudence]

Well done.

One thing I noticed yesterday was -

I therefore checked this replacement hob out electrically and made sure all plates had zero continuty to earth and there was no continuity between L / E, L / N, N / E on the hob with switches in the off position before reinstalling.

No continuity between L / N will mean it's broken.
It should read a few ohms depending on the Wattage. 2000W @ 240V is 28.8Ω
Use this calculator
http://www.tlc-direct.co.uk/Technical/Charts/OhmsLawCalculator.html


There should be mega-ohms between L / E and N / E.

 

[937carrera]

Ah, I wasn't perhaps as clear as I intended.

I was simply checking that there was no internal short at the mains connector block when the hob switches were in the off position. (i.e there should be no circuit so no continuity.

My next test was to check the resistance L / N for each ring as I rotated the temperature knob. The resistance values varied by element size but roughly went from 33 Ω to around 500 Ω. (probably a 2000W ring)

I didn't do any live testing, nor did I test the resistance of the elements within the rings, but that sounded like a good idea so I have just done that now.

The smaller ring offers 100 Ω , 300 Ω and 500 Ω elements. The link you gave reminded me of Ohms law and the difference between resistors in series & parallel, so I checked these results:

The simple maths says resistance should be 1000W/240V = 4.1667 A and resistance should be 57.6 Ω

Applying Ohms law to resistors in parallel comes out with a result of 65 Ω, if my maths is correct. I expect that the difference is just down to real world complexities / measuring issues. Many thanks for prompting me to do my O level refresher

I just checked the L / E for the same ring. That was showing around 7.9 M Ω ......... is that sufficient mega ohms to be good ???

 

[EFLImpudence]

Ah, I wasn't perhaps as clear as I intended.
I was simply checking that there was no internal short at the mains connector block when the hob switches were in the off position. (i.e there should be no circuit so no continuity.

Ah. Ok.

My next test was to check the resistance L / N for each ring as I rotated the temperature knob. The resistance values varied by element size but roughly went from 33 Ω to around 500 Ω. (probably a 2000W ring)

Seems reasonable.

I didn't do any live testing, nor did I test the resistance of the elements within the rings, but that sounded like a good idea so I have just done that now.
The smaller ring offers 100 Ω , 300 Ω and 500 Ω elements. The link you gave reminded me of Ohms law and the difference between resistors in series & parallel, so I checked these results:

Do you mean those are three of the rings?
If so, rather high.

The simple maths says resistance should be 1000W/240V = 4.1667 A and resistance should be 57.6 Ω

Yes,

Applying Ohms law to resistors in parallel comes out with a result of 65 Ω, if my maths is correct. I expect that the difference is just down to real world complexities / measuring issues. Many thanks for prompting me to do my O level refresher

That is the result of 100,300,500 but I'm not sure what you are comparing.

I just checked the L / E for the same ring. That was showing around 7.9 M Ω ......... is that sufficient mega ohms to be good ???

I don't know how reliable the value is with a multimeter but it is fine.
Even new ones aren't much better some times.

 

[937carrera]

That's great, thanks, some responses to your points

I didn't do any live testing, nor did I test the resistance of the elements within the rings, but that sounded like a good idea so I have just done that now.
The smaller ring offers 100 Ω , 300 Ω and 500 Ω elements. The link you gave reminded me of Ohms law and the difference between resistors in series & parallel, so I checked these results:

Do you mean those are three of the rings?
If so, rather high.

The values are for the three separate elements which are within each ring; This is how I think the hob works..............

Each of the rings has four electrical connections, which I believe are for three separate feeds for three elements within each hob with a common neutral. Six varying levels of heat are then available by selecting different combinations of heating elements via the temperature knob which has six positions. The resistance values quoted are for the three elements within one ring

That is the result of 100,300,500 but I'm not sure what you are comparing.

I was only comparing the results of calculations between resistance values based on the rated consumption of the ring and the equivalent value based on measured resistance values for the three elements within a ring. As much a test of my maths as anything else

I don't know how reliable the value is with a multimeter but it is fine.
Even new ones aren't much better some times.

That sounds like good news, thanks very much for the guidance, it seems like the wiring was the issue all along....I'm always prepared (perhaps too willing) to consider that there may have been more than one fault present.