Low voltage on 30 amp Cooker circuit

brianivor · 25 Oct 2021

All outputs from my cooker panel (Cooker and 13 amp socket) are dead.

The circuit is: Consumer Unit to the original Cooker Panel location in kitchen, which is now a
standard wall box from which a 30amp cable has been extended to a new Cooker Panel where the cooker is now located.

This has been working perfectly for over fourty years.

When this first happened last week, I checked the terminals in the wall box and obtained a reading of 155 volts. (the Consumer Unit fuse was sound but I cleaned the fuse contacts anyway.

I refitted the cover plates while I gave the matter some thought and, shortly after, everything started working.

A week later (today) the Cooker Panel has gone dead and again the reading at the wall box
is 155 volts.

No wiring in the circuit has been touched at any time other than prodding with the meter probes.

All lighting circuits and the ring main are working properly.

Can anyone advise me as to what the fault could be please ?

Thank you.

terryplumb · 25 Oct 2021

What exactly is a " cooker panel" ,and what is a" standard wall box", showing us pictures would helpful ?
Between what two points are you measuring 155 volts ?

magicmushroom666 · 25 Oct 2021

Check you meter works correctly with another known circuit

JohnD · 25 Oct 2021

was this with the cooker turned on? A high-resistance joint will drop the voltage under load.

what sort of test meter are you using? (photo please)

if you have safe test probes, test the voltage at the CU, both ends of the MCB, turn off the main switch and and verify with a safe insulated screwdriver (if you are competent) that all three terminal screws (including the neutral one) are tight

then test again at every point, especially junction boxes and switches.

A loose connection may show signs of overheating

if it started again after you had poked something, the fault may be near where you poked.

if not a loose connection, consider if a cable might have been damaged e.g. by a drill, screw or nail. This can be dangerous

If the cause is not readily apparent, you will need a qualified local electrician.

Harry Bloomfield · 25 Oct 2021

magicmushroom666 said:
Check you meter works correctly with another known circuit

and if that proves the meter is OK, I would suggest there is a bad connection somewhere. Likely with a cooker load, it will have heated up and be blackened. My first place to check, would be where it has been joined, to extend it.

brianivor · 25 Oct 2021

Thank you for your replies.
The cooker panel I believe is called a Cooker Control Unit, which feeds the cable to the cooker
The standard wall box is an 80mm switch box in which is enclosed a 30 amp screw terminal joining the cable from the consumer unit
to the CCU.
The meter is a £15 domestic item
No walls have been drilled
The 155 volts was measured with 500v ac set on the meter
and the voltage was measured between the red and black terminal screws on the screw terminal in the wall box junction.
The black terminal and earth terminal at this location registered zero.
I am going to double check my handiwork later this evening when the kitchen is empty just in case I have made an error of
some sort but as I have said, the complete circuit has been trouble free for decades and I cant get my head round this 155v
reading (I would have expected zero or 240v)
Again thank you for your advice

terryplumb · 25 Oct 2021

What's the voltage live to earth ?

JohnW2 · 25 Oct 2021

terryplumb said:
What's the voltage live to earth ?

In case this is not clear to Brian, that would presumably be (using his terminology) between the 'red terminal screw' and the 'earth terminal'.

It will be interesting to hear what he finds but if, as we have seemingly been told, there is 0V between N & E and 115V between L & N, then it's quite hard to see how he could get anything other than roughly 115V between L and E.

Kind Regards, John

sparkwright · 25 Oct 2021

JohnW2 said:
In case this is not clear to Brian, that would presumably be (using his terminology) between the 'red terminal screw' and the 'earth terminal'.

It will be interesting to hear what he finds but if, as we have seemingly been told, there is 0V between N & E and 115V between L & N, then it's quite hard to see how he could get anything other than roughly 115V between L and E.

Kind Regards, John

Is there not a good chance there could be 230v between L and E?

brianivor · 25 Oct 2021

I typed a long reply, but it seems to have gone missing (hope this one gets through)
Will retype as soon as I can
Thank you

JohnW2 · 26 Oct 2021

sparkwright said:
Is there not a good chance there could be 230v between L and E?

On the basis of what we've been told, how could there be?

If there were 230V between L and E and (as we appear to have been told) 0 V between N and E how could there be only 115V between L and N)?

Kind Regards, John

Harry Bloomfield · 26 Oct 2021

JohnW2 said:
If there were 230V between L and E and (as we appear to have been told) 0 V between N and E how could there be only 115V between L and N)?

Resistance of a poor/faulty connection along the N somewhere?

JohnW2 · 26 Oct 2021

Harry Bloomfield said:
Resistance of a poor/faulty connection along the N somewhere?

Well, for a start, my understanding is that the measurements were taken when the cooker was 'dead' (i.e. drawing no significant current), in which case there would be no voltage drop in the N, no matter how high its resistance.

In any event, as Messrs Ohm and Kirchoff tell us, the figures I mentioned are simply 'not possible', regardless of any considerations of resistances/voltage drops. Think of the potential differences being measured as 'distances above ground', and consider Tom and Dick (we don't need Harry

), with Tom's nickname being "L" and Dick's nickname being "N" - and ground being ground/earth. If Tom (L) were 230 feet above ground and (as we have been told) Dick (N) was 0 feet above ground, then it is inevitable that Tom (L) is 230 feet above Dick (N), and simply not possible that he could be only 115 feet above Dick.

Looked at the other way around, if the distance between Tom (L) and Dick (N) were 115 feet, and the distance between Dick (N) and ground were 0 feet, then it is again inevitable that Tom (L) is also 115 feet above ground - again, it's impossible that he could be 230 feet above ground.

Kind Regards, John

Harry Bloomfield · 26 Oct 2021

JohnW2 said:
Well, for a start, my understanding is that the measurements were taken when the cooker was 'dead' (i.e. drawing no significant current), in which case there would be no voltage drop in the N, no matter how high its resistance.

In any event, as Messrs Ohm and Kirchoff tell us, the figures I mentioned are simply 'not possible', regardless of any considerations of resistances/voltage drops. Think of the potential differences being measured as 'distances above ground', and consider Tom and Dick (we don't need Harry ), with Tom's nickname being "L" and Dick's nickname being "N" - and ground being ground/earth. If Tom (L) were 230 feet above ground and (as we have been told) Dick (N) was 0 feet above ground, then it is inevitable that Tom (L) is 230 feet above Dick (N), and simply not possible that he could be only 115 feet above Dick.

Looked at the other way around, if the distance between Tom (L) and Dick (N) were 115 feet, and the distance between Dick (N) and ground were 0 feet, then it is again inevitable that Tom (L) is also 115 feet above ground - again, it's impossible that he could be 230 feet above ground.

The point, or rather load you are missing - is the load of the meter itself. It might be a tiny load, but none the less it does form a load and the voltage it will see, depends very much upon the value of resistance at this poor connection we are assuming.

John - you must have come across situations where the testing process affects the measurement itself?

ericmark · 26 Oct 2021

Years ago it was easy, measure on 250 volt range and 500 volt range and see if reading changed, but the days of the analogue meter are gone, the new digital meter is more like the valve volt meter, it hardly draws any power.

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Low voltage on 30 amp Cooker circuit

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