Low voltage on 30 amp Cooker circuit

[Harry Bloomfield]

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.

10MOhm I think and supposing the break in the cable (assuming) is around 10MOhm then the measured volts will be around the 130v the OP claims to have measured.

 

[bernardgreen]

 

[JohnW2]

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?

I'm not missing that.

Even analogue meters were generally no worse than about 10kΩ/v - hence about 5MΩ on a 500V range, and the digital meters of today are at least that high - so the high resistance you are postulating would have to be 'megohms' to result in a 115 V drop - pretty unlikely in a circuit which 'sometimes works'.

I suppose it could be an intermittent 'complete disconnection', such that the meter was measuring between L and a 'floating' N.

However, even if that conceptual explanation were correct, your suggestion of measuring L-E voltage would not be 'foolproof'. The same mechanism would presumably apply if the 'megohms resistance' were in the L, rather than the N - and in that case the measured L-E pd as well as the L-N pd, would be 115 V (or whatever).

Kind Regards, John

 

[JohnW2]

View attachment 248461

True, but we're back to Mr Kirchoff again. If (per your diagram) the measured L-N voltage were 115 V because of a broken N conductor, resulting in a capacitive divider between N and E, then the N-E voltage would also be 115 V (co-incidental, since 230 = 115 x 2), not (the reported) 'zero volts', wouldn't it?

Kind Regards, John

 

[Harry Bloomfield]

However, even if that conceptual explanation were correct, your suggestion of measuring L-E voltage would not be 'foolproof'.

Not actually my suggestion.

I think we are all guessing and the OP seems to have disappeared.

 

[JohnW2]

Not actually my suggestion.

It wasn't - apologies for my imperfect memory. However, you did respond to my comments about sparkwright's suggestion with your suggestion as to how L-E could be measured as 230V, even though we had been told that L-N measured at 115V and N-E as 0 V - and all the subsequent discussion has related to that.

I think we are all guessing and the OP seems to have disappeared.

We're certainly all guessing. It's probably a bit premature to say that the OP has 'disappeared, since he was seen less than 24 hours ago - I do hope he returns, in the hope that he may be able to prevent us having to carry ion guessing for ever!

Kind Regards, John

 

[sparkwright]

we had been told that L-N measured at 115V
Kind Regards, John

155V.

 

[JohnW2]

155V.

You're right. I suppose that our minds were focussed on the fact that 115 is half of 230, and therefore (particularly me, but also even bernard's diagram) have been typing 115 V throughout this thread!

However, that oversight does not alter anything else which I (or anyone else) has written.

Kind Regards, John

 

[brianivor]

The comments about my problem were beginning to get a bit too technical for me!

Thanks to the initial help and advice I received from you good folk, I was able to resolve the problem,

the first mistake was to think "It can't possibly be that!".

The original 30 amp cable was extended via a 30amp 3 way terminal block and extra 30amp cable

to the new cooker position.

I could have sworn that all the screws were still tight but I removed the block and remade the connections

and everything has been fine since.

So, once again, thank you for all your contributions.

Brianivor

 

[brianivor]

O P here!

Curiouser and curiouser...

As stated previously, I dismantled the 30 amp wiring block being used to extend the existing cooker cable with new 30 Amp cable
to the resituated ccu.

On remaking the connections in the block, 240 volts resumed, the cooker circuit became live and the cooker worked again,

A few days later, the circuit failed again with a bit of on/of flickering of the washing machine plugged into the ccu (cooker wall panel)

Checking the ceramic fuse wire holder in the cu, it seemed a bit warm, so I removed the cartridge and rewired it with new fuse wire (30 Amp)
This got thing working again!

It has now happened again and so I purchased a new MEM 30 Amp fuse holder and replaced the old fuse holder but this has not been successful.

I did poke the new fuse holder (in situ) with an insulated screwdriver and got a response from the washing machine for a couple of seconds before it failed

I am wondering if there is a loose connection in the consumer unit itself?

All other circuits, lights, ring main do not seem to be affected at all.

Many thanks,

Brianivor

 

[JohnW2]

... I did poke the new fuse holder (in situ) with an insulated screwdriver and got a response from the washing machine for a couple of seconds before it failed ... I am wondering if there is a loose connection in the consumer unit itself?

From what you have described, that sounds like a distinct possibility - or perhaps a problem (maybe due to tarnishing/damage) to the contacts in the fuse holder.

Kind Regards, John

 

[JohnW2]

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.

I've only just noticed this one from last week. Although we tend to talk of the input resistance/impedance of digital meters as being 'extremely high', I'm not sure that it is all that much higher than was an analogue meter on a high volts range - as I wrote earlier in this thread, I think they were generally of the order of "10kΩ per volt" (implying a 100μA meter movement), which would equate to an input resistance/impedance of 5MΩ on a 500V range. Of course, on low volts ranges the input resistance would have been very much lower than that.

Kind Regards, John

 

[brianivor]

In reply to John particularly, further careful examination of the consumer unit (with great difficulty owing to its location),
has shown that the cable(s) entering the back of the 30 Amp socket are charred.

I guess this will be a job for a professional, but since the consumer unit (Mem wired) is out of the way,
and not tampered with, I am now wondering "why now?"

brianivor

 

[Harry Bloomfield]

I've only just noticed this one from last week. Although we tend to talk of the input resistance/impedance of digital meters as being 'extremely high', I'm not sure that it is all that much higher than was an analogue meter on a high volts range - as I wrote earlier in this thread, I think they were generally of the order of "10kΩ per volt" (implying a 100μA meter movement), which would equate to an input resistance/impedance of 5MΩ on a 500V range. Of course, on low volts ranges the input resistance would have been very much lower than that.

Whereas the modern digital ones tend to present the same resistance across the source, I think, irrespective of the range. Which is why some meters include some duplicated lower ranges, with a lower input resistance for checking the likes of batteries.

 

[JohnW2]

In reply to John particularly, further careful examination of the consumer unit (with great difficulty owing to its location), has shown that the cable(s) entering the back of the 30 Amp socket are charred. ... I guess this will be a job for a professional, but since the consumer unit (Mem wired) is out of the way, and not tampered with, I am now wondering "why now?"

I can't answer the "why now", but I do agree that you now really do need an electrician to get this sorted out - and probably sooner, rather than later.

Kind Regards, John