Strange voltage readings

ScottishGasMan · 20 Aug 2014

Working on a boiler today with intermittent electronic fault codes.

While boiler connected and live I had 240v L-N, 240v L-E, 0v N-E

On disconnecting the supply connector to the boiler (so boiler disconnected/no load, 240v L-N, 88v L-E, 63v N-E,

So clearly something not right with the supply to the boiler so as far as my remit goes in a customers house.

But out of curiosity, how can I get this reading on the cable, yet plug it into boiler and get normal reading under load (no other wiring/2nd supply etc) and what might cause this sort of voltage readings?

Checked with two different calibrated multimeters,never had an adaptor to take a reading from a plug socket to check for similar issues.

RF Lighting · 20 Aug 2014

There's no effective earth on the circuit supplying the boiler.

When you have the boiler connected there will be an earth via the metallic casing of the boiler pipework and bonding conductor(s)

When the boiler is disconnected, the connection to earth is removed too. As I'm sure you've worked out, the circuit is not safe and needs to be repaired by a competent person before it is used again.

ScottishGasMan · 21 Aug 2014

Thanks.

Should have figured the lack of earth and the earth when connected to the boiler out myself.

Just got thrown by the 60 odd volts between neural and earth when disconnected.

Certainly knew there was something far wrong with the supply, British gas electrical cover in the house so they were called but I was away before they arrived, Just dont like not being able to explain whats wrong to the customer, fine with reversed polarity/Broken neutrals etc etc but this one threw me.

ban-all-sheds · 21 Aug 2014

ScottishGasMan said:
British gas electrical cover in the house

What could go wrong.

Taylortwocities · 21 Aug 2014

JohnW2 · 21 Aug 2014

ScottishGasMan said:
Should have figured the lack of earth and the earth when connected to the boiler out myself. ... Just got thrown by the 60 odd volts between neural and earth when disconnected.

If the earth conductor to which you were measuring was 'floating' (i.e. not connected to earth) when the boiler was disconnected, then it's more-or-less pot luck as to what potential that floating conductor will have. Most commonly, it would be about half-way between L and N voltage, in which case measured L-E and N-E voltages would both be roughly half the L-N voltage (i.e. both about 115V with a 230V supply), but it doesn't have to be like that. What is a little surprising is that one would generally expect the measured L-E and N-E voltages to add up to roughly the L-N voltage, whereas yours only added to 151V.

Kind Regards, John

bernardgreen · 21 Aug 2014

JohnW2 said:
What is a little surprising is that one would generally expect the measured L-E and N-E voltages to add up to roughly the L-N voltage, whereas yours only added to 151V.

Suggests the meter has an impedance similar to the impedance of the capacitive coupling in the cable and hence the indicated voltage is reduced to about half the actual voltage.

JohnW2 · 21 Aug 2014

bernardgreen said:
JohnW2 said:

What is a little surprising is that one would generally expect the measured L-E and N-E voltages to add up to roughly the L-N voltage, whereas yours only added to 151V.

Click to expand...

Suggests the meter has an impedance similar to the impedance of the capacitive coupling in the cable and hence the indicated voltage is reduced to about half the actual voltage.

The arithmetic is obviously slightly more complicated than that but, yes, I presume that something like that must be the explanation. It may suggest that the length of floating conductor (i.e. the distance from 'break' to point of measurement) is quite short (hence capacitive reactance very high) since, IME, the impedance of modern digital meters (even cheap ones) is usually so high that it does not have much effect on indicated voltages when one is looking at the consequences of capacitive coupling in a decent length of T+E.

Kind Regards, John

securespark · 21 Aug 2014

ban-all-sheds said:
ScottishGasMan said:

British gas electrical cover in the house

Click to expand...

What could go wrong.

A bit of a general statement.

Spark123 · 21 Aug 2014

Yep, phone the AA instead

EddieM · 22 Aug 2014

Spark123 said:
Yep, phone the AA instead

AA ? Yes, I can fix it ..... hic!

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Strange voltage readings

EddieM

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