# Electrical test procedure on a gas boiler

#### Risteard

No need to remove the fuse if you've opened the double pole switch.

#### ekmdgrf

I have now noticed that you tried P&CH first.

Did he get told that he wasn't allowed to do it?

#### JohnW2

Earth and neutral= 17V
Would probably disappear with a low resistance meter.
UMM ... If the voltage between earth and neutral disappears with a low resistance meter than you have a problem because that would mean at least one of your neutral or earth is not properly connected.
Indeed, not to mention the fact that we've been told that the N-E resistance is only 0.7&#937;. One would hope that no voltage meter would have an input resistance lower than that (or, indeed, even remotely as low as that - the currents through the meter would be ridiculous, and frightening!)!

Kind Regards, John

M

#### martineire

I have now noticed that you tried P&CH first.

Did he get told that he wasn't allowed to do it?
its an electrical question no gas seals were broken nor was the seal to the combustion door broken. Not illegal

M

#### martineire

Earth and neutral= 17V
Would probably disappear with a low resistance meter.
UMM ... If the voltage between earth and neutral disappears with a low resistance meter than you have a problem because that would mean at least one of your neutral or earth is not properly connected.
Indeed, not to mention the fact that we've been told that the N-E resistance is only 0.7&#937;. One would hope that no voltage meter would have an input resistance lower than that (or, indeed, even remotely as low as that - the currents through the meter would be ridiculous, and frightening!)!

Kind Regards, John
Sorry I don't understand could you say that in layman terms

#### JohnW2

Sorry I don't understand could you say that in layman terms
In layman's terms ... I think EFLI's comment was probably incorrect, so you're probably best advised to ignore it (unless/until he clarifies).

Kind Regards, John

#### EFLImpudence

Is that not what is frequently referred to as 'ghost voltage' because of the high impedance ( >5M&#937; ) of 'normal' DMMs which disappears when using an analogue meter or DMM with LoZ ( 3k&#937; ) setting?

If not, bearing in mind the 0.7&#937; between N&E, how is it explained?

#### EFLImpudence

Did he get told that he wasn't allowed to do it?
No, does it matter? Often they do.

He was told to come here.

#### JohnW2

Is that not what is frequently referred to as 'ghost voltage' because of the high impedance ( >5M&#937; ) of 'normal' DMMs which disappears when using an analogue meter or DMM with LoZ ( 3k&#937; ) setting?
How can it be, if there really is only 0.7&#937; between the points being measured between? Your 'LoZ (3k) setting' meter would reduce the resistance between those points from 0.7&#937; to about 0.6998&#937; - do you really think that would make a difference? The phenomenon you're talking about is only seen when there is a very high impedance between the measurement points (usually with 'floating' conductors), in which case a LoZ meter will dramatically reduce that impedance, and thereby make the 'spurious' voltage reading go away.
If not, bearing in mind the 0.7&#937; between N&E, how is it explained?
Probably in terms of some sort of error! If there really were 17V between the two (N & E) conductors (as measured, say, with a high impedance meter), and there really was only 0.7&#937; between those conductors, then about 24A would be flowing (presumably permanently) through the N-E connection (wherever it was)! I assume that either the resistance measurement or the voltage measurement must be wrong - they simply aren't sensibly compatible.

Kind Regards, John

#### EFLImpudence

Perhaps it was only 1.7V.

Presumably not 0.17V as that would have been noticed.

#### JohnW2

Perhaps it was only 1.7V.
Perhaps - as I said, 'some sort of error'. However, if the N-E resistance really were 0.7&#937;, even 1.7V would imply a (presumably constantly) 2.4A flowing through the N-E connection, which seems not only very unlikely but also very wrong!

Indeed, if the circuit were protected by a 30mA RCD (I admit we haven't been told that it is!), and if the N-E resistance is 0.7&#937; at the point of measurement, then any measurable voltage greater than about 0.021V at that point ought to result in operation of the RCD.

Kind Regards, John

#### EFLImpudence

Ok.

My mistake - other than it may not be there with a more accurate meter.

#### JohnW2

Ok. My mistake - other than it may not be there with a more accurate meter.
Maybe - I think I'd be more inclined to suspect the way in which the measurement were undertaken, or the way the meter was read. It may even have been 17 mV - credible, and just about low enough to not trip an RCD!

Kind Regards, John

#### JohnW2

The phenomenon you're talking about is only seen when there is a very high impedance between the measurement points (usually with 'floating' conductors), in which case a LoZ meter will dramatically reduce that impedance, and thereby make the 'spurious' voltage reading go away.
To expand a little, in case anyone’s interested ....

... imagine that one has an essentially ‘floating’ conductor with an ‘incidental impedance’ (IR and capacitive coupling) about 10M&#937; to earth. Imagine also that the floating conductor has a capacitive coupling to a live (230V) source of impedance about 30M&#937;. The potential of that floating conductor relative to earth would be about 57.5V (230V * [10/(10+30)].

If you connect a voltmeter with a 5M&#937; input resistance between that floating conductor and earth, the impedance between that conductor and earth will reduce from 10M&#937; to 3.33M&#937;, so the potential of the conductor relative to earth will become about 23V - still seemingly quite high, but similar to the 17V reported in this thread.

However, if one then uses a voltmeter with an input resistance of 3k&#937;, the impedance between the conductor and earth falls to about 2.999k&#937;, and the measured voltage (potential from conductor to earth) falls to about 0.023V - i.e. almost zero.

[in case the pedantic police are around .... the above involves some simplification, since resistance and capacitive reactance, cannot be combined in that simplistic fashion, but it is close enough to demonstrate the principle!]

Kind Regards, John

#### AlanE

Theses test that I'm performing are standard tests that have to be preformed on a gas boiler before you service that boiler. The four tests to be completed are earth continuity, short circuit, resistance to earth and polarity. Three tests are done with the boiler isolated and obviously polarity is a live test. Theses tests are testing the boilers internal components and wiring looms within the boiler itself and not the source. Tests are being completed with a auto range multimeter.
Where did you discover the need to complete these four tests?

Earth continuity - yes; correct polarity - yes (flame failure often doesn't work with reverse polarity); Resistance to earth? Between where and earth?

Before I retired I used to repair gas, and oil, boilers for a living and I can assure you if you isolate the boiler from mains and measure the resistance of live to neutral/earth there will always be a reading unless PCB is isolated.

With a modernish boiler unless there is an on/off switch within the boiler there will always be a circuit between live and neutral through the electronic circuitry.

Now if you was referring to an old, and I mean old, boiler such as very basic boiler then true there should be virtually infinity resistance between live and neutral

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