What should the voltage be across the mains switch when off?

Xerxel · 3 Jan 2020

Hi

In my (new) consumer unit, there are 3V across the Neutral bar and the earth bar, when the mains is off. There is also 53V across the mains switch from L to N when it's off and 44V between the line supply and line out. Is this normal when the mains switch is off? Isn't N and Earth connected together at some point therefore should be 0 volts difference?

Could this explain why my LED bulbs never switch off but only go dim when switched off via a wall switch?

See pics for actual readings - does anyone know what the readings should be in a good installation?

Thanks in advance...

235

Xerxel
3 Jan 2020

53

Xerxel
3 Jan 2020

44

Xerxel
3 Jan 2020

3

Xerxel
3 Jan 2020

MainsSwitch

Xerxel
3 Jan 2020

ConsumerUnit

Xerxel
3 Jan 2020

flameport · 3 Jan 2020

Xerxel said:
Isn't N and Earth connected together at some point

Yes, but on the supply side, not within the installation.

Attempting to measure voltage using a high impedance digital meter between L and long lengths of disconnected wiring will give entirely useless results.

Xerxel said:
Could this explain why my LED bulbs never switch off but only go dim when switched off via a wall switch?

That effect is related to the spurious voltages, and is easily fixed by purchasing good quality LED lamps and not the cheapest tat from ebay.

John D v2.0 · 3 Jan 2020

Kind of, but not exactly. Those are just stray voltages picked up from wherever, other wiring etc. As soon as you connect something load wise the voltage will go away. Even a small filament bulb would do it.
Likewise an led only needs the tiniest current to light up perceptibly, so if there are live wires close to switched live the pick up could light them up enough to see.
One student house i lived in 20 years ago had that problem, those days it was energy saving fluorescent compact bulbs and they used to flash every few seconds when off. Very scary in the middle of the night on your own!

AndyPRK · 3 Jan 2020

What does it measure across bottom terminals

John D v2.0 · 3 Jan 2020

AndyPRK said:
What does it measure across bottom terminals

Should be more stray voltages, those are two isolated terminals connected together by any loads that happen to be present

333rocky333 · 4 Jan 2020

Not really the right probes to be prodding round in the consumer unit really.

Xerxel · 4 Jan 2020

333rocky333 said:
Not really the right probes to be prodding round in the consumer unit really.

ok - I'll change them. Which would be better ones?

Xerxel · 4 Jan 2020

flameport said:
Yes, but on the supply side, not within the installation.

Attempting to measure voltage using a high impedance digital meter between L and long lengths of disconnected wiring will give entirely useless results.

OK - I won't do that any more. I seem to remember in a youTube vid it mentioned using low impedance to let a tiny current trickle through - I guess that would be enough load to get a better reading? I don't have a LoZ switch on my multimeter however...

flameport said:
That effect is related to the spurious voltages, and is easily fixed by purchasing good quality LED lamps and not the cheapest tat from ebay.

Hmmm - they are quality LEDs. I suspect someone who took all the lights down for replastering put them back up again connected to any old wire poking out of the ceiling. For example, on one lamp, when the switch is on the lamp is off and when the switch is off the lamp is very dim. On the other lamp, switch is on and the lamp is on but switch is off and the lamp is dim. This is when everything is powered up. Is driving my wife nuts and she is blaming me for not having magically fixed it yet. I suppose the only way to fix that is to power down the whole house, disconnect every ceiling light on that circuit and every wire poking out of the ceiling and test them for continuity back to each light switch - is that correct?

333rocky333 · 4 Jan 2020

Xerxel said:
ok - I'll change them. Which would be better ones?

GS38 compliant, they only have up to 4mm of metal showing, when you bought the tester was there some slide on shrouds to go on the ends, as thats all you need.

DetlefSchmitz · 4 Jan 2020

Xerxel said:
I suppose the only way to fix that is to power down the whole house, disconnect every ceiling light on that circuit and every wire poking out of the ceiling and test them for continuity back to each light switch - is that correct?

Not really. LEDS light up through stray capacitance when they are supposed to be off, which is quite normal. It is possible to buy what used to be called 'snubbers' to connect across the lamps to reduce the stray voltages. A couple of people on this forum have a part number for this ready to hand and may come along and advise.

Xerxel · 4 Jan 2020

333rocky333 said:
GS38 compliant, they only have up to 4mm of metal showing, when you bought the tester was there some slide on shrouds to go on the ends, as thats all you need.

OK thanks - something like this? https://www.amazon.co.uk/Neoteck-Ac...s-Electricians/dp/B07BDGWNRM/ref=sr_1_14?th=1

The original came with covers but not shrouds.

SUNRAY · 4 Jan 2020

flameport said:
That effect is related to the spurious voltages, and is easily fixed by purchasing good quality LED lamps and not the cheapest tat from ebay.

Yeah right.

If it's that easy, why are there devices available to stop it happening?

John D v2.0 · 4 Jan 2020

Xerxel said:
OK thanks - something like this? https://www.amazon.co.uk/Neoteck-Ac...s-Electricians/dp/B07BDGWNRM/ref=sr_1_14?th=1

The original came with covers but not shrouds.

Yes this is the important bit:
Test Leads: CAT III 1000V, 20A (Max)
Test Probes: CAT III 1000V, 20A(Max)

bernardgreen · 5 Jan 2020

Capacitor 0.047uF micro Farads 250 volts AC in series with Resistor 100 ohms

Which come pre-packaged as a contact suppressor from RS Components.

As a contact suppressor it would be fitted between Live and Switched Live at the switch BUT as a snubber it has to be fitted between Switched Live and Neutral at the lamp..

( If there is a Neutral at the switch it may be more convenient ( space wise ) to fit it in the back box of the switch )

RS Stock No. 206-7847
Manufacturer Evox-Rifa
Manufacturers Part No. PMR209MB5470M100

There are other sources

SimonH2 · 5 Jan 2020

When you put your meter across the open contacts of the L side of the main switch, your meter forms part of a potential divider between the 240V mains and "earth". the circuit basically goes "L - meter - live cores in the installation plus N cores via any loads that are turned on - various capacitances between L&N cores and earth cores (correctly, Circuit Protective Conductors (CPCs)) & any earthed metalwork (eg pipework) - earth. As these capacitances are small, the voltage you see will be significantly lower than teh 240V mains supply.
When you measure across the N side of the switch, you are again measuring a complex system - but there should only be capacitive coupling between the L meter tail and any cabling that runs close to it (should be negligible).
And as above, when you measure between your N & E bars, the N is disconnected from the supply neutral, and hence from the upstream point (typically in the service head) where the supplier's PEN (Protective Earth & Neutral) conductor is split to separate E & N conductors.
TL;DR: As already said, the readings you have got are meaningless and of no relevance to your other problems.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

What should the voltage be across the mains switch when off?

Similar threads