Understanding voltage readings on dimmer and earthing

[John Z]

Hello there,

I am rewiring a metal dimmer switch after redecorating; before decorating there was no earth cable between
the cable from the wall and the earth terminal on the dimmer. The lighting circuit is protected by a RCD but as I understand it a metal dimmer switch is class 1 so should be connected to an earth cable.

So, after decorating I have connected an earth cable between the wall cable and the earth terminal on the dimmer, however when I use a multimeter to measure voltage I get the following readings:

Dimmer Switched On:

• Metal Body of dimmer / neutral wire = 74 volts
• Metal Body of dimmer / live wire = 74 volts
• neutral wire / live wire = 0 volts

Dimmer Switched Off:

• Metal Body of dimmer / neutral wire = 74 volts
• Metal Body of dimmer / live wire = 74 volts
• neutral wire / live wire = 240 volts

Can anyone give me the layman's explanation of what's going on here ? I would have though there would be 240v when I put the
multimeter terminals between neutral and live whether the switch was on or not; and I thought only a few volts would be present between the metal dimmer body and neutral is 74 volts safe/normal ?

TIA.

John.

 

[EFLImpudence]

Measure between Neutral and Earth using the meter on low ohms (Ω). It should be near zero.

It seems like the earth wire may not actually be connected to an actual earth.


With the switch ON full L and N are connected, therefore no potential difference between them.
In effect you are measuring between two points on the same wire.

 

[John Z]

thanks for the reply @EFLImpudence

Sorry for the stupid question but I tried all ohms settings on the multimeter between 200 up to 200M and there was never a reading between neutral and earth terminal.
However, setting the multimeter to 20V A/C I got a measurement of 0.385 V between Neutral and Earth (when the switch is off).

I still don't understand the voltages between live and earth and neutral and earth which are now showing 126V and 102V respectively (multimeter on 700V AC setting) - what am I actually measuring here ?

 

[flameport]

The dimmer does not have a neutral connection. The two wires are live and switched live. They are connected together when the light is on, and not connected when the light is off.

The earth isn't connected to anything, what you are measuring is a tiny leakage current caused by capacitance between the earth wire and the adjacent live wires.

You cannot use the metal dimmer switch without a proper earth connection.

 

[EFLImpudence]

The voltages between wires should be:
L - N ~240V
L - E ~240V
N - E ~0V

and the resistance with power OFF:
L - E open circuit
L - N a fews ohms through other appliances
N - E ~0Ω or 1 or 2.

Power OFF
You really need to measure between the earth wire at the switch and the earth at the consumer unit to determine the are actually joined.

If you don't have a long enough wire, you could try between earths at the switch and a nearby socket.

 

[EFLImpudence]

Don't forget to deduct the resistance of the leads and any wire you are using.

ALL power OFF
You could use an extension lead plugged into the socket and measure from earth on extension socket to switch.

 

[John Z]

Thanks @flameport and @EFLImpudence for your explanations.

@EFLImpudence the consumer unit is only a few metres from the switch so I will try your suggestion of measuring earth @dimmer to earth @Consumer unit.

John.

 

[John Z]

You cannot use the metal dimmer switch without a proper earth connection.

Sorry @flameport one more question, by "cannot" - you mean its absolutely unsafe to do so ?

Am I right to say that the purpose of the earth wire is, should the live wire become detached from its terminal and make contact with the metal dimmer switch's body, the current would flow through the earth wire to ground rather than make the dimmer switch's body an electric shock hazard? But wouldn't the RCD protection on the consumer unit trip if that were to happen ?

 

[EFLImpudence]

The dimmer does not have a neutral connection.

Yes, of course that is correct.
I usually tell off people who say that. I must have been thinking of something else.

However, if you get 240V L - SL when the switch is OFF it is connected to a Neutral through the lamp. Is it a filament lamp?
This will of course affect the resistance reading SL - E which could now be several ohms 50 or more.

 

[JohnW2]

Am I right to say that the purpose of the earth wire is, should the live wire become detached from its terminal and make contact with the metal dimmer switch's body, the current would flow through the earth wire to ground rather than make the dimmer switch's body an electric shock hazard?

Indeed (or any other fault which resulted in the dimmer's body becoming 'live')....

But wouldn't the RCD protection on the consumer unit trip if that were to happen ?

Quite so (or even the MCB in the CU) - BUT only if an earth wire is connected to the dimmer body, hence why the earth connection is essential (and the dimmer would be unsafe without it).

Kind Regards, John

 

[John Z]

OK, thanks to all who have replied.

So I created an extension cord, turned off the power on the ring main and the lighting circuit and plugged the extension cord into the wall outlet.

I connected the earth (or at least the green/yellow) wire of the extension cord to the wire protruding out of the wall which I'm testing whether its connected to the earth of the CU. The reading was infinity Ohms so presumably its not connected and useless as an earth.

So bottom line is I should change the metal dimmer switch to a plastic one yes?
The backbox is metal can I leave that in place or does that need to be replaced with a plastic one too?

TIA.

 

[EFLImpudence]

Yes, you should only use plastic switches.

Assuming the back box is sunk into the wall, it doesn't matter as it is covered by the switch and is (should be) only accessible when the power is off.
Some will say you should used nylon screws for the faceplate as they might be live in a fault - but they should not be unscrewed with the power on and are rather difficult to grab hold of.


You should find out why the earth wire is not connected to earth.

Perhaps the circuit is old and not earthed - with just a newer length of cable used for the switch, or
it could have become detached at the light fitting.

 

[333rocky333]

OK, thanks to all who have replied.

So I created an extension cord, turned off the power on the ring main and the lighting circuit and plugged the extension cord into the wall outlet.

I connected the earth (or at least the green/yellow) wire of the extension cord to the wire protruding out of the wall which I'm testing whether its connected to the earth of the CU. The reading was infinity Ohms so presumably its not connected and useless as an earth.

So bottom line is I should change the metal dimmer switch to a plastic one yes?
The backbox is metal can I leave that in place or does that need to be replaced with a plastic one too?

TIA.

You could find the other end of the earthwire and maybe reattach it, likely at the lighting point
EDIT
As EFL said