Voltage through earth to desk

[Fergus Macdonald]

I built a metal desk with wood top. I just wired in a 500W heat lamp on the desk.

13A plug goes to a single metalclad socket via 1.5mm2 3 core cable.
Same cable then goes from socket to 1 way metalclad switch.
Same cable then goes from switch to heat lamp.

Earths are all connected to each other and to back boxes.

When I take my multimeter and measure between the socket screw and the metal desk I get about 10V. When I turn the heat lamp on it reads about 25V.

Is that normal? Is that something to be worried about? Metal desk is not earthed.

Thanks in advance.

 

[Midwest]

Can't quite follow how you have you heat lamp working. Are you saying its on a trailing lead plugged into a socket outlet?
What is you multi meter? Even my MFT will display ghost voltages. A metal would not normally require bonding, in what contexts is it being used?

 

[Barry White]

Is this in an outbuilding or shed ? What made you measure between the metal desk leg and the fixing screw?

 

[Fergus Macdonald]

Hey.

They're all essentially in a line. In the back of the single socket both the incoming and outgoing cable are wired together into the screws.

The switch therefore only controls the heat lamp.

The end of the cable has a standard plug which goes into a wall socket.

Got a cheap Proster Multimeter from Amazon. Fairly consistent readings though - approx 9.8V every time and approx 25V every time. I checked the screw because I recently had a fault at my house where the screws were live. It's not an outbuilding or a shed, it's in an office.

 

[hightower]

Sounds to me like induced voltage which is creating a potential between your circuit earth and the desk (which I'm guessing has contact with the ground somehow. I'd wait for someone else to come along and confirm this though.

 

[Midwest]

Have you an analogue meter to test that 'voltage' with? What supply do you have? Why do have a single socket and then switch between plug top & lamp?

 

[Fergus Macdonald]

Don't have an analogue meter unfortunately.

Added a socket because thought it would be handy but if this isn't the best way to wire it then will remove it.

 

[Lucien Nunes]

If the desk metalwork is not connected to the earthed parts of the electrical fittings (which I presume are on the wood) or any other electrically connected objects, then it is floating i.e. not connected to anything. Voltage readings to floating items are meaningless as there is no circuit to measure. It's like asking the question: "What is the distance between point A?" All it tells you is that by the mere fact of a voltage being there, the desk is not earthed. But if it is supposed to be earthed, then you should be checking that with a continuity test, not voltage.

 

[Fergus Macdonald]

Good point. The desk is floating. However I don't get the same reading when I touch other metal floating things. Think it's safe to ignore?

Should the desk be earthed?

 

[Midwest]

Should the desk be earthed?

To determine that, you would need to assess whether the metal desk is an 'exposed conductive part or extraneous conductive part'. In the former as it's not metal as forming part of a conductive part, i.e. the casing of a washing machine and in the later, unlikely to introduce a potential, the likely answer is no.

If you think of it like a metal sink, metal radiator, or metal window, non of which require bonding. Unless somehow you metal desk forms part of the installation of your lamp heater, switches etc?

 

[Fergus Macdonald]

Ok thanks. Desk is not part of the install in any way.

Sounds like it's safe to ignore the reading.

Is the socket safe wired like that? It's essentially like an extension with a spur for the lamp.

 

[Pat H]

So you have made a sort of extension lead from a wall mounted 13A socket via a 13A plug and lead to a new socket and then a switch.
How are the new socket and switches mounted? into the desk wood?
As others have said you are measuring a known earth to a floating metal frame. The voltage is likely to be induced by nearby cables and has nowhere to go so just hangs around waiting for you to measure. The current is more important and I suspect the current would be unmeasurable. Touching the metal desk frame and earth has to be safe and won't be if the frame had a high enough voltage and current (that would have to be getting to the frame somehow)

 

[Fergus Macdonald]

Yes exactly. The back boxes are mounted onto the wood, not the metal.

Sorry, should've said. Did also try to get a current reading but couldn't get anything as you suggested.

 

[The Ghost]

If there is voltage around your structure and you are thinking of doing anything unusual on the desk it might add a bit of of frisson to the moment.

 

[Pat H]

One simple option would be to run an earth from your socket extension built into the desk to the metal frame.