Earth funnies and phantom voltages on lighting circuit

[Harry Bloomfield]

Furthermore, a good few of the lamp holders were brass (not 'insulated') and therefore would, per present-day thinking, need to be earthed.

and would be fitted with a screw, to earth the holder.

Indeed, but the slots in that 'timber trunking' were often multiple and quite wide. There are a few remnants still present in dusty and neglected corners of my present house and, as can be seen from photo, the slots are able to accommodate T+E ...

The predecessors to that!

 

[JohnW2]

and would be fitted with a screw, to earth the holder.

The screw might have been present, but there was often/usually no 'earth' in the wiring to connect to it!

The predecessors to that!

Fair enough!

 

[Harry Bloomfield]

The screw might have been present, but there was often/usually no 'earth' in the wiring to connect to it!

Which therefore meant they would not be permitted to be used, in the Regulations.

 

[JohnW2]

Which therefore meant they would not be permitted to be used, in the Regulations.

Maybe 'not permitted', but ....

@securespark , do you know what the regs said about metal lamp holders back then?

 

[securespark]

Sorry, back when?

You mean before the 14th Ed came in?

 

[JohnW2]

Sorry, back when?

When lighting circuits without CPCs were 'permitted'.

 

[securespark]

From Section 4 Earthing of the 13th Edition (1st September, 1955).

401.

Electrical apparatus operating at a voltage exceeding extra low voltage shall be protected against dangerous earth leakage currents by one of the means described in clauses (i), (ii), (iii) or (iv) below:

(iv) Isolation of metal in such a way that it is not liable to come into contact with live parts or with earthed metal (see Regulation 403).

403.

The application of Regulation 401 (iv) which exempts from the requirement of earthing metal which is so isolated that it is not liable to come into contact with live parts or with earthed metal, shall be restricted to the apparatus detailed in the following clauses (i) - (ix):

(vi) Lamp caps.
(ix) Lighting fittings using filament lamps installed in a room having a non-conducting floor mounted at such a height that they cannot readily be touched and are out of reach of earthed metal.

Edited because of my idiot "smart" phone.

 

[securespark]

Yet from Section 10 Earthing of the 11th Edition (June 1939), it states that lighting fittings must be earthed unless made "of non-conducting material."

So, the 11th ran from 39-50.
12th ran from 50-55.
13th from 55-66.

I can't say if the relaxation happened in the 12th or the 13th, as sadly I don't have a copy of the 12th Ed.

 

[JohnW2]

From Section 4 Earthing of the 13th Edition (1st September, 1955).

Thanks. That's interesting.

 

[Jurassicspark]

True, except that I don't understand the "especially old perished cables" bit - since I can't see how that would make 'induced voltages' any more likely (or greater)?

Get a LED bulb.

Fit a cheap LED bulb in a lighting circuit with rubber cabling or any other moth eaten cable with both of the wires in proximity (2 way circuit or an intermediate switch would be better) and see if the LED bulb remains on and glows faintly even with the switch off.

This has happened to me multiple of times.

With modern T&E, it does happen but not as frequent, well at least to me.

Well pragmatically, if you think about the equivalent circuit of the situation (disconnected earth?) you would have resistance, capacitance, inductance etc between the various conductors which can ultimately form a potential divider together with the impedance of the measuring instrument if measured, so you get a reading dependent on the ratio of those circuit elements. No volts are missing, greater or "more likely".

 

[securespark]

Interesting snippets regarding conductor colours in the 11th and 13th Eds.

In the 11th....the covering of an ECC in a flexible cable shall be of a brown colour easily distinguishable from red.
In the 13th, this changed to white with a green sleeve. And neutral in a 3 core flex was blue, but live was red.

In the 11th...three phase system of wiring....each conductor red, white and green respectively.
In the 13th, red, white and blue.

 

[flameport]

I've always assumed that the main reason was probably that there was, 'back then', no real perceived 'need' for earths on lighting circuits.

Pretty much.

Earthing was required to metal cases and metal frames of equipment, but not to anything which was all insulated.

That is from the 8th edition, 1924.
Lighting circuits were not a special case - anything else that didn't require earthing wouldn't have an earth conductor such as circuits for 2 pin sockets.
Most fixed equipment would have an earth as it was generally metal cased, but the requirement was that the equipment was earthed, not that the circuits contained a conductor for earthing.

Earth conductors to everything wasn't required until the 14th edition 1966, where it was specifically required for items such as lighting points to have an earth conductor regardless of what was connected.

 

[SUNRAY]

Interesting snippets regarding conductor colours in the 11th and 13th Eds.

In the 11th....the covering of an ECC in a flexible cable shall be of a brown colour easily distinguishable from red.
In the 13th, this changed to white with a green sleeve. And neutral in a 3 core flex was blue, but live was red.

In the 11th...three phase system of wiring....each conductor red, white and green respectively.
In the 13th, red, white and blue.

 

[Harry Bloomfield]

That's where I got the idea that red became brown, and black became blue from in 2001.

 

[JohnW2]

Get a LED bulb. .... Fit a cheap LED bulb in a lighting circuit with rubber cabling or any other moth eaten cable with both of the wires in proximity (2 way circuit or an intermediate switch would be better) and see if the LED bulb remains on and glows faintly even with the switch off. .... This has happened to me multiple of times. ... With modern T&E, it does happen but not as frequent, well at least to me.

As you say, these things happen, usually due to capacitive (but sometimes inductive) coupling between conductors. However, given that the conductor lengths remain the same (and, to a large extent, separation of the conductors remain the same) regardless of the nature and condition of the insulation of the conductors, I find it hard to understand why (by those mechanisms) the phenomenon you mention would be more common with "rubber or other moth-eaten" cable than with PVC T+E.

It is, of course, possible for deterioration of insulation in a "rubber or other moth-eaten cable" to result in a ('restive') leak between the conductors (as would be identified by IR testing), but that's a totally different ('fault') situation, and nothing to do with 'induced voltage/currents.