When did this earthing last meet standards?

[ben]

Looking round a house (with a kitchen refit done in 2013 and a boiler in 2010...) and came across some bare wire earths

[spuorgelgoog]

Very new service head and meter on a delightful old board ...

I think bare earths are pre 1966. Green earths 1966-77 (although not always sleeved inside back boxes etc).

Owain

[stephenten]

The name plate on top of the wood board is readable as Cambridge electricity supply company which according to

dates it between 1892 and 1948.

So I would date the bare earth wires to between the above years.

I personally would update the earth bonding and also I would check for lead water pipes and also for steel gas pipes which are prone to corrosion.

The consumer unit is of the rewireable cart type which is of a later date but clearly does not have MCB or RCD or RCBO devices.

I would also consider replacing the consumer unit.

The Henley block also wants checking as some of the twin and earth cables may have been wired into the Henley block rather than via the consumer unit.

[ARW]

It certainly looks that way.

[John Rumm]

We need a slightly wider angle shot to get a more complete picture. I can't quite see where that new green/yellow cable comes into the earth terminal from - it could be the main TN-S connection to the incoming supply sheath.

Other than the incomer and new meter, the rest of it is fairly old. The equipotential bonding looks poor (although on the bright side there probably is some). The main earth connection to the CU looks fairly poor, although its probably adequate to meet expected disconnection times on the main circuits (even if not for a fault in the CU itself).

I am not convinced about the T&E into the Henley - I can only see tails of various sizes. The T&E you can see just looks like that pair of them above the board clipped to the wall continuing down.

I can't see where those slimmer tails from the henley go either.

[tabbypurr]

It doesn't meet the regs now or since the 70s. Not sure why it's a big issue, the setup could do with some work is usually what counts when buying. You've got pvc cable, that's the main thing.

NT

[spuorgelgoog]

I once lived in a house that still had lead cable and ironclad DP fuseboxes.

Lovely stuff.

The phone line was twin bare wires to insulators and braided insulated cables internally to a bakelite bell.

Owain

[Brian Gaff]

Maybe the slaving has perished or just fallen off? I have a lot of bare wire earths in this house and nobody has died. Brian

[John Rumm]

Its not the fact that its bare as such that is the problem directly - although it does hint at the age of the installation, and hence other things you can expect to find. Its likely that much of the earthing will be done using undersized wires. Its also more likely for earths to be disconnected in places they are needed, and quite probable the lighting circuits are completely unearthed.

[John Rumm]

And should also add, the lack of any RCD protection will make things like equipotential bonding (both main and supplementary) for more important as a method of reducing shock injury risk.

[Scott]

What is the terminal block made of? Obviously before PVC. Was it porcelain or that stuff with a rough feel that looked like a stone? It doesn't look white enough to be porcelain.

[ARW]

I don't think that "slimmer" is the correct word.

If the meter tails are 25mm and double insulated (although I suspect that they are 16mm) a simple comparison of od diameter from the photo says that the "slimmer" cables are "seriously under sized".

[Scott]

Does it matter if earthing cables are undersized? They will only be used very occasionally and for short periods of time.

[ARW]

Yes it does matter. However I was talking about tail sizes (no sniggers from the back please Mr Pounder).

I'll see if I can get the photos of this weeks burnt out cables on the wiki later.

[ARW]

How do you know he has PVC cables? All you can see is that there are two PVC cables. The rest could be any old shit.

[Scott]

I realise that but the original question was whether the earthing was up to standard. My understanding was that the earth connection exists to create a short circuit and blow the fuse, which I assumed would happen faster than any overheating of the earth cable.

Does this apply to earthing cables or tails? I can understand that if the supply cable is undersized it would overheat. This may become more of an issue if we all start heating our homes with electricity and charging our cars from the domestic supply.

[Steve Walker]

But it could fail to blow the fuse in the required time if the earth is undersized, as the resistance may be too high and therefore the fault current too low.

Chances are that it won't be a problem, but it could.

Overheating is not the problem for earth cables. See above.

SteveW

[John Rumm]

You mean the exposed main earth terminal?

Usually plated brass:

[John Rumm]

Depends on what they are feeding - quite often you see very slim tails feeding things like Economy 7 time switches.

[John Rumm]

For earthing this is correct.

Although in this photo you also have protective conductors that are likely to be equipotential bonding wires rather than earth wires.

These are intended to reduce shock risk by electrically bonding together any bits of extraneous metal in the house such that under fault conditions you are unable to be exposed to conductive items at substantially different potentials. For this to work, they need to have a low enough resistance to limit any potential difference to <= 50V

There is a check you can do to check the survivability of the earth - under fault conditions (where you may have 1000's of amps flowing) they need to last long enough to open the protective device.

The equation used is s = sqrt( I^2 . t ) / k

Where s is the minimal cross sectional area of wire, I the fault current, t the time to clear the fault, and k a factor that models the cable type and its insulation under adiabatic (i.e. with no heat loss to the surroundings) heating.

So say you had a 1000A fault current on a ring protected by a B32 MCB. That would be adequate to operate it on the "instant" part of its trip, and clear the fault in <= 0.1 secs.

So s = sqrt( 1000^2 . 0.1 ) / 115 = ~2.75 mm^2 conductor area.

Now in your photo, some of those earth wires look like they could be galvanised steel rather than copper. Hence you have to use the "copper equivalent" conversion factor. To work out if the earth is adequate, multiple by the conversion factor (which is 2.255), and you get a minimum size of wire of 6.1 mm^2 - probably larger than some of those.

Both

Indeed.