earth conductor single or multistrand?

[RobertL]

I have two related questions about the earth conductor in household wiring.

1) in my 1960s house, most of the ring main has multistrand earth conductor; it looks as if it has the same x-sectional area as the L and N. However, one part (added later) uses cable with only a single strand earth. the L and N seem to be the same area (2.5 sqmm).

If a house uses a mixtrure of earth sizes does this not mess up a ring conductivity test? The expected earth resistance cannot be calculated properly so how can it be checked?

2) The cooker to wall outlet uses a 6 sqmm cable. the L and N are multistrand but the earth is a single strand. The cable came from Homebase and was sold for the purpose. Surely in an appliance cable should have multistrand earth to avoid fractures due to repeated flexing. the cooker is regularly pulled out for cleaning etc.

Nobody seems to sell 6sqmm cable designed for flexing (currys comet homebase bnQ). is it really normal to use "single strand earth" cable for a cooker?

thanks for any comments anyone cares to make.

Robert

[John Rumm]

You can do simple checks for continuity, and also check that it is not unexpectedly high, but that is about the limit for simple end to end tests on the ring.

For serious testing, an earth loop tester is required. This plugs into a socket position and tests the complete loop impedance including all the wiring involved upto to that point. It will also let you assess the maximum prospective fault current at each socket. Once you have results from those tests, you can calculate the minimum size of earth wire required to cope with the prospective fault currents - and check that you do in fact have that in place for the circuit (otherwise you run the risk of having the circuit fuse being protected by the wiring and not the other way round!)

It sounds like you have flat twin and earth cable rather than flex. If this is the case then even the stranded bit will only have relatively few strands, and it is not intended to be as flexible as a "flex" - but just easier to route and bend than if it had solid conductors of that size.

Have a look at the sizes chart here:

if that matches your cable.

It can be difficult to find large CSA flex, and hence it is quite "normal" (although not ideal) to find cookers connected with T&E. Unless you are pulling it in and out weekly then it is unlikely to be a problem.

[Owain]

Provided you always use cables with a greater than minimum csa, and calculate as though using the minimum, the end result will be compliant (albeit over-engineered)

Yes, it isn't pulled out often enough for it to be an issue, at least domestically. Commercial appliances that are pulled out at the end of every shift for cleaning behind would be different.

Owain

[Lurch]

On Thu, 21 Jun 2007 01:31:00 -0700, RobertL mused:

The resistance is the restistance, no matter what the cable is.

It's either good enough or it isn't, whether it's multistrand or not.

Twin and earth is designed to be used as fixed wiring cable, it is not designed for flexible final connections technically. You want 6mm 3 core flex.

No, the normal method is to use 6mm twin and earth. You won't find 6mm

3 core flex in any DIY stores.

How many times is it removed this cooker? Unless you're removing daily I can't see you're going to hit a problem.

[RobertL]

thank you for your comments gents. the cooker will come out once a month I guess so maybe the existing cable is OK.

Also thanks for comments on earth resistence. I will get the rings checked when I have the CU changed for one with RCDs. that will eb a pro job not DIY of course.

Robert

[Dave Plowman (News)]

It will likely be 7/0.29" strands for the line and neutral, and 3/0.29" strands for the earth.

Yup. That's the modern metric equivalent.

The ECC size did also change with 2.5 TW&E some years ago. But only really made a difference on very long runs unlikely to be found in most houses. The correct way to check is with an Earth Loop tester - this basically puts a brief pulsed short to earth and measures the current drawn. This measures the earth as installed and checks also for poor connections.

If it is a reasonable loop it will stand many removals of the cooker for cleaning as this is the common way of attaching a cooker - although you can buy flex from a specialist.

[Alfie]

Yes Thats normal these days

-- Alfie