Earth Bonding

[Tricky Dicky]

Got the new CU up and wired up today just a few cables which are coming down the wall which need tidying up and the final connection of the garage feed once the garage end is sorted.

I have connected the main earth bond which was fed into the CU using the same gland that I fed the 25mm2 tails however I have a slight problem feeding the gas bond, main water and CH bond. The gas bond as already installed is 16mm2 but too short to be fed in the trunking that all the cables coming through the floor come up before entering the CU from behind. However with a suitable gland could readily enter the CU from beneath. The two water pipe bonds in 10mm2 could be re-routed through the trunking but it is getting a bit crowded with two 2.5mm2 T&E still to be added when the kitchen ring main is installed. The simplest solution to me without using fresh cable is simply to route them together into the CU through a single gland. The outside diam. of the 16mm2 is approx. 7mm whilst the 10mm2 is just under 6mm. I have seen a gland that suits 10 - 16mm2 T&E and based on the cable dimensions in the wiki this gland seems suitable for all three.

The gland I think is my favoured option. Alternatively could I feed a single 16mm2 cable out of the CU to an external earth terminal strip and connect the three bonds to it? I would be interested in any other ideas remember whatever is suggested it needs to get by the BCO.

Richard

[ARW]

20mm knock outs on the CU and 20mm stuffing glands?

eg

Or if you use a separate earth strip please tell the BCO it's called a "main earth terminal busbar" it sound like he would like that,

[Tricky Dicky]

BTW I took some resistance readings for the main ring main in the house and the readings I got was 1.8 Ohms for the CPC and 0.8 Ohms for both Live and Neutral. Are these OK? Hope to finally permanently wire in the garage this weekend and will take readings for the garage ring main.

I do not know if this is common across LAs but in ours the BCO inspect the work but the actual testing is contracted out to electricians.

Richard

[Andy Burns]

Assuming 2.5 T&E with 1.5 CPE, you'd expect 0.8*1.67 = 1.3 ohms for the CPC

Worth checking for some terminals in need of tightening? An outside chance of a section of older T&E with 1mm CPC?

[spuorgelgoog]

I don't think so, as that would mean that the Main Earth Terminal would be inside the CU and the gas and water bonds would be jointed at the external strip which isn't allowed.

If you connect the main earth bond to the terminal strip, then the gas/water and the CU to the strip, the strip becomes the MET and the gas/water bonds are unbroken and that would be compliant.

Owain

[Robin]

FWIW I took the OP to mean an external MET from which from which only the /one/ 16mm2 cable would go into the CU - which I thought was a (if not the) bog standard installation - c.f.

[Tricky Dicky]

I can confirm that some of the wiring goes back to 1975 and I have come across cables with a CPC that looks thinner than todays standard, so that might explain the discrepancy in the readings.

To do the installation I knocked out three of the six rear knockouts, two are now quite stuffed with cables the third has still some scope and includes a 6mm2 T&E for our existing shower which will become defunct once we do the bathroom and move over to a digital shower. I still have to get at least 3 × 2.5mm2 cables in and taking a good look at it I should be able to get those cables plus the earth bonds through it. I know the easy answer is to use another knockout but I am trying minimise how much of the CU integrity is compromised making another 40mm diam. hole just for 3 cables seems OTT. All the bonding on the old plastic CU was done on the internal terminal strip so will just be repeating what was already correct.

[ARW]

I would go for a loose connection unless most of the cable has a 1mm CPC. Based on 0.8 x 2.5 = 2.

[ARW]

Well I have done testing for Doncaster council in such cases (some time ago)

[Tricky Dicky]

As nobody has commented on the Live and Neutral readings can I assume they are OK? As far as the CPC reading is concerned at this moment in time how critical is it and would it fail inspection? I intend to investigate it but access to many sockets is obstructed with having to move furniture around during the improvements. There could be other problems too, namely underfloor junction boxes. I have put three in, two to extend the ring main and one to provide a spur. I know where these are and can easily access them but when the previous owners had the conservatory built the three sockets and two spurs were wired into the existing ring main and I have seen some connections made with junction boxes which are not very accessible and somehow left a whole bunch of cables under the floor which do not appear connected to anything and according to my stick voltage/current tester have no current?

Richard

[Andy Burns]

Well, (0.8 ohms)/(0.0075 ohms/metre) implies you're pretty well at the recommended max cable length (or have more loose terminals)

[ARW]

But that is copied and pasted from the OSG:-) 106m max for a 2.5mm T&E ring with a 32A B type MCB is given as 106m and the limit is for voltage drop.

0.8 ohms end to end for LL and NN would not bother me in most cases for a RCD protected ring circuit. Especially when the kitchen is on it's a separate ring and the oven and hob are on their own radials.

[Andy Burns]

It did make me suspicious that Tricky was setting a perfect homework question ...

[ARW]

A LN short can be used as the limit for a RCD protected circuit if the circuit is not using high loads.

You can get a good 200m out of that.