Main supply earthing point question

Hi all,

Thanks to all those who've helped me out so far this week - really appreciated.

Now onto another question regarding my supply and earthing this time.

First, a bit of history. The house was built in 1959 and has, as far as I can tell, a TN-S supply arrangement.

In addition to the original CU that serves the main house circuits a small, dedicated CU has been added for an immersion heater, probably put in during the 60s. This second CU has a flat strap - not cable - which is connected to the sheath of the icoming supply in order to provide an earth.

Now, the main CU for the other circuits in the house is *NOT* connected to this earthing strap or the cable sheath in any visible way. It is possible that the CU is connected to the incoming water pipe which I know travels underneath the area on its way to the kitchen.

Now for my questions. Is this strap likely to be sufficient as a connection to the cable sheath? With it being flat it's difficult to judge its equivalent cable area which I know should be at least 16mm2. If sufficient, can I connect the original CU and the new CU that I'm installing to this strap? Should I be contacting the DNO to try and get them to come and sort out a decent earthing point?

Thanks in advance for any advice.

Mike

Mike,

It sounds like your original CU is not a TNS arrangement but more like a TT system. I would definitely contact your DNO and get them to provide you with a decent earthing point (which is their obligation). Also sounds like you don't have an RCD in between the meter and CU which is now a regulation but unlikely to be at the time your house was built

I personally would get a decent earthing point (whatever system it may be) and then get all earths going to that point. Its usually less of a ballache should you need to add further to the system in the future.

Also if possible swap both CU's for one large one and then earth to one point. May make things neater but I don't really know all the details of your situation.

Hope this helps

A photo paints a thousand words!

Thanks to both of you for your responses.

DyslexicGrimsbySparkie - When I said TN-S I suppose I was refering to what I thought it should be given what was available - ie the supply plus an obvious bond to the supply sheath for the immersion heater. I'll get the floorboards up tomorrow and check whether there is actually an earth bond to the water pipe or not - that should clear things up.

Situation is that I'm refitting the kitchen and doing the wiring for a new ring-circuit myself together with sorting out the currently poor garage circuit. Have bought new CU with RCD for this purpose and intend to move existing household circuits over to this at a later point. Because of Part P the council are involved with inspection of my new wiring so everything is getting a bit complicated now!

Lectrician - I'll try and post a photo tomorrow, but I think I may well be on the phone in the morning to the DNO to try and pursuade them to come and sort out the earthing point.

Thanks again,

Mike

Had a couple of issues regarding earthing recently

This is gospel (i have been told from several sources) so it's probably wrong !!!


1. Elec board do not have to supply an earth. Only to maintain an existing one if practicable.

2. They charge like a raging bull if you ask them to provide one a customer was quoted > £700 to do so.


My experience is with Scottish Power.

Just a quick update.

DyslexicGrimsbySparkie - you're quite right. The original arrangement of the supppy is TT with the earth originally provided by bonding to the water pipe. I've now found this bond next to the stop-cock in the kitchen.

I know that an earth provided by water pipe is no longer acceptable due to possible plastic further up the supply. As the supply was not originally provided with an earth point by the supplier my understanding from the regs is that I have to provide the earth? Should I be sinking an earth rod and connecting the exising CU and my new CU to this?

I also understand that an RCD on the main supply will be required. Now I'm back to my previous posts about connecting new equipment to the sealed meter!

Mike

i wouldn't advise diying an earth rod setup unless you really know what you are doing and have the proper test equipment.

the trouble with rods is from time to time you hammer them into ground that is basically insulating. therefore it is VITAL to properly test an earth rod after hammering it in.

talk to the supplier about the posisbility of them providing a TN-S or PME earth. if they can't do it or quote an insane price then get an electrician to sort out a proper TT installation with rod and twin rcd split.