High earth impedance

[carrerasprak]

OK - discovered yesterday (when British Gas came to install my new boiler) that the earth is very poor, up near 100ohms.

So I then start thinking and remember that the water pipe is now plastic and the earth comes from the water pipe.

So, first of all can I confirm what type of earthing arrangement I have will be TT? There is ONLY live and neutral coming from the main incoming cable, there is no collar or connection to the armour or a core (TN-S) and there is no link anywhere from the neutral to provide an earth (TN-C) and as before it is therefore not TN-C-S

So I need to resolve this ASAP. What are my options and the network owner are not interested unless I pay in the region of £2000.

Earth rod?

I would also like to add at this point that the incoming cable is the old fibre and metal wrapped stuff, could I put a collar round it and use it as my earth?

I have access to a megger so I can test the earth once I have done this to make sure it is working.

 

[johnboy6083]

Hi, regarding the option of creating TN-S by using the armour, can you not test the ZE of this first? If you get a satisfactory reading (max 0.8 ohms), then you can use that. In theory that is. Im sure that you have to have the cable owners permission. Also if the cable is old, which it sounds like, then the armour is probably badly corroded.

 

[carrerasprak]

Yes, I could just test the armour first and see what reading I get; I will get the megger tonight and see what it says.

I appreciate that the network owner are the only ones that are allowed to touch their cables but in the interests of safety and not having lots of spare cash sitting about to get them to do it I think it is probably a risk I am willing to take as at least it won't be fatal if it gives the house a good earth.

If it is no good I guess earth rod time and some 10mm will sort it.

I read a minute ago a BS which implied that in TT systems anything less than 200ohms is within spec, is that true? If so then should British gas still carry out the works as the readings are within spec? I said 200 but more accurately is was more like 189Ohms.

 

[Spudnik]

You must not connect a clamp to the incomming lead sheath cable to obtain your earth.

If no earth is present you will need to install an earth as per TT.

When installed as long as the reading is under 200ohms (better closer to 100) then that is acceptable.

 

[Notsosmart]

Hi, regarding the option of creating TN-S by using the armour, can you not test the ZE of this first? If you get a satisfactory reading (max 0.8 ohms), then you can use that.

As Jason has said no no no don't do it...also make sure an RCD is protecting the installation.

 

[johnr]

hi all, so just out of interest...the supplier is not duty bound to supply an earth? have the properties either side got tns/pme.?.its like in the osg it states electricity company isolator but when you want one fitting for safe isolation to rewire or change the c/u then most of them charge for what clearly states is theirs....moan over

 

[Spudnik]

They dont need an isolator. They have the fuse.

However, they are fitting them generally on new supplies, but these are normally in the form of a 'self connect' meter.

 

[carrerasprak]

You must not connect a clamp to the incomming lead sheath cable to obtain your earth.

If no earth is present you will need to install an earth as per TT.

When installed as long as the reading is under 200ohms (better closer to 100) then that is acceptable.

I'll blast in the biggest earth spike I can buy - any tips on locations for spikes, some people mention near stack joints (soil pipe collector).

Thanks

 

[johnboy6083]

Out of interest, why can you not use the lead sheath, with permission of course? is it because you cannot garauntee the correct connection of the sheath to earth at the other end of the supply cable?

 

[johnboy6083]

earth rod. somewhere wet? i worked on an installation last year when the ZE of the installation varied between 0.08 (winter and very wet) to 0.56 (summer).

 

[Des 56]

You could use the old obsolete water pipe as your earth connection if it exists
It does not have to be a rod 542.2.1

 

[Deleted member 9648]

Out of interest, why can you not use the lead sheath, with permission of course? is it because you cannot garauntee the correct connection of the sheath to earth at the other end of the supply cable?

I'ts because the only clamp generally available is a standard bonding strap which would crush the soft lead sheath into the paper insulated cores within........you really wouldnt want to be in the same room

 

[johnboy6083]

thanks wirepuller. i did not know that, as its well before my time. Im of the black, brown and grey, not red yellow blue variety.

 

[sparky1502]

i am in college just working through earth impedence. Aside from a meter to tell you what the ZE is, is there a forumlar for working it out...thanks

 

[Spudnik]

You could use the old obsolete water pipe as your earth connection if it exists
It does not have to be a rod 542.2.1

I thought that only referred to sub mains on your own land and not main incommers?

 

[Deleted member 9648]

i am in college just working through earth impedence. Aside from a meter to tell you what the ZE is, is there a forumlar for working it out...thanks

No , but in the case of TNCS ans TNS systems you can make an enquiry to the DNO who should be able to tell you the expected Ze.

 

[Sintra]

You could use the old obsolete water pipe as your earth connection if it exists
It does not have to be a rod 542.2.1

Earth Electrodes

All of the following can be used as earth electrodes :

Earth rods or pipes

Earth tapes or wires

Earth plates

Underground structural metalwork embedded in foundations

Welded metal reinforcement of concrete embedded in the Earth (excluding pre stressed concrete)

Lead sheaths & metal cable coverings provided the following conditions are met :
a - the cable covering must be in effective contact with Earth
b - the permission of the cable owner must be obtained
c - the owner of the cable must be able to inform the owner of the installation of any changes to the cable which may affect it suitability as an earth electrode 542.2.1, 542.2.5,

A metal gas or water pipe must not be used as an earth electrode. 542.2.4

 

[Spudnik]

Can someone pass me my regs book.

Its at the end of my table and ill have to get up.

Ta.

 

[Spudnik]

c - the owner of the cable must be able to inform the owner of the installation of any changes to the cable which may affect it suitability as an earth electrode 542.2.1, 542.2.5,

Like that will happen!

 

[netblindpaul]

Why are BG insisting on earthing connections better than that required in the regs?

 

[Des 56]

The earth electrode cant be a service pipe

It can be a pipe in the ground, for the main earth, where the ground may be too hard, for instance, to sink a rod

So as long as the pipe is an obsolete service pipe and under your control, and its not going to be removed by the water company
I would say that it may provide an approved means of earthing for the TT install
Not that I would recomend this in normal circumstances mind you