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When protecting a TT installation with an rcd (no less than 100 mA)due to the Ze reading being very poor (and unable to reposition earth rods or install new ones) where would you connect your main earthing terminal to if there is no main incoming earth?The only way i can think of is to connect it to the neutral bar to enable all earthed and bonded circuit to be included into the inbalance operation of the rcd?But this does not sound like good practice to me!any ideas?

[This message has been edited by james S (edited 04-10-2005).]

James,

When you say the neutral bar, I take it you mean the busbar in the distribution panel. You definitely would not connect to the neutral bar in the panel.

The whole principle behind using the RCD is that a fault current will cause an imbalance between phase and neutral currents. If you made the connection you suggest, then any fault current on the protective circuit conductors would be returning through the neutral side of the RCD, and there would be no imbalance to trip it.

The clue to the connection arrangement is given by the second letter of the "TT" designation. It means that the earthing is to a local earth only (T = terre, French for earth). If you earth to the neutral, then it is no longer a TT system.

If you really could not obtain a sufficiently low earth impedance to use TT (unlikely with modern RCDs), then you could convert to TN-C-S, otherwise known as PME.

In this case, the earth bar in the panel would be earthed to the incoming supply neutral ahead of the meter. This would, of course, need appropriate provision from the power company and require all the extra bonding requirements for a PME system.

In the early days, it was precisely the problem of high earth impedance values in certain areas which led to PME being used. Remember that this was when TT protection was generally by voltage and 500mA current-balance ELCBs, not the 30 and 100mA RCDs of today.


Qualifier:
Everything stated here applies to the wiring in the U.K. only. The precise arrangement of earthing is something which varies considerably from country to country.


[This message has been edited by pauluk (edited 04-10-2005).]

Thank for your reply paul i see your point,of course the rcd would not function seems obvious now thinking about it!!!
I think the conversion from TT to PME is the way to go about it.

can you explain what is meant by your following statement

If you really could not obtain a sufficiently low earth impedance to use TT (unlikely with modern RCDs), then you could convert to TN-C-S, otherwise known as PME.

My understanding is that the impedance of the external earth path is required to meet certain values i.e TT 21 ohms reguardless of what type or protection is used.
If like in my case the external fault path can not meet the 21 ohms or below then the use of a rcd is employed.

thanks again for your reply;-)

There are some areas where local soil conditions would make it very difficult to get the impedance down to anything like 21 ohms (keep in mind that at 240V that would mean a current of over 11 amps).

The loop impedance at, say, 100 ohms, would be far too high to rely on an MCB or fuse (a dead short would result in only 2.4 amps).
That's why we would then need to use an RCD if TT earthing was to be employed.

There are some areas where you couldn't even get the 4' rod in the ground.

This is a volts on earth wiring risk, rather than a fire risk?


[This message has been edited by gideonr (edited 04-13-2005).]

James,
The electrode resistance (as distinct from Ze) is required to be such tah the current required to operate the RCD, say 100ma, will not cause a voltage drop along cpc and over electrode resistance greater than 50v or 25v depending on the circumstances. Greater voltage drops may occur but with corresponding quicker disconnection. Effectively, this means a max electrode resistance of 500ohms for a 100ma device in normal locations and 250 ohms for other special locations such as farms. IEE GN3 also recommends values over 200 ohms should not be regarded as stable, so I guess that you have to look at both recommendations since using a 500ma rcd would not comply with the first provision found in BS7671 2001