Guten Tag Wolfgang,

Welcome to the forum.

In Britain we have TN-S, TN-C-S, and TT in use, which complicates earthing arrangements considerably.

TN-S was installed in the old urban areas, most of the distribution being by armored underground cables with the armor used as the separate earthing path back to the sub-station.

TT was the norm in rural areas for a long time, and necessitated the use of various earth-leakage devices just as for the TT systems common in France.

In residential applications, the voltage-operated ELCB (Earth-Leakage Circuit Breaker) was the norm for a long time, and many are still in use although now obsolete. These were superseded by the current-operated ELCB, or RCD to use the present-day terminology, which has gradually increased in sensitivity over the years.

As you say, with the high loop impedance which is possible on a TT system, the use of such a device is the only way to ensure that the power is disconnected at all, nevermind that it happens within the 0.4 second specified by the current regs.

TN-C-S was called PME here -- Protective Multiple Earthing. Its use goes back to at least the 1930s, but originally it was only used in specific rural areas in which local conditions made it very difficult to get a good ground connection. In fact the installation of a PME distribution system for an area back in the early days had to be by the explicit approval of the Secretary of State.

Gradually, TN-C-S has become more widely accepted and used, especially over the last 25 years or so. Extra earth connections have been installed on the neutrals of distribution lines, and it's now possible for any installation to be connected as TN-C-S.

Even though PME earthing is available now to all, of course there are still many properties using their own local earth and wired as TT. In my immediate neighborhood (rural Norfolk) I would estimate that at least 90% of domestic systems are still TT.

Where PME/TN-C-S is used here, the bond is at the service block just before the meter. From that point onward, neutral and earth are kept strictly separate.

There are some diagrams showing the various British arrangements here:
https://www.electrical-contractor.net/ubb/Forum15/HTML/000044.html


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Voltage of the bodies of faulty devices against ground shall be limited to 50VAC by harmonized norms. 50 Volts/100 Ohms equal 0.5 Ampere and zhis is exactly what you find on the EDF-RCD in the "branchement", also in yours, as you told us.

The 50-volt figure was also the limit set for the voltage-operated ELCB here. In practice, most of them would actually trip at a much lower voltage.


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So impedancies (is that English, = sum of all inductive, ohm and capacitive resistors in a loop)

Yes, in English:

R = resistance
X = reactance (capacitive or inductive)
Z = impedance, i.e. SQRT (R^2 + X^2)

Plural: impedances [Linked Image]

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Feel free to correct my technical terms in English, as this is not simple in German already.

Your English is far better than our German I'm sure (Texas_Ranger excepted [Linked Image] ).

Those long technical terms in German are scary! [Linked Image] [Linked Image]



[This message has been edited by pauluk (edited 10-01-2005).]