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Re: UK cooker/RCD problem #143869 09/28/05 07:06 PM
Joined: May 2004
Posts: 186
A
aland Offline
Member
Hi Wolfgang,
Welcome to the mad house.
You mention Belgium and France as having predominantly TT systems, what type do you have in the VERY WEST OF GERMANY? And where is the very West of Germany? Do you have Rcd's fitted as standard by the PoCo?
This thing about it being normal to have a current passing to earth still seems alien to me. Yes,if you have all the equipment to monitor it and logicaly deduce that it's O.K. fine. But so many people buy second and third hand equipment nowerdays nad it gets connected. For Instance the other day I did an inspection on a property; testing revealed that the cooker circuit did not have an earth connection to it,15 years it had been like that. The cooker had been changed once,re-conected by a well known supply house,it turned out to be an earth connection broken off for 15 years. If the cooker had developed a fault, who knows!!! If it trips out it needs proper investigation and thats the end of it as far as I am concerned. regards aland

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Re: UK cooker/RCD problem #143870 09/29/05 08:02 AM
Joined: Dec 2004
Posts: 354
K
kiwi Offline
Member
(France, Belgium for instance) earth leakage even in case of a full short to earth is that weak that a fuse will never trip.

Say it aint so Wolfgang. If I lived in France or Belgium where a full short to earth doesn't blow the fuse, I would emigrate

Re: UK cooker/RCD problem #143871 09/29/05 08:09 AM
Joined: Mar 2005
Posts: 1,803
Alan Belson Offline
Member
Course it don't blow the fuse! Ridiculous! The meter melts first!

Alan


Wood work but can't!
Re: UK cooker/RCD problem #143872 09/29/05 08:36 AM
Joined: Sep 2005
Posts: 153
W
Wolfgang Offline
Member
It possible that Alan has got an untypical installation in Mayenne.

But in a typical TT system the local Protective earth is connected to a ground electrode and notbonded to the neutral at any place. (It is forbidden!)

So any current taking the "earth path" has to enter the ground via local electrode and leave via transformer electrode.

So impedancies (is that English, = sum of all inductive, ohm and capacitive resistors in a loop) will be often 50 ohms or more in what we call the "fault loop". As Voltage against ground in France is 230 V (Belgium often 133V) a current of 230 V/50 Ohm = 4,6 Ampere will result. For any fuse or breaker this will be just a normal load.
So it is quite obvious that the only way to guarantee 0.4 seconds until circuit is switched off (harmonized European norms) can be achieved by means of one or nowadays two RCD in a row.

@ Alan Btw You mixed two locations: Sable d'Olonnes and Île d'Oléron

PS: As to France, I heard that formally it is not TT but IT, as often an impedance is between transformer neutral and transformer ground electrode. Usually this is to compensate capacities of the pole-mounted, but isolated cables they use. But practically it works as TT, as long as a definite voltage and current against ground is guaranteed.

Re: UK cooker/RCD problem #143873 09/29/05 09:40 AM
Joined: Sep 2005
Posts: 153
W
Wolfgang Offline
Member
@ Alan D:

The very West of Germany is just the western border to Belgium and the Netherlands around Aachen.

German power grids have a long history of many little power companies (here used to be 3 of them in one city).

Nevertheless the probably most typical grid is TN-C-S. This means that you got four wires entering your house. One of them is ground and neutral at the same time (in Europe called PEN). The other have 230V against ground and 400 against each other at 120°.

In older installation until 1971 the combined PEN usually continued 'til wall outlet or lamp. So you got just two wires, but one is the earthed Neutral, usually bridged in the outlet between one pole and earth brackets of Schuko! In pre-computer times this worked quite well, if only skilled persons worked on it. Main danger is a breaking PEN leading to full phase voltage on the (metal) bodies of faulty devices. Now it is obsolete as the difference in potentials by high currents using the PEN is a problem for electronic equipment.

Unearthed circuits are usually pre-WW2 or GDR.

Now in an actual installation you separate N and PE at the entrance in a house and bond it to a ground electrode in the foundations (mandatory for new houses). A PEN is today formally only allowed for more than 10mm2 but uncommon. In older houses it is very typical, however.

Actual minimum for a new meter is a 3 phase-63 A mains or 43,5 kW. This is usually enough for 6 of them ,but ...
Fuses become rare and more and more replaced by selective breakers. The N is normally never switched nor protected.

Protection by RCD is for TN-C-S in residences only mandatory for bathrooms and similar and outlets used also from the outside of a house (30 mA).

But in certain regions you also will find TT-grids with different designs.
For TT it is more or less necessary as the principal rule is the above mentioned 0.4 seconds. After that period of time fault voltage must be below 50 VAC. Without RCD it would need a very very good ground electrode system.

That might be enough for the moment. Feel free to correct my technical terms in English, as this is not simple in German already.


[This message has been edited by Wolfgang (edited 09-29-2005).]

Re: UK cooker/RCD problem #143874 09/29/05 11:53 AM
Joined: Dec 2001
Posts: 2,455
T
Texas_Ranger Offline
Member
Quote
In older installation until 1971 the combined PEN usually continued 'til wall outlet or lamp. So you got just two wires, but one is the earthed Neutral, usually bridged in the outlet between one pole and earth brackets of Schuko!

Basically TN-C, IMHO a scary system! Some parts of Austria did use it, but it was never very common, personally I haven't seen a TN-C system all my life! I was told about one by a friend, but that was definitely a hefty rural bodge job. He even seemed to be proud of it...

Re: UK cooker/RCD problem #143875 09/29/05 01:05 PM
Joined: Sep 2005
Posts: 153
W
Wolfgang Offline
Member
As a matter of fact this is all primarily a historical problem.

I suppose that the TN-C system was sort of trick to introduce a grounding in an existing ungrounded pre-war two wire installation at a time when RCD were merely ideas, as first working and payable RCDs were developped in the 60's.
And properly done, with a majority of devices with metal bodies or frames it was a lot safer than before.

Nevertheless, in Germany TN-C up to the level of an appartment or house main switch panel is still quite normal for everything that is more than 20 years old. And on the level of PoCo it is probably the majority.

Re: UK cooker/RCD problem #143876 09/29/05 01:26 PM
Joined: Mar 2005
Posts: 1,803
Alan Belson Offline
Member
Most French 230v domestic installations have a primary 500ma RCD and a relais de decouplage fitted before the consumer unit. The meter does not really melt, that was a just a joke. [Linked Image]
Les Stables d'Oblong. Yes, sorry, another typo!
Oh, and they weren't Lusitaniens either, ( according to my present wife ), but Pure-Race Spanish horses. The Lusitaniens were Portugese. Or was it French? Anyway, they had four legs.

Alan

[This message has been edited by Alan Belson (edited 09-29-2005).]


Wood work but can't!
Re: UK cooker/RCD problem #143877 09/30/05 01:12 PM
Joined: Mar 2005
Posts: 1,803
Alan Belson Offline
Member
Wolfgang,
50 ohms between a domestic earth arrangement and the transformer, leading to the assumption that only about 4.5A could flow at 230v. I don't think the current flow to earth needs to go back to the transformer per se. I would expect an earth rod, properly installed to a suitable depth, to achieve 5-10 ohms resistance.

Alan

[This message has been edited by Alan Belson (edited 09-30-2005).]


Wood work but can't!
Re: UK cooker/RCD problem #143878 09/30/05 02:08 PM
Joined: Sep 2005
Posts: 153
W
Wolfgang Offline
Member
@ Alan Belson

TT-system: transformer neutral grounded, Consumer neutral grounded, no conductor but ground/earth between both electrodes. Voltage against ground equal voltage against neutral.
Quite a lot of power grids use this system all over Europe, particularly in rural areas.


IT-system(ideal): transformer neutral not grounded at all. No voltage against ground (at least in small systems without capacitive connections). First fault allows to continue service.
Typical for hospital emergency sections, sensitive industrial processes, isolation(insulation?) checking device (continuously, with alarming) mandatory.

Conclusion: Without a grounded transformer neutral there will no reasonable current on PE conductor to ground at all in case of a fault.

In France maximum resistance of ground electrode is limited to 100 Ohms by Normes Francaises. They typically use tiny, handy 1m long rods and a 6mm2(?) conductor to connect the PE of a domestic installation.

I got a small house in the Gard (30). There I measured the loop impedance of all the houses around by means of my Amprobe Genius 5080E installation tester:
My house has got a loop impedance of 314 Ohms or a maximum current of 0.8A on PE in case of short to PE! This is really above limit, but protected by a second 30mA RCD.
My neighbours got a very new installation and about 57 ohms. A friend downhill 143 Ohms.

Okay this region is rather dry, but I made the measurements at Easter after a wet winter. As it is down there, nobody bothers as CONSUEL has passed already.

For the 100 Ohms limit I have a very simple explanation:
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.

To achieve 5 Ohms on a ground electrode is not that simple at all. More difficult furthermore, to guarantee this value all over the year. Even in much wetter Germany no one expects that value for much longer and thicker electrodes than used in France.

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