Cross-posted for jooles (Belgium):

Aha,
I see we have another Member from Belgium, come to join us.
Welcome to ECN, jooles!.
I've never heard of a Distribution system that uses a fuse in it's Earthing conductors before.
And personally, I would think, that if the Dist. system was referenced to Earth for safety reasons, this sets a very dangerous precedent indeed!.
Should the fuse operate, you effectively lose your earth continuity and therefore any path for Fault currents.
Fusing of Earth conductors, over here, (NZ) is a practice that is dis-allowed by Regulation, in fact not even the Neutral is allowed to be fused.
I'd be very interested to see what a few of our other European members have to say on this Fused Earth system, is this a European thing?.

Mike :]

Just a small point, jooles.
Are you sure that these devices aren't Surge Diverters, for Lightning protection?.

Could also be a resistance for a high resistance ground, but they are pretty rare.

This strikes me as very odd, as a fuse in an earthing conductor is unheard of normally.

It sounds as though jooles' system is the standard 230/400V 4-wire wye European system, which as far as I'm aware always has the neutral solidly grounded.

I can't quite figure what TN-NET means either. European systems are usually TN-C-S or TT. As the xfmr is right there in the buildings served, I would have thought TN-C-S would be the most likely choice.

There should be a camera around the place and I might try to take a photo to show everyone exactly what I mean.

Screwed on the wall of each building, outside the transformer room, the earth cable runs in a loop to a pair of white ceramic insulators, and there is a strip of thin metal in the circuit in between them. The earth cables are the very thick ones, I think perhaps 10 or 15 mm. I assume it is a fuse, because I can't think of any other reason for it and in any case.

There is also a box outside the transformer room with what I think may be a ground fault detector -- three lamps in a row, always unlit.

To explain the context, I work at a chemical factory comprising many large buildings, and there are many kms of pipelines on metal gantries connecting the buildings, so I don't know if this unusual earthing system is maybe to do with those. There is definitely nothing like it in other places I have worked. There is a high demand for power and as I say most buildings have their own transformer room.

You have found a real headscratcher! I can't think of anything logical. The only thing I could come up with was that the fuse is there to break the bond between lightning arrester and the electric system in the event of a direct lightning strike. This could be an attempt to prevent lightning from wrecking the electric system and meet the requirement for equipotential/mains bonding under normal conditions.

Picture to explain my silly idea:

Lightning rod on roof
I
I
Earth electrode #1
I
Fuse
I
Earth electrode #2
I
I
Electrical system

It doesn't really make much sense to me.

jooles,
It wouldn't by any chance be a Test Link for the Earthing system would it?.
We use such a thing here on power poles to test the transformer earthing, without losing an Earth connection while the testing is being done.
The Main Earthing lead is merely swapped over between the two seperate wires, that would run to an electrode or grid system, in the ground.

Jooles welcome to ECN, what a starter post, I agree with Mike and all other members if it is a fuse it can only spell danger of sorts. The other thing about fusing an earth wire what fuse rating do you fuse it at? In the event of an earth fault could you accurately predict the path that the current will flow along especially in a large industrial situation with metal pipes and support gantries everywear. Look forward to the pics.

Photos received from jooles:




The labeling on the switch on that cabinet may be a little too small to read on some monitors with the reduced image size. It says "HOOFDSCHAKELAR", and has the standard European "0" and "1" markings for on and off.



[This message has been edited by pauluk (edited 09-23-2004).]

It looks to me as though these are indeed test links to allow the earth conductors to be opened for testing. I notice that one cable has a piece of blue tape on it. It could just be there by chance, or maybe it indicates the earth cable which is running to the neutral to ground the supply system?

I assume the "3 x 380V + N" designation is a Continental way of saying it's a 220/380V wye system.

Ah Mystery solved, they undoubtedly look like test links.

Thanks for the clarification -- I thought they looked so much thinner than the cables that they could well be fuses. I often think what would happen to them if a serious fault occurred and one of them went phut! so tend to keep a bit of distance.

In any case, I still wonder if they would not be best inside the secure transformer room, instead of out in the open like they are. If any troublemaker thought they would borrow a spanner and an extension lead from one of the workshops, they could enliven the entire building's earth points, no?

On every floor, in the labs, there are large metal sinks, about 2m X 1m, which are not bonded so far as one is able to see. This seems unusual to me, and I wonder if it is part of this strange TN-NET designation. I've never seen it anywhere else.

Beginning the Dutch word on the switch label is "hoofd" which is "head"; I suppose the usual English is "Master". "Schakelaar" is a switch. So, it turns off all the power in that area :-)

Thanks again for the info :-)

j

Whew!,
Thank God for that!.
I thought there must have been some nasty person going around, fusing people's Earth Conductors, while they were't looking.

 
Long shot — Top picture / 4 cables might be a test point for cathodic-protection monitoring.
Even longer shot — Lower picture could be a cathodic-protection rectifier. Both are used for large underground metal pipe, casings or tanks. Example.. www.vniigaz.com/english/production/pde/pde-tech.htm www.farwestcorrosion.com/fwst/dcpower/univers02.htm

One thing does have to be said about them wire clamps.
Considering that they are clamping 10mm earthing wires, they look a little on the light side.
And also, are they made of steel?.
Any earth busbar or clamp is made of Brass
here.

Hmmm. TN net simply means the neutral is actually a PEN, that means it may serve as a PE conductor. Usually this is split up into TN-C-S at the service entrance point, that means running a separate PE conductor. TN-C means just a PEN. If it's actually a TN-C system there _might_ be a reason for not bonding the sinks (corrosion issues?), otherwise there isn't.
Hoofdschakelar means main switch as you already said.
The 3x380V+N thing is weird and definitely not typically European. 220/380V is far more common. Maybe it has some special meaning.