Competition time... Lowest TT Zdb

[aptsys]

TN-C-S involves a bit more for the DNO than simply linking N+E at the cutout. There must also be DNO earth electrodes at various points throughout the distribution network (hence Protective Multiple Earthing), and out in the sticks where TT is commonly encountered, this would be difficult to achieve.

TN-C-S doesn't necessarily mean PME though. You can have a TN-C-S supply that isn't earthed at multiple points.

 

[ericmark]

I worked in Point of Ayr gas terminal and must have sunk 100's of earth rods each one less than 8 ohm these were all bonded together and to all metal work and at incomer there was a earth pit with 4 rods all at 8 ohm and connected with bare copper underground cable. All these were then connected to boards earth vie a huge resistor to limit earth current flow.
Never measured all together but guess loads of zeros after decimal point. Not even sure how one would measure it? Or what it is called? Only system shown with resistance is IT system yet we are told it is not permitted in UK and this was in Wales. Also of course back up gas powered generators think around the 1.5MW each so when one is told IT is not permitted in UK well maybe not for small users but those resistors are used in gas terminals all to do of course with stopping any sparks.

So there is an exception to every rule.

 

[Spark123]

I don't think we are not allowed to use IT systems, I used to work in a dept which had an IT setup in a test area.

 

[electronicsuk]

TN-C-S involves a bit more for the DNO than simply linking N+E at the cutout. There must also be DNO earth electrodes at various points throughout the distribution network (hence Protective Multiple Earthing), and out in the sticks where TT is commonly encountered, this would be difficult to achieve.

TN-C-S doesn't necessarily mean PME though. You can have a TN-C-S supply that isn't earthed at multiple points.

I have to admit that I wasn't aware UK electricity suppliers were offering TN-C-S without PME. Still, always good to learn something new.

 

[DCC]

I worked in Point of Ayr gas terminal and must have sunk 100's of earth rods each one less than 8 ohm these were all bonded together and to all metal work and at incomer there was a earth pit with 4 rods all at 8 ohm and connected with bare copper underground cable. All these were then connected to boards earth vie a huge resistor to limit earth current flow.

*curiosity well and truly sparked*

Enlighten us interested amateurs? why? (not the parallel connection bit)

Why have a super low earth resistance if you then pop a resistor in to limit current flow?

Given that domestic installations can be made 'safe' with resistances much higher what is the design aim going on here? (understood its a pretty special location but what does this setup allow you to do that's different)

Tell us more!

 

[plugwash]

It is also illegal to use the protective conductor from a TN-CS supply for earthing caravan sites and marinas.

What I find a bit amusing is that you can't legally use TN-C-S to supply caravans but the railways seem quite happy to use TN-C to run thier trains.

Both mobile metal boxes with wheels and lots of exposed metalwork yet totally opposite rules..........

 

[ericmark]

As to resistor in earth it limits current flow to what cables can withstand it was very low I would guess less than 1 ohm size of box I would think it could sink around 100KVA without too much problem considering size of load banks I have used.

As to railways. Can you see any factory ever being allowed to install a live rail at ground level? How they can get away with new extensions to system with live rail I don't know. OK I would accept in the underground section live rail may be best option. But Wirral to Chester is all countryside with in some areas fencing with much to be desired and it was fitted in last 20 years with live rail.

 

[nozspark]

FWIW mines and railways (and others) don't come under BS7671...

Eric...
where abouts are you??

Used to live in a village not far from Point Of Ayr and knew a few of the lads there...
one a shift charge electrician and a face machine operator (Dooby) particularly

I've since moved to Prestatyn!

 

[ericmark]

I now live in Bryn-y-baal just outside Mold. When working on Gas terminal it was during building not running. And at that time I had not worked under ground. I did later in Hong Kong very dangerous work but good money. And yes TBM did have IT system but because part of the machine (A Robin) they got away with it.

Yes I know they can get away without following BS7671 same as where my son lives on a narrow boat very few rules apply. But point was anywhere else it would never be allowed.

 

[Adam_151]

As far as I know there is nothing to stop you using an IT system in the UK eric

If yo google blakley unit, you'll fnd a device thats in use in science labs in schools etc in the UK to minimis shock risk from bench outlets, which is effectivly a IT system derived from a double wound transformer... this tends to confuse sparks bearing loop testers

Its TNC thats forbidden (though according to an old college lecturer of mine... it was in use for SP submains in one chain of department stores in the uk, using single core MI cables)

 

[nozspark]

When you said Point of Ayr I thought you mean't the pit not the gas terminal!!