Taking cables from a house to a garage

[ianmacd]

The bottom line is:BS 7671 doesn't say you can't.

It doesn't say you can't walk up to someone in the street and stab them with an earth rod either but are you going to do it? Deviation only means no defence if needed.

No-one on this forum has offered a valid (electrical) reason why you shouldn't.

An additional earth path can only make the installation safer.

How can you say that? What proof have you got? Its a deviation from 7671 so, again no defence. What about the fire thats started as you only put a 4mm cable on the earth rod and it tried to push the pfc though it and it melted?

Ze won't change - the main earthing conductor (MET to cutout) is disconnected for this measurement (no rod).

Well thats wrong as YOU have installed it as an earth path so should be left in.

Zs will be better (rod included)

Should the dreaded happen - loss of supply neutral - then you have an alternate path for the current to flow.

And this is why it should be at the PEN as you may well get all your neighbours too!!!

That's the arguments for - what have you got against??

In red.

 

[ADS]

I read it twice and struggled to find a constructive argument amongst that lot;\

Except maybe the 4mm thing - but if you were too cheap to run a 10mm to it, that's your own lookout - bearing in mind, in that scenario, it won't be the full fault current flowing, it will be the 'load' current - no one mentioned a fault on the installation.

So still no valid reason not to do it.

 

[ianmacd]

10mm may not be enough either so you may have just caused a fire. Should be the same size as the DNO supplied earth (542.1.8).

 

[spinlondon]

I would have thought, that all these things could happen anyway, with bonded services.

 

[steptoe]

This is an excerpt from the Guidance on the Provision of Earthing Terminals for Eastern & London Regional Electricity Company Connections Street Furniture & Lighting Single items of electrical street furniture. issued around 2000.EPN

 

[PBC_1966]

Its a deviation from 7671 so, again no defence.

There is no rule in BS7671 which says you cannot have an earth electrode in combination with a TN-C-S system, so how can it be a deviation?

Well thats wrong as YOU have installed it as an earth path so should be left in.

When you make a bond from the MET to a buried water pipe you have also installed another earth path, but you don't leave that in for the measurement.

10mm may not be enough either so you may have just caused a fire. Should be the same size as the DNO supplied earth (542.1.8).

How would it be any different from the 10 sq. mm. bonding to a water service, which BS7671 says is acceptable?

The possibility of the bonding having to carry such current is the reason for the increased size over bonding on non-TN-C-S systems, but if it's dangerous on a purpose-made earth rod, then it's equally dangerous on a bonded supply pipe. The cable may well be too small, but if BS7671 considers 10 sq. mm. to be adequate against the potential risk in the latter situation, why not the former?

 

[bluetof]

There is no rule in BS7671 which says you cannot have an earth electrode in combination with a TN-C-S system, so how can it be a deviation? When you make a bond from the MET to a buried water pipe you have also installed another earth path, but you don't leave that in for the measurement.

How would it be any different from the 10 sq. mm. bonding to a water service, which BS7671 says is acceptable?

The possibility of the bonding having to carry such current is the reason for the increased size over bonding on non-TN-C-S systems, but if it's dangerous on a purpose-made earth rod, then it's equally dangerous on a bonded supply pipe. The cable may well be too small, but if BS7671 considers 10 sq. mm. to be adequate against the potential risk in the latter situation, why not the former?

AGREED Applaud Smiley

 

[steptoe]

There is no rule in BS7671 which says you cannot have an earth electrode in combination with a TN-C-S system, so how can it be a deviation? When you make a bond from the MET to a buried water pipe you have also installed another earth path, but you don't leave that in for the measurement.

no you havent, 7671 2008 strictly prohibits water pipes and such like being used for earth electrodes

How would it be any different from the 10 sq. mm. bonding to a water service, which BS7671 says is acceptable?

The possibility of the bonding having to carry such current is the reason for the increased size over bonding on non-TN-C-S systems, but if it's dangerous on a purpose-made earth rod, then it's equally dangerous on a bonded supply pipe. The cable may well be too small, but if BS7671 considers 10 sq. mm. to be adequate against the potential risk in the latter situation, why not the former?

because you have provided the earthing system with your newly installed earth electrode,

Im now starting to get really fed up with people claiming to know what they are doing and clearly dont understand the basics of earthing.

have you read the exported earth thread? perhaps that will assist your understanding.

this I suppose is the one of the things that separates qualified people from being competent persons.

 

[ianmacd]

There is no rule in BS7671 which says you cannot have an earth electrode in combination with a TN-C-S system, so how can it be a deviation? When you make a bond from the MET to a buried water pipe you have also installed another earth path, but you don't leave that in for the measurement.

How would it be any different from the 10 sq. mm. bonding to a water service, which BS7671 says is acceptable?

The possibility of the bonding having to carry such current is the reason for the increased size over bonding on non-TN-C-S systems, but if it's dangerous on a purpose-made earth rod, then it's equally dangerous on a bonded supply pipe. The cable may well be too small, but if BS7671 considers 10 sq. mm. to be adequate against the potential risk in the latter situation, why not the former?

I can see why steps gets so stressed with people on here. Its not in 7671 then its a deviation. I can not see how you can not understand that. It makes me wonder what else you can not understand.

 

[PBC_1966]

decide on the earthing system and dont mix them.

dual earthing is prohibited

Can you quote the regulation which you believe says that you can't install a local earth rod on a TN-C-S system?

And for another try on this one, please explain how you think that a bonded metallic water pipe running multiple tens of feet underground is any less of an earth electrode than a rod you drive into the ground specifically for that purpose.

 

[ianmacd]

Show me one that says you can.

Its not what I think or what you think is safer, its the regulations. Follow them or you are on your own in the dock when you kill someone.

 

[spinlondon]

Show me one that says you can.Its not what I think or what you think is safer, its the regulations. Follow them or you are on your own in the dock when you kill someone.

Regulation 531.3.1.

 

[PBC_1966]

When you make a bond from the MET to a buried water pipe you have also installed another earth path, but you don't leave that in for the measurement.

no you havent, 7671 2008 strictly prohibits water pipes and such like being used for earth electrodes

Yes, and the Wiring Regs. have prohibited such use as the means of earthing since the 14th edition due to the possibility of metallic pipes being replaced with plastic.

But that doesn't change the laws of physics: If you have 40 ft. of metallic water pipe buried in the ground, then it is going to be an effective electrode whether you like it or not. More effective than a typical 4 ft. earth rod in fact. Your fault current is not magically going to refuse to flow through the bonding conductor and that pipe to earth just because BS7671 says that the pipe is not to be used as the earthing means due to the chance of being replaced with plastic at some point. If the earth path is there, the current will take it.

because you have provided the earthing system with your newly installed earth electrode,

You have provided an extra path back to source, just as you do when you bond that buried metallic water pipe. In the event of an earth fault on the installation, where do you think the bulk of that fault current is going to flow, given the resistances involved?

have you read the exported earth thread?

Yes. And I've still been unable to find any clear statement from you as to why you think that merely providing an extra earth electrode (subject to suitable conductor size) is such a problem.

Edit to add: Beyond a rather unconvincing argument that some DNO guy might see the rod and remove the N-E bond. Given that any installation could be made dangerous if somebody starts disconnecting things without knowing what's going on, I think we can dismiss that one.

 

[bluetof]

Yes, and the Wiring Regs. have prohibited such use as the means of earthing since the 14th edition due to the possibility of metallic pipes being replaced with plastic.But that doesn't change the laws of physics: If you have 40 ft. of metallic water pipe buried in the ground, then it is going to be an effective electrode whether you like it or not. More effective than a typical 4 ft. earth rod in fact. Your fault current is not magically going to refuse to flow through the bonding conductor and that pipe to earth just because BS7671 says that the pipe is not to be used as the earthing means due to the chance of being replaced with plastic at some point. If the earth path is there, the current will take it.

You have provided an extra path back to source, just as you do when you bond that buried metallic water pipe. In the event of an earth fault on the installation, where do you think the bulk of that fault current is going to flow, given the resistances involved?

Yes. And I've still been unable to find any clear statement from you as to why you think that merely providing an extra earth electrode (subject to suitable conductor size) is such a problem.

Edit to add: Beyond a rather unconvincing argument that some DNO guy might see the rod and remove the N-E bond. Given that any installation could be made dangerous if somebody starts disconnecting things without knowing what's going on, I think we can dismiss that one.

AGREED :Y

 

[ianmacd]

Regulation 531.3.1.

Thanks for making our case spin. That says treat system at TT.

 

[spinlondon]

Thanks for making our case spin. That says treat system at TT.

No actually, it doesn't.

It says the circuit should be treated as TT.

The rest is still treated as TN-C-S.

 

[ianmacd]

No actually, it doesn't.It says the circuit should be treated as TT.

The rest is still treated as TN-C-S.

So care to explain how you can treat a single circuit at TT and the rest as something else? Are you advocating the use of 2 'MET's?

 

[SPECIAL LOCATION]

But nowhere does it say that there cannot also be a local earth electrode.

Read the whole of 542.1 Earthing arrangements, general requirements.

i.e. page 126...

542.1.1

542.1.2

542.1.3

542.1.4

542.1.5

542.1.6

542.1.7

&

542.1.8

All in context with each other!

 

[SPECIAL LOCATION]

The bottom line is:BS 7671 doesn't say you can't.

??

Someone else who hasn't read the whole of 542.1 Earthing arrangements, general requirements.

i.e. page 126

542.1.1

542.1.2

542.1.3

542.1.4

542.1.5

542.1.6

542.1.7

542.1.8

All in context with each other!

:|

 

[spinlondon]

So care to explain how you can treat a single circuit at TT and the rest as something else? Are you advocating the use of 2 'MET's?

Don't blame me, not my fault it's what it says in BS7671.

There doesn't seem to be a regulation for TN-C-S, where PME isn't provided.

Why's that then?

Allthough, there is one that says the earthing arrangements may be used jointly or separately?