cross bonding

[bernardgreen]

TT system with earth rod.

There has to be an earth wire from the earth rod to connect the supply transformer star point to the CPC wiring around the house. If that is not there then the RCDs will not operate when there is a live or neutral fault to the CPC. This earth cable can be ? well.... what size is required for the earth in order to carry the 30 or so milliamps of fault current that will trip the RCDs

But the earth rod is also introducing an external potential to the building. So it requires to be bonded to the MET of the system. I understand this wire has to be 10mm as it is a bond to an incoming service and may need to carry a lot of current to ensure the MCB operate on over current.

Perhaps there must be two wires from the earth rod to the MET.

 

[JohnW2]

TT system with earth rod. ... what size is required for the earth in order to carry the 30 or so milliamps of fault current that will trip the RCDs ... But the earth rod is also introducing an external potential to the building. So it requires to be bonded to the MET of the system. I understand this wire has to be 10mm as it is a bond to an incoming service and may need to carry a lot of current to ensure the MCB operate on over current.
Perhaps there must be two wires from the earth rod to the MET.

Quite ... and maybe two wires of different colours, in the hope that would help the electrons to know which path to take

Of course, it is pretty optimistic to believe that any TT earth would have low enough impedance to make any MCB operate (hence the reliance on RCDs in TT installations). 50Ω would be pretty good for a TT earth, and the 5-6 A fault current that would result in would not even trip a 6A MCB, let alone one of higher rating.

Kind Regards, John.

 

[westie101]

OK a thought, could we define an earth wire as one that connects an installation to the general mass of earth either directly of via the supply system.
i.e. the wire from the MET to either the TT earth rod or the DNO earth.
Everything else is bonded to the MET for a variety of reasons that, in respect of an individual wire, could change under fault conditions!

 

[EFLImpudence]

could we define an earth wire as one that connects an installation to the general mass of earth either directly of via the supply system
i.e. the wire from the MET to either the TT earth rod or the DNO earth.

Yes, this is the Earthing Conductor.

Everything else is bonded to the MET for a variety of reasons that, in respect of an individual wire, could change under fault conditions!

Not everything else - only extraneous conductive parts. Incoming services by Main Bonding and other parts by Supplementary Bonding.

Plus Exposed conductive parts by CPCs.

 

[Electrifying]

I can honestly say this is one of the scariest threads I have ever read on any electrical forum - I hope to god most of the posters are DIY and not electricians.

 

[RF Lighting]

TT system with earth rod.

There has to be an earth wire from the earth rod to connect the supply transformer star point to the CPC wiring around the house. If that is not there then the RCDs will not operate when there is a live or neutral fault to the CPC. This earth cable can be ? well.... what size is required for the earth in order to carry the 30 or so milliamps of fault current that will trip the RCDs

Correct. The minimum required size is 2.5mm² providing it is protected from mechanical damage. If not then 4.0mm² minimum is required.

But the earth rod is also introducing an external potential to the building. So it requires to be bonded to the MET of the system.

No it doesn't. It is at the potential we want everything in the house to be at.

I understand this wire has to be 10mm as it is a bond to an incoming service and may need to carry a lot of current to ensure the MCB operate on over current.

Well then you understand wrongly, on how to size protective equipotential bonds, and also the purpose of why they are installed.

Perhaps there must be two wires from the earth rod to the MET.

Wrong again. There will be an earth to the earth rod, an earth to the CU, and PEBs to any extraneous metal work that can introduce a potential into the building whether that is the same potential as the rod under normal conditions, or some other potential altogether, is entirely irrelevant.

 

[RF Lighting]

I can honestly say this is one of the scariest threads I have ever read on any electrical forum - I hope to god most of the posters are DIY and not electricians.

Yes they are, but they are probably still more clued up than many practicing electricians on earthing and bonding.

 

[westie101]

True in one way, I am not and never have been termed as an Electrician, but am not a DIYer in the strictest sense!

 

[RF Lighting]

I think we all know that you're a special case

 

[westie101]

a special case

'Tis amazing how many folk (usually friends) say that!

 

[JohnW2]

I can honestly say this is one of the scariest threads I have ever read on any electrical forum - I hope to god most of the posters are DIY and not electricians.

Yes they are, but they are probably still more clued up than many practicing electricians on earthing and bonding.

If not only for the benefit of outsiders/newcomers to this thread, I think we should try to put it in perspective ....

Apart from some minor differences of opinion regarding ‘grey areas’, I think it’s probably true to say that all of us involved in this thread (with the exception of the op), whether electricians, DIYers or whatever, know (in terms of the regulations, engineering and safety) ‘what needs to be connected to what’, with what cable and why the connections are necessary. The discussion is essentially all about what we call the process and the cables – a question which, as I’ve said, I find essentially unhelpful. The terminology used is far less important than an understanding of the underlying principles involved.

Kind Regards, John.

 

[JohnW2]

Perhaps there must be two wires from the earth rod to the MET.

Wrong again. .....

I don't think Bernard uses emoticons enough - since it sometimes seems difficult for some to see the extent to which his tongue is sometime in his cheek

Kind Regards, John.

 

[westie101]

Sometimes a long involved discussion can help all to ensure we are reading from the same page, the discussions, often on minor points. we can end up in at work are always useful.

If anyone can, it isn't too difficult, follow them there is much to learn

 

[JohnW2]

There has to be an earth wire from the earth rod to connect the supply transformer star point to the CPC wiring around the house. ... what size is required for the earth in order to carry the 30 or so milliamps of fault current that will trip the RCDs

Correct. The minimum required size is 2.5mm² providing it is protected from mechanical damage. If not then 4.0mm² minimum is required.

Indeed, or 16mm² if not protected and buried. Mine appears to be 16mm² even though it is not buried!

I once wondered why, given this, a TT installation still needs a minimum cable size of 6mm² (and commonly 10mm²) for main bonding, even if the earthing conductor is only 2.5mm². However, albeit presumably a pretty rare situation, the idiosyncracies of my TT installation afford one explanantion...

...as I've described before, although the Ze of my TT earth electrode is usually around 75-80Ω, the Ze of my incoming water pipes is only about 0.25Ω - I strongly suspect because of connection via the underground pipework and a neighbour's main bonding to their MET and hence to the earth of what I suspect is a TN supply.

This being the case, whilst a L-E fault close to the origin of my installation would not result in more than about 3A flowing down my earthing conductor (until an RCD operated), a several hundred amps could be trying to flow down the main bonding conductor (very briefly, until RCD and/or MCB operated).

Put another way, athough I am not 'allowed' to rely upon the water pipes as my earth, the reality is that they effectively are my earth, since they provide a path to true earth with something like a 300-fold less impedence than my poor little earth rod! Getting back to the thread, I guess that really does (in this exceptional situation) throw a few spanners into the question of whether my MEB/MPB really is bonding, rather than an earthing conductor - given that virtually all of any fault current would flow down that conductor

Kind Regards, John.

 

[bernardgreen]

I don't think Bernard uses emoticons enough - since it sometimes seems difficult for some to see the extent to which his tongue is sometime in his cheek

In this thread it has been a lot of tongue in cheek. But it does seem strange that cable installers cannot see that if two wires are connected to the same length of pipe they are both connected to the same galvanic (metal) node and a node can only have one designation.

A cable installer with a book of regs says a water pipe to a boiler cannot not be earthed but must be bonded to the MET. He then cannot cope with the following.

[1] FACT The metal work of the boiler must be earthed via the CPC in the electrical supply to the boiler

[2] FACT The pipe that must not be earthed is has a metal to metal contact with the earthed metal of the boiler.

[3] FACT The the pipe is earthed via the boiler's CPC when the regs say it must not be earthed, bonded yes, earthed NO