Exporting PME.

[securespark]

I have tested an outhouse fed via a PME- supplied DB. I know about exporting PME - that if you have extraneous conductive parts, you should either make the cpc feeding the sub db the same as the incoming CSA or use TT.

However, I am puzzling this point a great deal...

We all know the definition of an extraneous conductive part.

Does a copper water pipe introduced into the outhouse via a polypipe count as an extraneous CP?

If so why?

If not, why do we bother fitting PEB's to copper pipework fed via non-conducting pipes?

 

[equitum]

My understanding is that a water supply, even if fed in HEP, contains water, which is conductive, albeit depending on the level of impurity of the water not a particularly good one. If copper is then used after the HEP supply, then it becomes an ECP and must be bonded etc.

 

[securespark]

So what happens if that copper is then broken by plastic joints? The main water pipe in the dwelling is copper, but then run out to the outhouse is poly, then it changes to copper again.

 

[Pensdown]

Does a copper water pipe introduced into the outhouse via a polypipe count as an extraneous CP?

That would depend on what resistance reading you get when you test between the MET and the copper pipe but in general no.

If not, why do we bother fitting PEB's to copper pipework fed via non-conducting pipes

Good question.

We don't bond the internal copper pipes within bathrooms if the feeds are plastic so why bond the mains water if it's fed out the ground in plastic?

 

[equitum]

I spose the point is that if at some point a water service is in the ground in copper, then the water within it has an "earthy" potential. At any point where copper is used within the equipotial zone, then it becomes conductive and requires bonding.

 

[Pensdown]

I think it's more the case of "one reg fits all".

Petrol stations are TT and one of the ways they isolate themselves from the earthing network is by using all plastic pipes onto the sites.

 

[ban-all-sheds]

My understanding is that a water supply, even if fed in HEP, contains water, which is conductive, albeit depending on the level of impurity of the water not a particularly good one.

Assuming drinking-water levels of impurities (and we only have a single tier water infrastructure in this country), it's actually a very poor conductor - about 54k ohms/m for 22mm pipe...

I too have always wondered why we put main bonding on copper pipes where the supply comes in in plastic.

 

[securespark]

I'm glad it's not just me, then, wondering about the usefulness of bonding copper pipes fed in plastic incoming supply...

The water pipe currently has a bond to the cpc of the sub-main supply.

If the pipe is an ex-CP, then this is not required. Presumably, as things stand at the moment, this pipe being bonded is not an ideal situation?

I'm going to call the IET later. See what they say!

 

[davelx]

My understanding is that a water supply, even if fed in HEP, contains water, which is conductive, albeit depending on the level of impurity of the water not a particularly good one. If copper is then used after the HEP supply, then it becomes an ECP and must be bonded etc.

There are two "Wiring Matters" on this subject:

http://www2.theiet.org/Publish/Wire...tumn_plastic_pipes_to_bond_or_not_to_bond.pdf

and

http://www2.theiet.org/Publish/Wire...tumn_wiring_matters_section_601_revisited.pdf

which is focussed on bathrooms

The first one states that even if the supply pipes are plastic, if the piping inside the installation is metal it must be main bonded. IMO this refers to an installation where the metal pipes traverse multiple areas (rooms), so that if there was a fault anywhere in the installation that resulted in the pipework becoming connected to a live conductor, the entire installation pipework would rise to the potential of that conductor.

I don't believe it means that metallic piping stubs that are, for example, use solely where visible in, say, a bathroom, for aesthetic reasons, needs to be bonded back to the MET if the rest of the installation is in plastic, even if these are the only metallic sections and there is no other MEB the water system.

How this applies in, say an outhouse, is subject to interpretation, I guess.

 

[securespark]

OK. The exact situation is this:

The pipe enters the outhouse in poly, then changes for a few feet to copper, terminating in a tap. That is the full extent of the pipework.

Should that be bonded?

As PME has been exported to the outhouse & the copper pipe is bonded, does this constitute a risk?

 

[securespark]

OK. Here is the definitive reply from the IET.

In a building that has other services (gas, water) where PME has been exported, there are the following risks:

In the event of loss of neutral, there is a risk of electric shock due to increased touch voltage and further a risk of fire due to the cpc burning out as it cannot withstand the fault conditions.

Either a full size bonding conductor should be connected, or the supply to the outhouse converted to TT.

 

[ban-all-sheds]

FYI - from an earlier enquiry with them (forum consensus, not technical helpline) if the cpc of your submain cable is big enough to be a bonding conductor it can perform both functions.

But the definitive reply above doesn't address the Q of is a copper pipe in the outhouse that's on the end of several metres of plastic an e-c-p....

 

[securespark]

It isn't. It's only a 6mm²/2.5mm² feed.

I explained exactly the scenario to him. His interpretation made the water pipe an ex-CP, thus requiring bonding.

BUT, regardless of the bonding/PME debate, the situation is dangerous as the cpc is too small to withstand fault conditions should the neutral go open circuit: there is a fire risk.

 

[ban-all-sheds]

The first one states that even if the supply pipes are plastic, if the piping inside the installation is metal it must be main bonded.

I can see how internal pipework might become an extraneous-conductive-part, as it might in places be buried. But if not, aren't you making things more hazardous by connecting something that was isolated to the earth of your supply?

IMO this refers to an installation where the metal pipes traverse multiple areas (rooms), so that if there was a fault anywhere in the installation that resulted in the pipework becoming connected to a live conductor, the entire installation pipework would rise to the potential of that conductor.

And that argument is not regarding the pipe as an extraneous-conductive-part, its treating it an an exposed-conductive-part, and the bonding cable becomes an earthing one.

And once you accept that argument then you'd have to go around earthing anything else metal that "could become live if there was a fault"...

I don't believe it means that metallic piping stubs that are, for example, use solely where visible in, say, a bathroom, for aesthetic reasons, needs to be bonded back to the MET if the rest of the installation is in plastic, even if these are the only metallic sections and there is no other MEB the water system.

The man from IET, he say yes:

I explained exactly the scenario to him. His interpretation made the water pipe an ex-CP, thus requiring bonding.

Hmmmmmmmmmmmmmm....

BUT, regardless of the bonding/PME debate, the situation is dangerous as the cpc is too small to withstand fault conditions should the neutral go open circuit: there is a fire risk.

So would you question the very existence of 6/2.5 cable, and say that any use of it is wrong?

What if it was supplying a shower in the bathroom - would the cpc be adequate?

What if it went to the outhouse but wasn't a sub-main - just had an industrial socket on the end to supply something industrial there - would the cpc be adequate?

According to 54G undersized cpcs are never allowed, but they are used all the time - I assume they are OK according to the calculation in 543-01-03.

So is 6/2.5 ever OK?

 

[Spark123]

Replace the short length of copper pipe with poly