Earthing/bonding question

Does it introduce an earth potential from outside to inside? If it is not in the ground, and just on the wall, then it wont introduce an earth potential from outside to inside and wont need bonding in any way.

However, if this copper is connected to earth inside the building somehow, by bonding or parallel paths of some kind, and you have PME earthing, you are advised to insulated the copper pipe from this earth path with an insulating section of pipe.

PME by it's nature, due to circulating network currents, may at times have it's N/E at a slight potential above true earth, resulting in "perceived shock" to anyone stood on the ground and touching the pipework.

This is seen quite often in coastal towns where a surfer comes back from surfing and washes himself or his board down with a hose before taking his wetsuit off. He touches a metal tap outside, stood on mother earth. He receives a tingly shock. An electrician is called to have a look, and bonds the tap as he assumes this will sort the problem - it wont. It needs insulating from earth.

 

Hi Lec,

So, ideally, in the OP's situation take it outside the equipotential zone in plastic?

Do you know what this potential can rise to?

Cheers
Fats

 

Hmmm 50v max - however in the event of neutral tie down being lost then 200v + not nice tbqh.

Or it used to be. 30mA rcd does stuff but not enough in Lec's instance.

 

''you are advised to insulated the copper pipe from this earth path with an insulating section of pipe''

I always was under the impression that this was a requirement anyway Lec- be it in any electrical deployment clime.

A simple way of doing it is using linked -

http://www.screwfix.com/p/jg-speedfit-15esotp-emergency-shut-off-tap-15mm/13133

This can be fitted inside obviously - normally I would never use this kind of stuff but as it is solidly linked between CT there is no blow off risk. The isolator tap would negate any pd differentials with respect to standing on terra firma outside, and touching the tap which comes from inside.

 

It might work in some areas of where the rod is planted - but not in others. As for wether a rod can be used to supplement PME - on one job we did the DNO asked us to bolster with a rod. Just cabled a piece of 16mm to it and chucked it in with the clay/backfill and stuff in trench..what do you say? nuffin really.

 

It's not a requirement JP.

I would advise the customer to insulate the pipe work from the PME earthing.

I would first however double check your loop impedance to make sure this is nice and low.

With PME, you can have current flowing through your bonding conductors under "normal" conditions. Current can flow from the network, into your home, and through your bonding to earth, or to a neighbouring cutout etc.

PME is not advisable in communal showering, swimming pools etc for this reason. IT is also prohibited on petrol forcourts due to circulating currents in the steel tank and pipework.

An RCD is not going to help with perceived shock, nor will any additional bonding.

Insulating the pipework is the way to go, once you have proved your earth connection to the DNO is fine. (test in isolation, with the main switch off).

 

Ideally, yep.

In reality, it is not much, but when your outside and wet, you can get a tingle from 20 odd volts. JP's 50v assumption is a tad high I would say.

 

With your main switch(s) off, you are likely to still see a spark if you disconnect the earth (you could clamp meter it), if it is a circulating network current. The suppliers neutral is still able to "leak" back through your bonding with your main switches off.

 

Are you definitely sure that it is not a requirement to negate continuity of CT feeding outside tap Lec? I find the whole thing quite surprising. I think the regs need looking at tbqh - glad I only use 7671 as a base wiring deployment vehicle - and conjoin my wiring regs with it.

 

Mind you saying that who the heck is going to negate PRV ct continuity? One blast and the plastic fecking tap would melt..

 

I'm sure it is not a requirement, just a recommendation. The only thing set in stone is the use of PME on petrol forcourts. Everything else is just a recommendation.

I am sure there was an article on this at some point in the NIC mag.

These days, it is becoming quite common to see outside showers in posh homes, or sea side homes. It is the good old "tap" while standing on wet grass that seem to be the main culprets though.

 

I know what you mean about PME in petrol stations Lec.

I'll come clean and admit that I haven't isolated my garden tap only because I had loads of metal iso valves (ballofix) and no plastic iso valves to hand (because I don't like plastic stuff tbqh) but I will whip one in one day..and that electrical inspection chap might insist on it.

I might do a test..what I will do is bang a lump of metal down into well sodden ground away from the tap (about 4.5 mtrs from tap to electrode because my patio is concrete) and then with my wander in action see if there is a pd - the tap is electrically continuous with the met..ie below 0.05

Only out of interest - I expect to see what I don't know 5v max, but judging by the soil around my parts and my intake type it might be 0v - maybe I will do the test this weekend and post the results.

 

Your lecky inspector wont find anything wrong with the tap. There is nothing in the regs. It is recommendation only.

 

That's true Lec but he wants me to cross bond the boiler in 10mm..flipping last thing I need clagging up my pipes is clamps and 10mm cable, they are all below 0.05..and all rcd protection will be in place.

 

Nope would only tick PME box Sparks, and measure ze only with respect to the PME - bearing in mind also that if you did augment it with a rod (not really sure you could call it a TT rod) it might be best to just dig a hole and sling the rod down said and bury it. Otherwise you are maintaining a pit..it could over complicate matters tbqh.

 

If you put a rod in, it's essentially an extraneous conductive part and needs its conductor sized as such ;-)

Nothing wrong with adding a rod, it helps. No different to adding a rod or tape to a gen backup install.