Main earth and bonding issues

[marc8]

Not wishing to dis some of your advice, but struggling a tad with some.

Metal services must be bonded. Plastic services do not need bonding, and wouldn't waste my time checking them. The metal pipe work that runs from the plastic services may need bonding (528.3).

Changing from TT to TN-C-S may have a cost implication to the client which may not be necessary (although I would), and I wonder why installing 100mA S Type in this situation, would be a consideration. And OP doesn't have to test the whole installation, if he is doing an alteration or addition (minor works).

I'm always learning, so I willing to stand corrected.

might have read slightly wrong with plastic pipe & not mentioning the metal part after.

changing from TT to PME or TN-C-S has always been free for me & have done quite a few

the 100ma trip S type is normally used to protect cabling in the infrastructure more important in what was 16th edition as lights ect did not require RCD protection now most circuits are 30ma protected. but I would always do a TT system with 100ma trip & the reason for s is the time delay so the 30ma will trip first if you only use 100ma without S type they can go at the same time as the 30ma & take out whole board.
me now with metal clad board cables in there unprotected I would install the 100ma trip on a TT.

testing meaning the one circuit or more if pumped shower (running of sockets) & lights running of lighting, i would want to test the whole circuit before i put an RCD at the mains hence the mention of RCD spur above the door & protecting just the work i would be installing so i don't have to test the whole circuit.

 

[Midwest]

It might be just the way your wording your replies then.

I have never been involved with costing of conversions to PME, but I've read of different responses by DNO's with regarding to charges, something to consider before going ahead with it perhaps.

Agree with you re 100mA S Type on TT supplies and A3 CU's, the OSG gives some suggestions for all supplies (3.6.3.3). There are other considerations for TT, distribution circuits for example.

Still not clear what you mean in your last paragraph. Installing a local RCD, would still need the series of tests conducted for the whole circuit, not just the piece after the RCD.

 

[The Ghost]

Still would like to know why Ze changed when you disconnected water bonding.

 

[leep82]

Still would like to know why Ze changed when you disconnected water bonding.

Parrallel paths?

 

[The Ghost]

If you did not disconnect the main earth from the MET you would have the water pipe as parrallel earth causing a misleading Ze reading

 

[westward10]

Typical TT the water bond is probably providing the earth and the connection to the electrode is probably doing no more than earthing it.

 

[leep82]

If you did not disconnect the main earth from the MET you would have the water pipe as parrallel earth causing a misleading Ze reading

My original ze test was with main bonding and main earth both connected in the MET and a reading of around 4ohms obtained. I then removed the main bonding to the incoming water performed a ze test again and got a reading of 20ohms

 

[The Ghost]

water bond is probably providing the earth

Thats very naughty then

 

[The Ghost]

So that second reading is your Ze which is ok then. I think that might be a bit of a poser then bacause the water sounds like a better earth than the main earth so it would then act as earth in fault conditions....hmmmmm

 

[leep82]

So that second reading is your Ze which is ok then.

Yes

 

[Des 56]

So that second reading is your Ze which is ok then.
Yes

and a reading of around 4ohms obtainedMy original ze test was with main bonding and main earth both connected in the MET. I then removed the main bonding to the incoming water performed a ze test again and got a reading of 20ohms

Leep82,you keep stating that you measure at the met at a time with bonds connected then disconnected

Has a loop reading been taken off the assumed 10mm cable with it actually disconnected from the consumer unit?

It seems to my reading that a Ze has always included parallel paths on each occasion if that reading has been taken at the met,it seems no assurance of any kind of electrode connection has been carried out

 

[leep82]

Leep82,you keep stating that you measure at the met at a time with bonds connected then disconnected

Has a loop reading been taken off the assumed 10mm cable with it actually disconnected from the consumer unit?

It seems to my reading that a Ze has always included parallel paths on each occasion if that reading has been taken at the met,it seems no assurance of any kind of electrode connection has been carried out

No I havnt disconnected the assumed main earth from the MET to do a Ze only the known main bond to the water. I assumed with the main bond disconnected it would in essence be a true Ze reading, what other parrallel paths could there be? The c.p.c's for the connected circuits were also in the MET at time of test. Should i have actually disconnected the assumed main earth form the MET and done a Ze test that way?

 

[SparkyChick]

You should disconnect it to be absolutely sure there are no parallel paths. Could be supplementary bonding for example which could bring the water pipe into play through some convoluted path.

I'd also be inclined to double check that the other 10mm isn't actually connected to the gas pipe. I did a job the other day two 10mm bonds, one traced to water, the other to the gas (verified by continuity) but the clamp was nowhere to be found and the cable just disappeared off somewhere.

 

[The Ghost]

Should i have actually disconnected the assumed main earth form the MET

Well really this is what I was sneakily alluding to without wishing to cause concern. So you are measuring external E ohms hence mains off and Main E disconnected from MET so you can only be measuring Ze. But clearly if its one of them MET on the board outside the DB then you can see there is nothing connected to it, it is the same really.

 

[The Ghost]

cable just disappeared off somewhere

Well I hope you found it and put it where it belongs.

 

[leep82]

You should disconnect it to be absolutely sure there are no parallel paths. Could be supplementary bonding for example which could bring the water pipe into play through some convoluted path.

I'd also be inclined to double check that the other 10mm isn't actually connected to the gas pipe. I did a job the other day two 10mm bonds, one traced to water, the other to the gas (verified by continuity) but the clamp was nowhere to be found and the cable just disappeared off somewhere.

There is definately no bond to the gas. The gas supply is visible its entire length on the outside of the building from the meter at the front

You should disconnect it to be absolutely sure there are no parallel paths. Could be supplementary bonding for example which could bring the water pipe into play through some convoluted path.

I'd also be inclined to double check that the other 10mm isn't actually connected to the gas pipe. I did a job the other day two 10mm bonds, one traced to water, the other to the gas (verified by continuity) but the clamp was nowhere to be found and the cable just disappeared off somewhere.

There is definately no bond to the gas. The gas supply runs externally down the side of the house and is visable for its entire run. It enters the property at two points the boiler and the gas hob. I checked at both points and no bonding present. I also checked in the meter enclosure before anyone asks

 

[westward10]

So that second reading is your Ze which is ok then. I think that might be a bit of a poser then bacause the water sounds like a better earth than the main earth so it would then act as earth in fault conditions....hmmmmm

This is par for the course on TT systems with metal piped services bond the gas, assuming a metal service pipe the Ze will probably be even lower.

 

[The Ghost]

This is par for the course on TT systems

Ok, so very rarely come across TT and usually arrange PME when I do. Suppose it could be interesting having a shower then when there is a fault.

 

[davesparks]

Still would like to know why Ze changed when you disconnected water bonding.

Technically Ze didn't change, he just measured a Zs and a Ze. Ze can only be measured with all bonding disconnected.