TT System Query

[sbrown2]

Good Morning

I would like some advice.

Working on a Domestic TT System. Primarily a kitchen refurb. The customer in my case is the contractor. I told him there should be an earth electrode around. I tried to locate and could only locate 6mm earth cable to a disused lead water pipe. Eventually found the earth electrode with the cable disconnected. The original consumer unit has no RCD protection and is around/at least 25 years old. I've been told to ignore this consumer unit and fit new for the kitchen with 30mA protection.
Questions

1. If resistance readings are ok can the lead pipe suffice as the earth electrode?
2. Would you put the disconnected electrode back into service if the tests are good?
3. Is it normal practice to leave customer CU and fit new one to specification?

Any help would be appreciated.

 

[somersetsparks]

Indeed of course I have not, hence the smiley face. Just trying to highlight the fact that if you get a reading of less than 0.35 ohms P-N, then that would mean the neutral has been earthed at the relevant points.

Well not really, It just means you have a good path back to the transformer, which could ber on a pole outside the house!

 

[dwe]

Phew!!! was a bit concerned there! I dont normally get involved with this sort of thing but had to this time!!! lol. But can confirm i have had new installations done in the past few years, and living out on the sticks, not all have been PME. cheers for setting my mind at rest.

 

[IQ Electrical]

Well not really, It just means you have a good path back to the transformer, which could ber on a pole outside the house!

Thankyou, I wasn't really getting through and it's a rather important point!

 

[somersetsparks]

Thankyou, I wasn't really getting through and it's a rather important point!

Haha, to right IQ, scarily enough its not the first time somebody has suggested just 'PME'ing it' I've heard it down the pub before.

There is alot more to consider then just the phase to Neutral Loop, inevitably you will always get a good phase to neutral reading, regardless of whether it is staked, as it is a copper or ally conductor all the way back to the tranny. Every 3rd pole must be staked
for PME, most joints into the mains for a new supply are also staked in the pits etc.
At the end of day the stakes are only there to provide protection in case of a lost neutral, they will never improve the phase-neutral reading.
To be fair I don't think any or possibly very few TT supplies are put in now but in reality how much of the network is new?

I even no of a sparks who rung up to complain about a high ze on a PME system, the DNO promptly arrived to fix it, by means of sticking a 'This is Not A valid Earth Terminal' Onto the cutout!

 

[IQ Electrical]

Haha, to right IQ, scarily enough its not the first time somebody has suggested just 'PME'ing it' I've heard it down the pub before.

There is alot more to consider then just the phase to Neutral Loop, inevitably you will always get a good phase to neutral reading, regardless of whether it is staked, as it is a copper or ally conductor all the way back to the tranny. Every 3rd pole must be staked
for PME, most joints into the mains for a new supply are also staked in the pits etc.
At the end of day the stakes are only there to provide protection in case of a lost neutral, they will never improve the phase-neutral reading.
To be fair I don't think any or possibly very few TT supplies are put in now but in reality how much of the network is new?

I even no of a sparks who rung up to complain about a high ze on a PME system, the DNO promptly arrived to fix it, by means of sticking a 'This is Not A valid Earth Terminal' Onto the cutout!

Oh I know all about the requirements for PME, the post above read "Just trying to highlight the fact that if you get a reading of less than 0.35 ohms P-N, then that would mean the neutral has been earthed at the relevant points."

A reckless statement, how can you possibly think that when you consider the actual requirements for a PME system?

 

[somersetsparks]

Some people ay! Its worrying really isn't it?
Seems to be a pretty widespread dangerous myth though, wonder how many properties are connected only to neutral (In theory, I suppose, No earth)

 

[Wiredspark]

I even no of a sparks who rung up to complain about a high ze on a PME system, the DNO promptly arrived to fix it, by means of sticking a 'This is Not A valid Earth Terminal' Onto the cutout!

ha ha ha, the poor thing is, they are sparks. Time for some reading up guys lol

 

[sbrown2]

I think the convesation you are having points out what I thought. That being you have to test and interpret the regulations to allow a safe installation. And if not sure as in my case........ask.

Cheers

 

[drew35]

PME it, and I've never known DNO refuse as long as their regs are met, usually distance of consumer unit from cut out, and length of their cables inside the property, usual charge about £50 + vat.

The draw back with TT, as you're found, is that the connection is all to easily disconnected, people see a cable going into the ground, don't know what it is, so they just remove it. I've never seen anyone remove an earth connection from the service head.

 

[Engineer54]

PME it, and I've never known DNO refuse as long as their regs are met, usually distance of consumer unit from cut out, and length of their cables inside the property, usual charge about £50 + vat.

The draw back with TT, as you're found, is that the connection is all to easily disconnected, people see a cable going into the ground, don't know what it is, so they just remove it. I've never seen anyone remove an earth connection from the service head.

That is why a ground level Earth pit should ''Always'' be provided over an earth rod connection!! How many actually get one these days, is somewhat questionable...

In my view, if the DNO supply has already been PME'd, then there shouldn't be any charge for simply linking the neutral to the MET with a few inches of earth cable!!

 

[tvrelectrical]

I've had this a few times on TT systems. Call up the DNO, its up to them to confirm PME is in place. Some DNO's have this as a simple yes/ no on the phone because the have the earthing details on file. Some will send an engineer who will confirm. All free of charge.

 

[aaelectric]

HI
I have been to look at a job and gave the client a tempoary board with 4 sockets on 2mcb' 20amp and an rcd. edf was coming to connect . The new supply cable they have put in is 3phase pme system. they would not connect the board as they now said we have to put an earth rod in. As its a pme i thought they would connect to neutral. why would they ask for an earth rod? and what type and size shall i put does it matter where it goes and what test can you do with no supply/ and with a supply. As this is london area i have not have much experience on earth rods etc
Thanks in advance

 

[Engineer54]

I have no idea why they are refusing to connect a PME supply without an earth rod being present. But you can always ring them, and talk to someone in authority, asking them why they require a rod on a PME incoming cable....


Sorry, you asked for information about size of earth rod and location...

Well i wouldn't use anything less than 5/8'' x 2.4m threaded end rods and using at least 2 of them preferably screwed together and deep driven. Alternatively they can be driven singularly, ....spaced apart by at least the length of the rod. Don't install them close to outside house walls, that's where all the building rubble will be. Try and go out at least 1.5m out from any house walls and don't forget to protect the earth rod connection with a purpose made ground level earth pit enclosure...

 

[Gravid]

HI
I have been to look at a job and gave the client a tempoary board with 4 sockets on 2mcb' 20amp and an rcd. edf was coming to connect . The new supply cable they have put in is 3phase pme system. they would not connect the board as they now said we have to put an earth rod in. As its a pme i thought they would connect to neutral. why would they ask for an earth rod? and what type and size shall i put does it matter where it goes and what test can you do with no supply/ and with a supply. As this is london area i have not have much experience on earth rods etc
Thanks in advance

I this something to do with them not providing an earth to temporary structures or remote out buildings seperate from the site?

 

[tvrelectrical]

HI
I have been to look at a job and gave the client a tempoary board with 4 sockets on 2mcb' 20amp and an rcd. edf was coming to connect . The new supply cable they have put in is 3phase pme system. they would not connect the board as they now said we have to put an earth rod in. As its a pme i thought they would connect to neutral. why would they ask for an earth rod? and what type and size shall i put does it matter where it goes and what test can you do with no supply/ and with a supply. As this is london area i have not have much experience on earth rods etc
Thanks in advance

What type of building is the installation in? Is it something temporary? Site power?

There are various special locations TN-C-S is not allowable under 7671