First time working on a TT Sytem

[lukeish]

Hi all, I have a few questions regarding a TT system. First time ive worked on one (Self employed now and throughout my apprenticeship and self employed career I haven't had one before) 16mm2 Tails coming in from the rec. There is 2x 2.5 (looks about right) Single earths going to the incoming gas and one disappears towards water boards stop cock in the pavement. The property has a 30mA RCD main switch. I'm right in thinking I need to upgrade tails to 25mm2 and bonding to 10mm2. I will install a Earth Stake with 16mm2 earth to the fuseboard. Does the RCD man switch need to be changed to a lower mA? Is there a maximum length that the earth stake can go? Is it acceptable to take 16mm2 from stake to the DB then take bonding from there or do they have to come from the stake? or can I put a MET in for them?

Any advice appreciated. Thanks a lot.

 

[Richard Burns]

If the main fuse is <100A then 16mm² tails should be OK, main bonding should be 10mm².
30mA main switch is a bit low but does cover any circuits needing 30mA protection, it also does not provide segregation of circuits to avoid nuisance. The main earthing conductor can be calculated from the adiabatic equation, with reference to table 54.1 if applicable, absolute minimum is 2.5mm²and is unlikely to need to be 16mm². Is this a new system as surely there is already an earth electrode in place?
Put an MET in.

 

[lukeish]

House is s very old house (before 1960's) so not a new installation. No Earth stake in place but I notice 2doors down has had 16mm Main Earth and a new Earth stake installed. I did think about asking the homeowner how long ago she had the work completed. I don't know what the main fuse is yet, but its an old porcelain fuse holder presumably containing asbestos - maybe 60A normally? What would you recommend mA for a TT system? The whole property doesn't looks brilliant, No CPC on lights, DB added to the rear to feed the kitchen and an SWA incorrectly made off in it to feed 3x sheds miles away at the top of the garden. When was there any updates in the regs regarding TT Systems? Most if not 95% of the wiring is old colours (older than 2004?) but there has been a loft conversion with lights and sockets in new harmonised colours yet no cert available? Also the new owner ( How I got the work) only got keys last Friday and paid for 2x £7-800 surveys but they didn't flag the wiring up? I presume I'm right in saying that its not right having no earth stake and only 2.5mm2 to gas and water? Bear in mind the water isn't bonded at the stop cock consumer side... its bonded in the pavement on the supplier side?

 

[ruston]

Are you sure that the cable that disappears into the pavement is not to an earth stake.
The water would be bonded inside the property , not outside.

 

[Midwest]

House is s very old house (before 1960's) so not a new installation. No Earth stake in place but I notice 2doors down has had 16mm Main Earth and a new Earth stake installed. I did think about asking the homeowner how long ago she had the work completed. I don't know what the main fuse is yet, but its an old porcelain fuse holder presumably containing asbestos - maybe 60A normally? What would you recommend mA for a TT system? The whole property doesn't looks brilliant, No CPC on lights, DB added to the rear to feed the kitchen and an SWA incorrectly made off in it to feed 3x sheds miles away at the top of the garden. When was there any updates in the regs regarding TT Systems? Most if not 95% of the wiring is old colours (older than 2004?) but there has been a loft conversion with lights and sockets in new harmonised colours yet no cert available? Also the new owner ( How I got the work) only got keys last Friday and paid for 2x £7-800 surveys but they didn't flag the wiring up? I presume I'm right in saying that its not right having no earth stake and only 2.5mm2 to gas and water? Bear in mind the water isn't bonded at the stop cock consumer side... its bonded in the pavement on the supplier side?

I have never installed a 'new' TT system, but have worked on several existing ones. If you have a copy of the latest yellow OSG, it gives a couple of suggestions for a new TT CU installation. Somewhere you will have an 'earth stake', either a rod or more probably an old metal pipe. Have you consider asking the DNO for conversion to PME, if the customer is willing to pay? The way that you have described the main fuse, I would be getting them to have a butchers anyway. If you do install a new earth stake, you may wish to ask members advise on sitting, and may consider testing first (if your MFT provides facility), or it may prove expensive in terms of numbers of stakes!

Think most sparks run a 10mm cable to a stake, only because that's what they have in the back of the van, but Richards way is the correct way to calculate the size of earthing conductor.

Reg 544.1.1 specifies the size of main protective bonding conductors, 544.1.2 details where services should be bonded.

Sounds like some of the property is due a rewire, certainly the circuits with no cpc. What did your survey of the property suggest?

 

[telectrix]

good point. richard. i think a more thorough investigation is required. maybe see if he can get a Ze for the installation.

 

[Richard Burns]

If the "bonding" to the water goes to the suppliers side then this is probably the main earthing for the property; using the metal water pipe as the main earth, which is no longer allowed.

 

[Midwest]

using the metal water pipe as the main earth, which is no longer allowed.

'unless precautions are taken against its removal and it has been considered for such use' (542.2.6)

 

[Richard Burns]

'unless precautions are taken against its removal and it has been considered for such use' (542.2.6)

Only for private supplies not for public water supplies.

 

[telectrix]

but probably a better Ra than any rod that he could bang in.

 

[Midwest]

Only for private supplies not for public water supplies.

Doesn't actually say that in the reg, but agree with you. Just pointing it out, to act as a smart arse and as we don't know what the OP has his earth connected to.

 

[telectrix]

but what sort of a Ra would you get when you rodded a smart arse?

 

[Andy78]

Are you sure that the cable that disappears into the pavement is not to an earth stake.
The water would be bonded inside the property , not outside.

Aye that could be possible. They are nearly all unused now round here, but it is very common to see the old earth stake hugging the front brickwork of the house with it's unsheathed earth wrapped round it.

I'd be tapping in a temp earth stake and getting the DNO to quote for a PME if available. Always the best option in my book.

 

[telectrix]

personally i prefer a TN-S any day.

 

[rolyberkin]

Onsite guide page 45 4.4ii gives you your csa for TT supplies if you have one to hand.

 

[Deleted member 26818]

Minimum CSA for bonding is 6mm2.
Minimum CSA for installation earth is 2.5mm2.

 

[bigspark17]

personally i prefer a TN-S any day.

Here here! Seconded .

 

[Midwest]

but what sort of a Ra would you get when you rodded a smart arse?

Don't know, as I said have little experience of such installs, perhaps you can provide such info with your wealth of experience on the subject

 

[ruston]

but probably a better Ra than any rod that he could bang in.

On a lovely lead pipe probably.

 

[ruston]

If the "bonding" to the water goes to the suppliers side then this is probably the main earthing for the property; using the metal water pipe as the main earth, which is no longer allowed.

I've had one on the old lead pipe, The Ra was 6 ohms.