TT system

[dal_87]

Hi doing a cu change at weekend 8 way dual rcd on a 100amp fuse and first time doing one on a tt system noticed that main bonding looks to be a 6mm and so is bonding to gas and water looked at some posts and people say is ok but regs state 16mm main earth and 10mm bonding do I change it all and follow regs book as I should do or is earthing ok as it and will I affect it if I upgrade earthing size. Also are the 16 mm tails fine ? Thanks

 

[telectrix]

with TT earthing systems, the fault current is relatively low and so it's common to have a MEC of only 6mm. if you check your regs. the 16mm for TT is the requirement if it'd buried without protection. also, you need to replace the main switch with a 100mA time delay RCD as you will not otherwise meet disconnection times. the 16mm tails will be fine if they show no signs of damage.

edit: the bonding conductors will also be OK if sound and give decent resistances.

 

[dal_87]

Thanks for the help so replace the 2 rcd that come with the board to 100 ma and the rest will comply with 6mm earthing far as I no earth rod is underneath house

 

[Knobhead]

with TT earthing systems, the fault current is relatively low and so it's common to have a MEC of only 6mm. if you check your regs. the 16mm for TT is the requirement if it'd buried without protection. also, you need to replace the main switch with a 100mA time delay RCD as you will not otherwise meet disconnection times. the 16mm tails will be fine if they show no signs of damage.

edit: the bonding conductors will also be OK if sound and give decent resistances.

Why ? ? ?

 

[telectrix]

well, not an absolute must, if all circuits are 30mA RCD protected, but IMO, a good back up.

 

[dal_87]

Will it comply with dual 30 ma rcds as I said first time doing on tt need much info as possible

 

[Knobhead]

well, not an absolute must, if all circuits are 30mA RCD protected, but IMO, a good back up.

An expensive and unnecessary back up.

 

[dal_87]

Right to clarify on a tt it' ok to put a dual rcd cu in with no 100ma time delay rcd just as long there is 2 rcd covering all circuits

 

[davesparks]

An expensive and unnecessary back up.

I think this must be the first time i've disagreed with you about something!

Modern RCDs just aren't reliable, they need their test buttons pushed regularly to keep them working and people just don't do it. I'm sure in a few years time we'll be seeing the cheap and cheerful RCDs in cheap dual RCD boards failing on EICRs on a regular basis.

For some reason that all important sticker which tells you to push the test button regularly just doesn't have the desired effect.

 

[Chris sparky]

Right to clarify on a tt it' ok to put a dual rcd cu in with no 100ma time delay rcd just as long there is 2 rcd covering all circuits

Yes in a nuttshell. What telectrix is saying is maybe put a 100ma time delay as main switch for some back up as further elaborated on by davesparks

 

[Richard Burns]

Yes, so long as the board is not made of metal.
All circuits in a TT installation need RCD (not necessarily 30mA) protection, so long as that is covered you are OK.

 

[dal_87]

Yes new one is insulated .....the board iam taking out is an old rewireable fuse board with a 30year 100ma rcd in front of main switch which saved the house from catching on fire the other day so done its job well after all this time. Great dual cu fine ! but what if I need a feed to garage rcd cu in swa normally I try not put this on house rcd but in this situation will have to ?

 

[davesparks]

If the existing 100mA RCD is working then keep it, I bet it'll outlast any modern cheapy job you fit.

 

[dal_87]

Ok I leave it in and still have dual cu it just looks rather dated but it can't be seen

 

[Knobhead]

I think this must be the first time i've disagreed with you about something!

Modern RCDs just aren't reliable, they need their test buttons pushed regularly to keep them working and people just don't do it. I'm sure in a few years time we'll be seeing the cheap and cheerful RCDs in cheap dual RCD boards failing on EICRs on a regular basis.

For some reason that all important sticker which tells you to push the test button regularly just doesn't have the desired effect.

I’m not going to disagree with you but a 30mA and a non-time delayed 100mA in series isn’t going to show much discrimination unless it’s low level cumulative leakage. Under fault conditions it will be a ---- up which goes first.

As for being more reliable, some were, some unfortunately weren’t. The Hackbridge Faraday AKA Wylex 100mA were a pain in the bum.

As for the label, I‘ve told you before, you’re got getting anywhere near my CU!

(OP, it’s a long standing joke about my hatred of labels plastered all over CU’s)