TT Earth

[Power Ball]

Any Ideas on this please. I went to look at a small job today and the TT earth ran into the comsumer unit earth bar along with the Incoming water and gas..

Should this not have been taken into an earting block along with water and gas bonding then 1 16mm earth to the CU..?

Chas

 

[jl-heating]

i take all 3 to the cu, or if unbroken 10mm i take 2 to the cu.

 

[Andy™]

all earths can go to one central point - this can be block in CU. and with TT, you dont always need 16mm

 

[green-hornet]

I concur with the previous answers

 

[Power Ball]

Thanks Guys.. Thought i would just make sure before i go ahead.

Great replies and very helpfull

Chas

 

[Apache]

Do we even need 10mm earth on a TT?

 

[Power Ball]

On Site Guide shows 10mm from water and gas to earth block then 16mm to CU ..?

 

[Apache]

On Site Guide shows 10mm from water and gas to earth block then 16mm to CU ..?

I'm only a vet, and one drinking whisky at that, but the reason AFAIK for 10mm bonding was sharing neighbours fault current on a PME supply. On your own rod the biggest fault current you could have would not require a 10mm earth?

Please correct me them that know :|

 

[SPECIAL LOCATION]

Any Ideas on this please. I went to look at a small job today and the TT earth ran into the comsumer unit earth bar along with the Incoming water and gas..Should this not have been taken into an earting block along with water and gas bonding then 1 16mm earth to the CU..?

Chas

As the others have said its all ok being inside CU, however, the main reason for using an external earth block on domestic as MET, is more a convenience of access for later testing.

e.g. easier to diss off parallel earth paths for testing your Ze or in the case of TT your Ra.

But it is perfectly ok to use the internal earth bar as the MET,

Most modern CU's normally have three connections set aside for this purpose.

Just it can sometimes be a bit of a bum for anyone coming along later to do a PIR to get the earths disconnected!

personally I tend to stick external earth blocks.

 

[batty]

Most DNO ask for 16mm main earth on all systems including TT.

Batty

 

[Andy™]

Do we even need 10mm earth on a TT?

sometimes. depending on mechanical protection etc. i mostly use 10mm on TT anyway - saves carrying 6mm for the odd use

 

[Evans Electric]

I think it is just the mechanical protection thing ,. If you protected the cable with conduit you could probably use 2.5 . ( See OSG) Your main switch will be an RCD which will limit the current to the rod to mA .

It does'nt look right and TBH I use 10mm but its overkill.

Deke

 

[Andy™]

I think it is just the mechanical protection thing ,. If you protected the cable with conduit you could probably use 2.5 . ( See OSG) Your main switch will be an RCD which will limit the current to the rod to mA .It does'nt look right and TBH I use 10mm but its overkill.

Deke

an RCD will not limit current. it simply takes less earth leakage to trip than an MCB/other. either way, its probably only 10A flowing (with a very good reading), so 10mm is OTT

and what happens when the RCD fails

 

[Evans Electric]

Bit pedantic there Andy ? Thats what I meant , the fault current will be in milli amps therefore 10mm is overkill. ( See page 32 OSG) As regarding the RCD failing , that is in the hands of the IEE , they make the rules, we do as we are told ,do we not ?

By the way , you said you used 10mm yourself !!!

What do you suggest then , should the RCD fail , what should we be doing ?

Deke

 

[Evans Electric]

Most DNO ask for 16mm main earth on all systems including TT.Batty

Don't forget , Batts, the DNO would not be involved with the main earth on a TT as they were unable to provide an earth. Its down to the contractor with TT .

Deke

 

[Andy™]

Bit pedantic there Andy ? Thats what I meant , the fault current will be in milli amps therefore 10mm is overkill. ( See page 32 OSG) As regarding the RCD failing , that is in the hands of the IEE , they make the rules, we do as we are told ,do we not ? By the way , you said you used 10mm yourself !!!

What do you suggest then , should the RCD fail , what should we be doing ?

Deke

If the RCD fails... it fails. BRB says there OK to use so they must be used!

Maybe we need a new design of RCD - like fail proof designs. we all know MCBs dont get tested, and rarely fail, so someone has got to come up with an RCD that doesnt stick and wont fail

dont come across many TT round here, mostly TNS or TNCS, so dont keep 6mm in the van. so 10mm gets used. Customer is paying for cable anyway

 

[Evans Electric]

You raise a good point Andy, I have never heard of a failed MCB ,that is to say , it won't trip. However I 've replaced numerous RCD,s , often they do not even trip with the test button. And lets face it no one tests them as recommended, do they?

As I said in an earlier post , we get more TTs in the city these days as the network cast joints are rusting away. That brings me to another point, it is obvious that other sparks have been to these properties, new kitchen, shower etc but they never check or do anything about the high Ze that we find now on TNS supplys.

 

[Andy™]

You raise a good point Andy, I have never heard of a failed MCB ,that is to say , it won't trip. However I 've replaced numerous RCD,s , often they do not even trip with the test button. And lets face it no one tests them as recommended, do they? As I said in an earlier post , we get more TTs in the city these days as the network cast joints are rusting away. That brings me to another point, it is obvious that other sparks have been to these properties, new kitchen, shower etc but they never check or do anything about the high Ze that we find now on TNS supplys.

but when was the last time you 'test tripped' an MCB?!

Most failed RCD's ive came across, flick it on/off a few times usually gets it to trip/within times, but still not good.

Rarely have a high Ze on TNS round here (aint i lucky!), but when it does happen, cant fault NEDL. gets repaired straight away

 

[green-hornet]

Very strange to hear of so many rcd failures.

There was a study carried out by the electrical safety council and out of say 12% of failures they sent to the lab for testing only 2% or 4% actually failed there.

This is because of the fact that the removal and transporting of the rcd was enough to dislodge any dust build up that could of caused the malfunction.

They concluded that the regular testing via the test button was enough to maintain a good working rcd.

I could give you the exact figures but that means looking and its a bit late.

 

[Evans Electric]

but when was the last time you 'test tripped' an MCB?!Most failed RCD's ive came across, flick it on/off a few times usually gets it to trip/within times, but still not good.

Rarely have a high Ze on TNS round here (aint i lucky!), but when it does happen, cant fault NEDL. gets repaired straight away

Interesting , in the Midlands , usually, its down to you, can't give you PME or cable sheath , its your problem. On commercial jobs there is always an earth but not domestic !!