High Loop Impedance Readings (Tt System)

[whudean]

hi there, im an electrician for a local council so havent really ever had to work on TT systems

my auntie called me up to say she was having her boiler repaired and the gas man tested the boiler point and said there was a earthing problem

i go round and plug in my testers and get 260ohms, i plug in various sockets and get the same readings knowing full well that its far too high.

im after any help and suggestions on what i can do and if its a common problem

any more questions please feel free to ask. thankyou

 

[steptoe]

260 ohms isnt that bad a reading on TT at all, its nice to have it under 200 ohms , and, obviously the lower the better, but 260 could be seen as acceptable.

is there RCD protection on all circuits?  at least 100mA

I know a lot of local councils where TT is the predominate earthing system BTW, 

 

[whudean]

hi steptoe thanks for your reply.

my local council unfortunately doesnt have many TT system properties, i sometimes wish they did and im not afraid to admit that i dont know alot about TTsystems as i have never had to work on them.

the 260ohm readings steptoe is on all the sockets when i plug in my earth fault loop impedance tester, not at the fuseboard itself, so your saying these readings are generally ok??

the tails and 25mm earth go to the fuseboard which is an old crabtree D/B and has a main rcd switch which conrols 8circuits

i was only there for a short while and didnt check the RCD is 100mA? will get that informations shortly

thanks for your reply

 

[SPECIAL LOCATION]

Have a read of regs 411.5.1 thro 411.5.4 in your BS7671  page 57..

and 3.5.3  in On Site guide page 25..

and 10.3.5.1  in OSG  page 99...

that should refresh your mind about TT's

Guinness

 

[batty]

I would first check the connections on the rod if you can find it if they are OK maybe try putting another rod in.

I have just finished a job TT system 6mm going to rod tested and have over 400 ohms. Now this is on a main switch GE consumer unit with no RCD protection anywhere. They did have problems but all now has been upgraded new rod, bonding to water and gas now with a reading of 60 ohms on sockets which I am happy with. I have also put in RCBO consumer unit.

 

[Doc Hudson]

Welcome to the forum, you do not say what RCD protection (if any) is in place on this installation. If there is 30ma then BS7671 states that in relation to touch voltage the max permissible is 1667ohms,  (1667x30ma=50v). However the proviso that a value above 200ohms may be unstable is also given. So your 260ohms, if the ground is dry, may be acceptable. It is not uncommon for Gas men to have limited knowledge and misinterpret wiring regulations and the readings they have taken. So I would not worry about what the Gas man says, just do your own tests and check them back with the wiring regulations. Any bits you are unclear about just come back and ask on the forum. We have some very knowledgeable guys/girls who are very willing to help.

Doc H.

 

[whudean]

Doc H, thanks for your reply. I will pull out the regs book now and have a good look.

The consumer unit had a main RCD switch which is 30mA

I did tell her not to worry about the gas man, but was unclear on what is acceptable and unacceptable on TTsystems. But will have a good read up today.

Thanks

 

[dave2]

Had exactly the same problem today.

Customer calls to say BG have failed the installation as EFLI is >200 ohms.

TT system wired to 16th edition with 100mA main switch and 30mA protecting sockets.

Called in and my reading  was 112 ohms.

Methinks the gasmen need some extra training so as not to unduly worry clients.

 

[Steve3948]

BT used to do this when installing powered systems in large manor houses, basically doing what they are programmed to do and not understand why.