Earth fault loop impedance reading on TT System in domestic premises

[whitebeetle]

Hi All , I am new to this forum but am an industrial electrician (20 years) but have been using my Robin KTS1620 Multi tester in my house to do a few tests as i am considering returning to partp work as my industrial work is quiet.
The house is supplied by a TT System through a 100mA RCD.
I connected my tester to the ring circuit and found the earth fault loop impedance to be 40 ohms and the prospective short circuit current to be only about 5 Amps !. I decided to investigate the earth and found the earth spike to be buried near to the road. I disconnected the lead from the main earthing terminal that goes to the earth rod but cannot get an earth loop impedance reading through the rod ? Does this mean the rod is failing to connect to earth , if so am i right in assuming the 40 ohm reading must be got through the gas / water pipe bonding ?
Also i assume i should now be fitting a 30 mA RCD in series with the 100mA RCD for greater protection ?
Any help appreciated
Chris

 

[Guest123]

Hi Chris, and welcome along.

Sounds like you have a bit of a problem there. With no earth being provided by the earth rod then yes, the earth you are getting is via the services pipework.

You really need to be installing a new rod/rods ASAP to provide you with a good earth path. Regs say should be less than 200ohms, some electrical bodies such as NIC say less than 100ohms.

What type of DB do you have at the mo?? as installing a 30mA RCD up front to protect all circuits isn't really the best option and infact contravenes the regs.

Cheers

 

[Darkwood]

If you find difficulty achieving a permitted earth reading try a few earth rods in series as far apart as possible and as with bonding not cutting the earth wire at each rod but looping unbroken this might help, i do recall from my apprentice days i was told to pee on the ground by the rod to improve the conductivity of the ground but not really sure thats acceptable lol.

 

[whitebeetle]

Hi Thanks for fast response - most helpful- typical that my house that i was gonna replace consumer unit on for assessment happens to be on a TT System with dodgy earths !

I can't understand why i can't get a reading on the earth rod at all ? I connected the three wire input to the tester with live/neutral connected to incomer to main switch (with switch off) and earth lead to end of earth rod but just reads overload - thought i'd get something.

Whats the situation with regs then regarding 30mA RCD - Is the 100mA rcd OK then because its TT Because i know with TNC / TNCS protection required now is 30mA.

Sorry for all questions but not worked much on TT
Chris.

 

[Guest123]

Hi.

I dont mean to tell you to suck eggs, but have you gone up through the ranges on the meter i.e 20, 200, 2000.

I would find it very hard to believe that the RA was over 2000ohms!! unless of course it's totally broken off.

Basically with the new 17th edition, near enough all circuits now need 30mA RCD protection it's best to use either a dual 30mA RCD board with circuits spread accross the two RCD's i.e upstairs lights and downstairs sockets on one and vice versa.

You could also use a main switch board with individual RCBO's for each circuit (a litttle more expensive) but this does minimise the inconvenience in the event of a fault.

 

[whitebeetle]

Yep have gone up through 20,200,2000 and nothing ? Perhaps like you say its broken of but what a pain as its buried in concrete at the side of the road !
Think what i'll do is bury a piece of 10mm cable in the garden and perform a test just to check i get a reading before redoing the job properley - what you think

 

[Spudnik]

Just install a new rod.

Even if you had a good reading on the old one, it would still need to be accessible for inspection.

Now you know why!

 

[electricAl]

Hi Whitebeetle.

I assume you meant your PEFC was 5amps & not PSCC ?

Also, is your 100mA RCD slugged (S-Type) If not then putting it in series with the 30 won`t give you discrimination.
If not & depending on other factors being acceptable (& you`re currently struggling with one of the major ones) you could do away with the 100 if you fit the split dual RCD board as recommended (basically treat as TN system)

You`ll need that twin board or RCBOs irrespective of what type the 100 is unless the customer doesn`t want lighting in the bathroom or you can mechanically protect some other light cct

 

[calv68]

dont mess about mate just bang a new rod in like jason said by the time you have spent trying to get a reading of a rod thats been there for years you could have had the new one in