Does a TT Earth 0.13Ohms require a 100mA incomer?

[Energetic]

Had an interesting one this week, customer assures me he's on TT, DNO days the same but i'm getting 0.13Ohms on the Ze, never seen it that low befor.

There's no 100mA RCD on the supply and it doesn't look as if there's ever been one, so with that reading i'd assumed it was a TN system... so my question is do i now need to put on it as its TT? I'm assuming i do but with a reading that low is it still required?

My Regs book is in the van elsewhere... Thanks.

 

[Engineer54]

0ne of your figure relates to network substation earthing, that is frankly always exceeded and the other is just total nonsense!! lol!!

 

[tbs1983]

ok sorry! my bad, hope everyone enjoyed a chuckle at my expense!

Clearly the lower the better but I just had 21 ohms in my head from somewhere. anyone care to explain the significance of this figure I would greatly appreciate it!!!

 

[Richard Burns]

The OSG mentions the 21 ohms for TT but it is referring to DNO supplies and the 21 ohms is a technical maximum value that would never be accepted for a distribution network..

 

[tbs1983]

Thanks Richard, i must have been on another planet when I posted my first comment!
Although Engineer54 claims my max 200 ohms comment is nonsense. Is this true? BS7671 states 200 ohms does it not? although I believe the NICEIC like to specify 100 ohms max.
Perhaps Engineer54 has been brainwashed by Tony Cable and his band of merry 'reg making' men.

 

[trev]

I recall the figure 21 ohms is the lowest a TT system can be and 200 ohms the highest. would need to check up on that though

Oh Bugger! I'd best nip up to Cleadon and pull out a couple of rods that gave me an Ra of 0.29 then

 

[tbs1983]

kick me while I'm down Trev

 

[trev]

kick me while I'm down Trev

Absolutely mate

 

[trev]

I'd struggle like hell to get those rods out now anyway mate, the ground will have consolidated around them by now and Ra may well be even lower.

 

[tbs1983]

Ive heard of people sprinkling copper filings in the soil around earth rod to improve reading, was this common practice at any time or just a 'trick of the trade' ???

 

[trev]

I've not heard that one before

 

[SPARTYKUS]

Weeing on it i've heard of. Not copper filings.

 

[jimjimjim]

Sounds like a cheap form of marconite!!

Jim

 

[Engineer54]

Ive heard of people sprinkling copper filings in the soil around earth rod to improve reading, was this common practice at any time or just a 'trick of the trade' ???

Nope it's not any trick of the trade that i've ever seen or heard about. Powered carbon in a mortar mix i know of , but never heard of copper fillings. Can't see as that will make any difference myself, copper isn't exactly moisture retaining is it!! lol!!

 

[tbs1983]

fair comment, just heard of it a few years back but never actually seen it done or have any science to back it up

 

[Silly Sausage]

.... copper fillings. Can't see as that will make any difference myself, copper isn't exactly moisture retaining is it!! lol!!

I reckon they'd rapidly sulphate, ie salt, thereby increasing the ground conductivity.