TT testing question

[driverman]

Hi, Doing 2395 revision and doing a small temp builders supply therefore TT . Installed 2 x 13a sockets protected by 16amp breaker and 30ma RCD.
Installed 1 x earth rod immediately by the meter / con-unit position. No other circuits. No additional main equipotential bonding as nothing there.
My Ra (Ze) reading was 10 ohms using an ELI tester method 3 BGB. I'm under the impression that this reading is OK as under 21 ohms but I'm unable to find this figure in writing. But reading other threads I see figures of 200 ohms. The figure of MAX 1667 ohms baffles me for 30ma RCD. RaIn<50v. Clarification required please. Apologies in advance as I'm sure this subject has been flogged to death on here.

 

[kingeri]

200ohms is the max. allowable result on a TT, but you should be aiming to get it as low as possible. 10ohms is pretty good for a starter! Don't forget that that figure would likely come down over time on a permanent install, as the dirt settles and compacts around the rod. The max figure of 1667 for an RCD illustrates why RCDs are so essential on TT installs, as you'd be hard pushed to meet disconnection times if using breakers or fuses alone. Just don't fall into the trap of thinking that RCDs are quick fixes in TN installs!

 

[KAS1]

50/0.03=1666 ohms

 

[driverman]

200ohms is the max. allowable result on a TT, but you should be aiming to get it as low as possible. 10ohms is pretty good for a starter! Don't forget that that figure would likely come down over time on a permanent install, as the dirt settles and compacts around the rod. The max figure of 1667 for an RCD illustrates why RCDs are so essential on TT installs, as you'd be hard pushed to meet disconnection times if using breakers or fuses alone. Just don't fall into the trap of thinking that RCDs are quick fixes in TN installs!

Thanks for that........does this fIgure of 21 ohms exist? i can remember the guy teaching us saying 21ohms as he referred to the 21 year old age joke, thats how I remember it.

 

[kingeri]

21ohms is the max. value for the DNO's earth electrode, and isn't a figure that concerns you in terms of installing a rod for TT!

 

[Sparks1962]

10 ohms is bloody good, what did you do, chuck a bucket of water on it? lol

 

[telectrix]

beaten by kock it up kingeri. exactly what he said.

 

[kingeri]

Thankyou Tel.

 

[telectrix]

yr. welcome!. at least you ain't drilled a gas pipe.LOL.

 

[driverman]

10 ohms is bloody good, what did you do, chuck a bucket of water on it? lol

Well i was surprised myself.....It's the first TT that I've done under BS7671. The earth rod went in easily using lump hammer, only took a few mins! I was half expecting it to take a while to get it knocked in! I used to put rods in many many years ago into chalk, took forever in those days! i've been out of contracting for 16 yrs.

 

[Flanders]

See page 57 of the BGB table 41.5.

 

[MDJ]

beaten by kock it up kingeri. exactly what he said.

Yup agree

 

[driverman]

See page 57 of the BGB table 41.5.

Thanks, yes I've been reading that and OSG book.....But was still confused by the 21 Ohm figure. Kingeri has explained that part and how 1667 is obtained.

 

[Sparks1962]

Well i was surprised myself.....It's the first TT that I've done under BS7671. The earth rod went in easily using lump hammer, only took a few mins! I was half expecting it to take a while to get it knocked in! I used to put rods in many many years ago into chalk, took forever in those days! i've been out of contracting for 16 yrs.

We get all sorts here.. sand, heavy clay in some areas. Ok to knock in and never need much over 50mm CSA rods apart from in sand. You've been away quite a while, I keep thinking about it but that's as far as it gets!