TT installation queries

[harrymonkfish]

Hi there.

I'm a bit confused about the values taken while testing a TT installation. I need enlightening.

Sub main from house 16mm SWA into 4 way CU with 30Ma main switch/RCD.
16mm earthing conductor.

1 ring
1 radial sockets
1 light circuit

Ze = 164 ohms
PFC = 1.4 amps
Zs of ring = 184 ohms

R1+R2 ring 0.34 ohms
R1+R2 radial 0.05 ohms
R1+R2 lights 0.85 ohms

Comments appreciated.
Harry

 

[calv68]

nothing wrong with them mate your ze is a little high but anything under 200 ohms is deemed to be safe

 

[Amp David]

If your Zs of ring is 184 ohms, then how can the r1+r2 only be 0.34 ohms?

 

[Des 56]

The Zs would usually be less than the Ze and R1+R2 values.(especially, much reduced, if there are parallel earth paths)

Is the rod local to the sub main load end or at the supply side ? (did the testing include the R1+R2 of the SWA) It still would not make up the 20 ohm excess

If not, was this value obtained with an earth loop tester, with the" no trip" function, which is sometimes very inaccurate and therefore ignorable?

Your reading is within recomendations.but there is a question mark against those stated values

The PFC of 1.4 amps ,unless you meant 1400 Amps and forgot the K or you are measuring the prospective earth fault current only
The prospective short circuit current would give a much higher reading than the earth fault current in a TT system

 

[calv68]

thats what he said pfc 1.4amps x ze of 164ohms = 229.6volts those readings are correct

 

[harrymonkfish]

The rod is local to the second CU, about 3 metres.

The value was obtaineed with a loop tester with the no trip function on. should i test again with it off?

 

[calv68]

it dosent make a difference mate with the trip on the test just takes a bit longer you should only have it on when doing your zs readings though

 

[harrymonkfish]

so apart from the discrepency between the r1r2 and zs compared to the ze reading, everything is ok you think?

on the test results cert, the section which asks for the max Zs permitted by 7671, what value should i put? 1667?

thanks

 

[calv68]

thats fine and you dont have a discrepency on your ring circuit because thats what your reading is .you will not get the same results by calculating zs