PIR

[Andy™]

Thanks Andy, thats a lot clearer. Dont come across TT much up here.

not much TT and your in wales!? even here in a built up area, there are lots of TT

 

[rob196]

[quote name='Andy

 

[rob196]

Have just done a PIR and now looking for some help if possible guys.... house with a TT system put no access to the Earthing rod, earth cable just dissapeares under the front of the house, no garden just a public pavement so all concrete and tarmac.......Ze reading is 157ohms .....

Zs reading range from 19.4-20 ohms

all other readings are fine R1+R2 etc, etc

6 circuits, four 5A, 32A & a 45A.........With a 80A main switch/30ma ELCB ( please see pics) not sure if ELCB'S are even in the regs?.....can anyone tell me what the breakers are? bs 3871 perhaps? or even the type of cut out fuses used? And any ideas for coding this PIR?

Forgot to say the PFC value is 12KA.........

 

[Andy™]

So the max zs is a bit high but is rcd protected does this make it safe and to regs with the max being 1667? Table 41.5.............411.4.9.....althought it,s only for final circuits not exceeding 32A?

if using a 30mA RCD, max Zs can be 1667 ohms.... but in reality, if you have anywhere near that, something is seriously wrong

 

[Andy™]

Forgot to say the PFC value is 12KA.........

thats a high PFC. substation next door?

the MCB's are rated to 6KA, but the service fuse will be rated to much higher so covers your PFC

 

[netblindpaul]

[quote name='Andy

 

[revjames]

[quote name='Andy

 

[sparkytim]

Andy,I had heard/knew this, but I can't for the life of me track down the origin of the statement/info.

Any suggestions please?

Ta,

BS 7671 REG 434.5.1 page 76

Characteristics of a fault current protective device

A lower breaking capacity is permiteed if another protective device having the necessary rated short circuit breaking capacity is installed on the supply side.

i.e BS 1361 TYPE 2 MAIN FUSE RATED AT 33 KA

 

[Andy™]

^^ what he says

 

[spinlondon]

Strange, a lot of the properties in Neath are TT.

Would the hidden rod be a code 4, as the earth conductor connection cannot be checked now?

 

[rob196]

BS 7671 REG 434.5.1 page 76 Characteristics of a fault current protective device

A lower breaking capacity is permiteed if another protective device having the necessary rated short circuit breaking capacity is installed on the supply side.

i.e BS 1361 TYPE 2 MAIN FUSE RATED AT 33 KA

Why is it when you read the regs it always sounds DOUBLE DUTCH, have to read it at least three time before i can even get the jist of what it means....anybody else have this trouble?.......... headbang

 

[rob196]

Strange, a lot of the properties in Neath are TT.Would the hidden rod be a code 4, as the earth conductor connection cannot be checked now?

I'm thinking of putting it as a 2 ( needs improving ) or a 3 ( requires further investigation ) after all with a reading of 157ohms i think it can only get worse not better........

I know there are are lot of different views but could it be possible to change the earthing system?

 

[Andy™]

Why is it when you read the regs it always sounds DOUBLE DUTCH, have to read it at least three time before i can even get the jist of what it means....anybody else have this trouble?.......... headbang

that reg is fairly straight forward IMO. but there are many others which are open to debate

 

[rob196]

PSC was 6ka ( L-N ).................PFC was 12 KA ( L-E ) Think this would suggest that there is a major problem with the earth rod that i cant see????

 

[Andy™]

PSC was 6ka ( L-N ).................PFC was 12 KA ( L-E ) Think this would suggest that there is a major problem with the earth rod that i cant see????

was this taken with bonding in place?

 

[rob196]

[quote name='Andy

 

[Andy™]

YES........

may be correct then. but its unusual for loop to be that ow/PFC so high

 

[rob196]

[quote name='Andy

 

[Andy™]

your looking at it the wrong way.....

if you have such a high PFC, then its because Ze is very low - with bonding in place, it may be sharing a metallic pipe to next property wich may be TN, so you get Zs of what you would find on TN

if it was a corroded connection on rod, then Ra would be high, but unless you can visually check it, the high resistance may just be the rod and not a dodgy connection