Domestic PIR & maximum permitted Zs on a TT system

[Revved Up Sparky]

I have recently completed a certificate for a PIR on a TT system.

Another spark challenged me today for putting 200 ohms in the column where you enter the maximum permissible Zs. He said I should be entering the values as stated on the table which is on the inside page of the NICEIC book of certificates I use, eg 1.15 ohms for a 60898 B 32A MCB, 6.13 ohms for a 60898 6A MCB etc.

Well I responded by suggesting that he was incorrect and that those values on that table on the inside of that page are for TN systems only.

What is the point of putting 1.15 ohms down as the max permitted Zs when you know it's TT and the reading will be something like 147 ohms ?. Surely I would then have to enter a note that the circuit has failed the Zs test ?

Are the values on that table on the inside of the test certificate for TN systems only ?..... who is right ?..... please say me .... -- otherwise I wil have to tell a joke.

 

[steptoe]

I have recently completed a certificate for a PIR on a TT system.Another spark idiot challenged me today for putting 200 ohms in the column where you enter the maximum permissible Zs. He said I should be entering the values as stated on the table which is on the inside page of the NICEIC book of certificates I use, eg 1.15 ohms for a 60898 B 32A MCB, 6.13 ohms for a 60898 6A MCB etc.

Well I responded by suggesting that he was incorrect and that those values on that table on the inside of that page are for TN systems only.

What is the point of putting 1.15 ohms down as the max permitted Zs when you know it's TT and the reading will be something like 147 ohms ?. Surely I would then have to enter a note that the circuit has failed the Zs test ?

Are the values on that table on the inside of the test certificate for TN systems only ?..... who is right ?..... please say me .... -- otherwise I wil have to tell a joke.

you are kinda half right, but more half than he is,

the tables are based on TN systems only,

have you looked at the notes on tables 41.2 41.3 & 41.4 ?

you are technically allowed much more than 200 ohms ,

usually when I am doing PIRs on TT I simply put down N/A in that column.

tell your 'mate' to have a look at table 41.5

 

[Revved Up Sparky]

Thanks for that Steps, much appreciated.

 

[steptoe]

you are very welcome rev,

I think IIRC its Ra x In <= 50v (maybe) :|

 

[batty]

That was lucky bad joke avoided.

O) (hells)

 

[NozSpark]

I always put 1667 ohms for 30mA protected circuits

The 200ohms is just what they'd like it to be below, not what it should be below IYKWIM?

 

[m4tty]

I got pulled last year on my assessment for putting 1667 in instead of the lower figures. I was told the lower figures go on the cert. Prob just my assessors opinion tho.

 

[NozSpark]

What do you mean by "lower figure" Matty?

200ohms or 1.44ohms (say for a B32)?

While your (R1+R2) should comply if it were connected to a TN earth, it isn't and those values have no relevance on a TT installation.

 

[m4tty]

Yeah noz I meant 1.44 etc and this was on a tt and I put 1667 in the column and he said I had to put 1.44 etc. I hope I havent got the wrong end of the stick as I'm quite nervous as got assessment in a minute lol

 

[Sidewinder]

I put the 1,44 type values on my certs so it makes the people behind me think about the job, my AE is fine with this

 

[kme]

As do I; though I wouldn`t slate someone for putting 200; or even 1666 (1667 is technically too high, by a third of an ohm)

 

[gselectrical]

I usually put 1666 ohms (for a 30mA RCD). My assesor is quite happy with this. If I record the tabulated value and then record the actual reading obtained, Easycert usually flags it up. So in my opinion it is acceptable to record the 1666 ohms value.

GS

 

[ALFRED]

If you go table 41.5 in the regs it gives you the values of Zs for a T.T. installation. It all depends on the RCD in use i.e from a 30mA ( 1667 ohms ) to a 500mA ( 100 ohms ). It then warns you in a note below the table that the rod resistance shouldn't be above 200 ohms because it could be unstable. ( The NICEIC ask for a 100 ohms )

Alfred ( the new bloke )

 

[batty]

If you go table 41.5 in the regs it gives you the values of Zs for a T.T. installation. It all depends on the RCD in use i.e from a 30mA ( 1667 ohms ) to a 500mA ( 100 ohms ). It then warns you in a note below the table that the rod resistance shouldn't be above 200 ohms because it could be unstable. ( The NICEIC ask for a 100 ohms )Alfred ( the new bloke )

Thats great Alfred are you trying to confuse us and welcome to the forum.

 

[ADS]

If you're using a RCD for earth fault protection,(regardless of earthing system), you can't really put 'maximum allowed' as 1.44 and then put a higher figure down as your measurement.........how can your reading be higher than the maximum allowed??

C'mon!!

 

[Badger]

One wordAnd it begins with a 'B'

:slap

b !! CK $ :^O

 

[steptoe]

BEER

 

[batty]

Use the iee certs that way you don't have to put anything down. Have they changed them in the greeny book.

 

[ADS]

Which side of the fence are you on ADS ? Would you put 1.44 or 100,200 or 1666.666 ?

If I was using a 30 mA RCD as earth fault protection, I'd put 1667.

Just as I would put 500 ohms, if I was using a 100 mA RCD for earth fault protection.

 

[ADS]

I don't understand what you're getting at - my cert has a 'Maximum permitted Zs', so, if I was using a RCD for earth fault protection, that's where it would go.

If my earth fault protection was provided by an MCB, and the RCDs were only 'additional protection', then I would put the standard figures in the column.

What are you playing at??