Zs values on TT system

[pburley]

Hi guys new to testing and about to do my first TT system tomorrow. Popped in today and did a quick Ze test and got 16.98 ohms. Zs values will not meet max values for 7671. Whats your thoughts guys. (what have I forgotten?). Pete

 

[telectrix]

Ze on a TT system should be no more than 200ohms (100 if you believe the NICE boys). As this means your Zswill be too high to meet disconnection times RCD all circuits.

 

[Guest123]

411.5 onwards, page 50 BS 7671.

 

[pburley]

Hi Telectrix, so if my R1+R2 for a ring circuit is say 0.40, this would give me a figure of 17.38 for my Zs. 7671 states for a bs60898 MCB max Zs of 1.16. This is way over but my RCD covers circuit, is that right?. Pete

 

[pburley]

Cheers Lenny reading it now. Ta

 

[Ponty Massive]

It's due to the fact your on a TT system that your Zs is way over, hence the need for RCD on a TT system.

 

[pburley]

Cheers guys. Many thanks.

 

[telectrix]

yep, max. Zs on TT is 1667ohms. that's what you put on the test sheet as max. Zs allowed. but as we have all said RCD is necessary.to achieve disconnectin time of 0.2sec.

 

[dochisholm]

isnt it 0.4 secs telectrix

 

[dochisholm]

my mistake 0.2 on tt sorry