Getting confused with a Zs / time - regs in contradiction??

[Rockingit]

Ok, I'm trying to work out whether or not I can put a C50 protected final circuit onto an SNE supply which already has a Ze of .67

Section 411 Table 41.3 (new book) gives me a max value of Zs as .46, so we need to use an RCD...right?? 411.4.9 goes on to say that you can do that for a non time delayed RCD, and use the values for a BS88-3 (.72 which still doesnt get me very far) - so why then does the list in 41.3 include RCBO's to 61009??

Or am I reading the whole thing wrong and need to go back to school?!

R.

 

[telectrix]

41.3 refers to the overcurrent characteristics of RCBOs. refer to table 41.1 and 41.5

 

[Rockingit]

OK, but 41.5 refers to Zs's for TT systems as note 1 refers to 411.5.3 (i) and (ii), both of which refer to TT systems and circuits not exceeding 32A.

Ah! Got it. Reg 411.3.2.3 is the get-out clause for something not covered, so long as it doesn't exceed 5s.

So.... can anyone see an issue with using a 300mA RCD and 50A C type as protection for a 1ph final circuit to a 12kW ground source heat pump fed on 10mm singles in trunking, with a Ze at origin of .67?

 

[telectrix]

well, as it's a circuit >32A and cables not buried in walls <50mm, you won't need a 30mA RCD. you would be using the 300mA RCD for fault protection so the max. Zs would be 167 ohms. sounds good.

 

[Rockingit]

Cheers Tel, I 'knew it should be' but trying to prove it was just defeating me - goes to show, two heads better than one (especially with the morning I'm having!!!).

This is why we went to sparky school and don't do £50 back pocket bungs in the pub. At least we know how to make something catch fire properly!!