Max Zs value

[Luke1989]

Hi

Would anyone be able to help me,
Max zs value for type c 32amp RCBO on PME system- 0.72/0.58 or 1776 ohms?

Any help would be appreciated

 

[kingeri]

The RCD element of the RCBO is their as additional protection on a PME system. You need to meet the disconnection times for the MCB element.

 

[Octopus]

0.72/0.58 imho

 

[Luke1989]

Thanks, I agree but was wondering (as I have to explain to a customer) if/what the reason you can't use 1667

 

[kingeri]

The RCD is additional protection only. What would happen if the RCD failed? Whilst they are great as additional protection, they should not be relied upon as the sole means of protection, as you effectively would be doing if you went with such a high Zs figure! The MCB would become basically just a switch!

 

[telectrix]

you need to design the circuit so that the max. Zs is as specified for the OCPD, but there's nothing to say you can't enter 1667 in your SOTR.

 

[silverfox1]

Have a read through this thread Luke .

http://www.electriciansforums.co.uk...-electrical-forum/81261-exceeding-max-zs.html

 

[GLENNSPARK]

yet again we have posters thinking its ok to `cheat`....using an RCD to get round Zs issues....and thus not design circuits properly....

 

[Engineer54]

This i suppose very much depends on whether this is a ''New Circuit'', or an existing circuit that's been altered etc. It seems if existing it's OK to rely on the 1667 ohms of an RCD!! Not my way of doing things, i might add, i would always find the reason for exceeding Max Zs and rectify as needed/required. But if this is for say, a periodic test report purpose, going by BS7671 the circuit conforms as such!!!!! lol!!

 

[TC SPARK]

Agree with previous post. The RCD is there for additional protection, correct installation methods being meet would mean correct/complying Zs values. Don't use RCD's as a cheat to justify high Zs readings, Investigate as there could be loose connection etc......