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......

 

[GLENNSPARK]

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!!

not in my world it dont...

i would certainly be noting it...

 

[sledgehammer]

There's right, there's wrong and then there's the grey area. As has been said before I'm sure it's used to comply for tt installs!! But I ask this, if the Zs is too high then why? R1&R2? So what about short circuit impedance? If single phase and all the same conductor size and you can't meet Zs then you ain't gonna meet SC.

 

[GLENNSPARK]

There's right, there's wrong and then there's the grey area. As has been said before I'm sure it's used to comply for tt installs!! But I ask this, if the Zs is too high then why? R1&R2? So what about short circuit impedance? If single phase and all the same conductor size and you can't meet Zs then you ain't gonna meet SC.

no...

whats Zs got to do with Zpn/Zpp?

 

[Engineer54]

not in my world it dont...

i would certainly be noting it...

Nor in mine, and you're right i'd certainly be noting it too!!

I was basically just pre empting wirepuller coming in and stating (quite correctly) that you would only be testing against BS7671 conformity, and as such, RCD values conform... lol!!

 

[Luke1989]

His Reply "Your comment is noted but look at 411.4.9 which refers to 411.3.2.2 which obviously enable use of table 41.5, regarding MCB part of the RCBO this will only used for against overload currents and fault currents in accordance with chapter 43. Which means RCBO is not just a switch. Which is all subject to RCBO tripping times in accordance with reg's" Thoughts?

 

[Richard Burns]

In a TN system you can use an RCD for fault protection under ADS in compliance with the minimum standards for installation as provided in BS7671.
However good practice would be to use all the available means of protection, which would include designing a circuit that can use both the RCD and the MCB for fault protection and so allows greater protection thatn the reliance on a single device.
This is a simple matter and does not cause any problems in design so why not do this.
Obviously in a TT system you do rely on the single RCD unless you can get TN levels of Ra, but this is costly and difficult to do in general domestic situations.