max zs and rcds

[sellers]

if a circuit is a bit over its max zs but is rcd protected does this make it still safe and to regs with the max being 1667ohms? thanks

 

[Brian 157]

The RCD ensures your circuit meets the requirements for disconnection times, (as on most TT installations). How much over the maximum Zs is the circuit & why is it over? If it's a bit over the OSG or "rule of thumb" value then calculations applying the relevant correction factors may prove it to be within limits. If the circuit's Zs is too high is it because the circuit is too long, in which case is it meeting the requirements for volt drop?

 

[ADS]

if a circuit is a bit over its max zs but is rcd protected does this make it still safe and to regs with the max being 1667ohms? thanks

Hi,

I was going to reply:

"In a word, No":yellow card

Then I read 411.4.9 again

Where an RCD is used to satisfy the requirements of Regulation 411.3.2.2, the maximum values of earth fault loop impedance in Table 41.5 may be applied for non-delayed RCD's to BS EN 61008-1 and BS EN 61009-1 for final circuits not exceeding 32A.

411.3.2.2 ...Maximum disconnection times for final circuits

Table 41.5 ...Max Zs (i.e. 1667)

So now I would have to say yes, it's fine

Although you have to wonder why we measure Zs at all - "just bang an RCD on it" ha ha

 

[Brian 157]

"just bang an RCD on it"

Pretty much sums up the 17th Edition really...

 

[ianmacd]

The RCD ensures your circuit meets the requirements for disconnection times, (as on most TT installations). How much over the maximum Zs is the circuit & why is it over? If it's a bit over the OSG or "rule of thumb" value then calculations applying the relevant correction factors may prove it to be within limits. If the circuit's Zs is too high is it because the circuit is too long, in which case is it meeting the requirements for volt drop?

I guess it could be older cable that has a reduced CSA of the CPC so VD may not be an issue but having problems with Zs still.

Agree though that its not good practise to have Zs too high.