Max length of cable runs.

Ray Pooley · Apr 5, 2022

Hi

I'm looking at Table 7.1(i) in the On Site Guide. Max length of cable runs.

In some cases where there is no RCD protection the length is given as a number with a zs suffix. eg 47zs

The zs suffix means "limitted by Zs" the final circuit line-earth loop impedence.

In these cases is the number part the actual max length? For example 47zs = 47 meters?

Thanks

Julie. · Apr 5, 2022

Ray Pooley said:
Hi

I'm looking at Table 7.1(i) in the On Site Guide. Max length of cable runs.

In some cases where there is no RCD protection the length is given as a number with a zs suffix. eg 47zs

The zs suffix means "limitted by Zs" the final circuit line-earth loop impedence.

In these cases is the number part the actual max length? For example 47zs = 47 meters?

ThanksView attachment 96622

Yes 47 metres max length, - but as this is due to Zs then if the Ze is bigger then normal, then obviously may be much shorter.

Ray Pooley · Apr 5, 2022

Julie. said:
Yes 47 metres max length, - but as this is due to Zs then if the Ze is bigger then normal, then obviously may be much shorter.

Thanks Julie. Appreciate it.

pc1966 · Apr 5, 2022

The "unlimited" case is voltage drop and you see it has no change between RCD/not and TN-S / TN-C-S

Most other limits are 'zs' for circuit Zs as above, however, there are a few of cases where it lists 'ad' for adiabatic limit being reached (in most cases simply not permitted, and if mentioned at any length then a reason to consider the design in more detail).

Oddly some cases just say 'NP' without a reason (e.g. 20A BS3036 with 2.5 T&E)!

Ray Pooley · Apr 5, 2022

pc1966 said:
The "unlimited" case is voltage drop and you see it has no change between RCD/not and TN-S / TN-C-S

Most other limits are 'zs' for circuit Zs as above, however, there are a few of cases where it lists 'ad' for adiabatic limit being reached (in most cases simply not permitted, and if mentioned at any length then a reason to consider the design in more detail).

Oddly some cases just say 'NP' without a reason (e.g. 20A BS3036 with 2.5 T&E)!