Max length of cable runs.

[Ray Pooley]

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.]

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]

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]

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]

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)!

Voltage drop obviously.

I'm sure if I stare at it long enough I'll spot some kind of rational pattern.

Not yet though.

Cheers