UK Maximum Zs Value Exceeded

[toms22]

Hi

I have recently done some testing on an installation. A submain is fed from a B50 mcb and has a zs of 1.10ohms. The maximum for this is 0.87. The cable supplying the submain is a 25mm 2 core swa with the armourings used as the cpc and is roughly a 70m run. Both glands and earth leads have been checked and are okay.

Would it be viable to fit a 300ma rcd to protect this circuit as the disconnection time cannot be achieved. The problem I have is the building supplied by this has two large heaters and a number of vacuums and there is an earth leakage of around 150ma.

Each circuit in the submain has individual 30ma protection.

Upgrading the cable/running a separate earth alongside the swa would not be possible due to the nature of site.

 

[mainline]

Hi

I have recently done some testing on an installation. A submain is fed from a B50 mcb and has a zs of 1.10ohms. The maximum for this is 0.87. The cable supplying the submain is a 25mm 2 core swa with the armourings used as the cpc and is roughly a 70m run. Both glands and earth leads have been checked and are okay.

Would it be viable to fit a 300ma rcd to protect this circuit as the disconnection time cannot be achieved. The problem I have is the building supplied by this has two large heaters and a number of vacuums and there is an earth leakage of around 150ma.

Each circuit in the submain has individual 30ma protection.

Upgrading the cable/running a separate earth alongside the swa would not be possible due to the nature of site.

Yes an upfront time delayed rcd is an option.
What is the Ze ?
How many Rcd circuits are there being supplied by this sub main. ?

 

[toms22]

Yes an upfront time delayed rcd is an option.
What is the Ze ?
How many Rcd circuits are there being supplied by this sub main. ?

Hi mate, ze was 0.23! And there are 6 circuits on the sub main, 2 heater circuits, 2 lighting circuits, cctv supply & a ring

 

[Tom256]

What readings were you expecting for the cable prior to testing?
What is the make of the protective device?
Is there any previous paperwork [test or design] for that distribution leg?

By a quick estimation (estimation because no figures for resistance with me for the armour so somewhat ball-park) and based on 20 C temperature at time of test that R1 / R2 would seem to be too high for the length of run on a 25mm cable.