UK High Max Zs

[newfutile]

Most socket outlets should be on (RCD) Residual Current Device especially if they could be used outside, the maximum ZS for the lighting should be less than 1.84 ohms for a 13 amp bs1362 fuse,though a 3 or 5 amp may be better if loads permit

 

[mainline]

Thanks for the heads up. Please could you help unpack that some more for me or provide a reference for further reading.

The Rcd doesn't provide over current protection.

 

[timhoward]

I would add:
Where an RCD is used for automatic disconnection because of high Zs readings, it must be confirmed that the L-N loop impedance is low enough to ensure that the overcurrent protective device operates before thermal damage occurs to the cable.

Thanks for the heads up. Please could you help unpack that some more for me or provide a reference for further reading.

This is one of my bug bears as it's very easily overlooked with temporary installations unless they are properly inspected and tested under BS7909. I'm happy to unpack.

I'll turn up at a site and find lots of distribution boxes strung together with long cable between them, normally starting at a 63amp socket that is part of the fixed wiring. Sometimes a generator.

(production manager) "Every distro has an RCD, it will be fine"....
(me) "We'll see"

The most recent case was a distro box near the end of a chain that had a Zs of 0.99 ohms and a Line to Neutral loop impedance of 1.14 ohms.
The protective device was a C32 MCB.

So as a tester I'm mainly bothered about 3 things:
1 - line to earth faults clearing
2 - line to neutral faults clearing
3 - loading and volt drop

1 - The Zs (0.99 ohms) exceeds 0.68 ohms which is the max Zs of a C32 MCB for ADS to occur using the MCB. So 'normal' ADS is out of the window. But there are multiple RCD's between here and the supply, and these will take care of line-earth fault protection. Not first choice, but safe.

2 - L-N loop impedance is 1.14 ohms. The prospective short circuit current (PSCC) is 230/1.14 = 202 amps. Can you see the problem with this, using the graph at the back of the regs book:

(Note it isn't a given that the nominal voltage, 230v is actually present due to volt drop of long runs, so in reality it might be even lower fault current)

This is why BS7909 certificates usually have a specific column for PSCC:

The point @mainline is making is that an RCD doesn't do anything at all to help with line to neutral faults. If the impedance is too high will take a long time to trip the MCB and things will get toasty; about 8 seconds of toasty-ness in the example above.

Solutions here would be include changing the protective device (a B32 would be ok), finding a nearer supply point. or using higher rated cables to lower the cable resistance.

Hope that helps.
Tim