Earth fly lead size & requirement

[Gringoking88]

Hi all,

Hope you are all well and all is good!

I am installing a sub board being fed by a 80A SP+N Metal Switch Fuse at source, this is feeding a 25mm 3 core SWA at a length of about 2 meters to the main DB (both boards are metal). I am being challenged in saying that I have to add an earth fly lead at both ends, preferably the same size as the CPC e.g. 25mm, as the glands will be fine for continuity of CPC into the armour.

Am I being super safe or just a moron, whats the right answer?

Thanks all.

 

[telectrix]

i would add fly leads to ensure that the steel armour is reliably earthed. seen a couple of sites where the continuity from earth bar to metal casing is poor. if you do the adiabatic, bearing in mind the restivity of the armour, 10mm should suffice.

 

[Mike Johnson]

Are you really competent to carry out this work, if so you would be looking at what your earthing arrangement is to the existing installation.

 

[Gringoking88]

i would add fly leads to ensure that the steel armour is reliably earthed. seen a couple of sites where the continuity from earth bar to metal casing is poor. if you do the adiabatic, bearing in mind the restivity of the armour, 10mm should suffice.

Thanks Telectric, my thoughts exactly, but was just going to be over kill with the fly lead.

 

[Gringoking88]

Are you really competent to carry out this work, if so you would be looking at what your earthing arrangement is to the existing installation.

Hi Mike,

Not sure how this is relevant to the question asked?

 

[Mike Johnson]

Because you are in the DIY section and messing with the DNO switch fuse? and digging around inside the existing CU therefore dangerous and maybe notifiable?

 

[Gringoking88]

Because you are in the DIY section and messing with the DNO switch fuse? and digging around inside the existing CU therefore dangerous and maybe notifiable?

Ah apologies, I shouldn't be in DIY as am registered as DI, not sure how to get moved tbh

 

[timbobelfast]

Use the adiabatic equation, but as you have a core as cpc and the armour a 6mm flylead from the earth tag would be my approach.

 

[pc1966]

Are you talking about leads from the glands' banjo (or better still Earthing nut / Piranha nut) to the related earth terminals?

Yes, you should do it as you can't always be sure the box will be in great contact, not corrode a bit with time, or even (probably not here) be able to cope with the fault currents.

• Simple answer is to go with the CSA from Table 54.7 in the regs, which in your case is 16mm.
• Or you can go with the adiabatic calculation, and the fastest way to check that would be Table B4 from the OSG which should no change in Zs 0f 0.4 ohms from from 10mm to 16mm, implying 10mm is fully OK for a 80A fuse and 5s disconnection times.
• Also you also really should consider the limits of Table 54.8 for the TN-C-S case in case there is extraneous stuff bonded to the CU earths, which typically means a 10mm minimum.

TL;DR Use 10mm or 16mm as you have lying around in this case!

 

[pc1966]

Just looked over again and realised you have a CPC in the 3 cores of the SWA, that takes away the issue of TN-C-S bonding as you already have the sizeable end-end path.

However, you should still be designing to cope with an internal cable fault/penetration incident even if very unlikely, and so you should have the armour ends rated for fault-clearing of the 80A fuse.

You could get away with just 10mm at the only feed end, but I would say it is still good practice to have both ends able to cope with any fault currents. In reality 6mm at both ends should be sufficient, but 10mm would be meeting all requirements without assumptions of current sharing, etc.