SWA and earthing

[Cav43]

Why do people (customers) buy the bits and not complete the job?

Two questions:

How to terminate 3 core 4mm SWA for a garage supply in to a domestic C.U.? (I was going to terminate the SWA in a 3x3 box with gland and then run 4mm T+E to the CU) Y/N?

Should the braid on SWA be earthed (I'm sure this has been questioned before!)

 

[securespark]

Yes, it should be glanded off.

The method I use is to gland the cable, leaving plenty of tails to reach the CU terminals, then fit the gland to a galv through box. This avoids having to join the SWA to another cable: you can attach the SWA tails direct to the CU, avoiding the need for a further joint.

Then attach the galv box to the CU with a brass bush. Run a piece of 4mm² (or other suitable size) earth from the CU earth terminal to the galv box & attach using a ring crimp terminal & brass M4 screw/ washer.

Finally, put a lid on the galv box.

 

[Cav43]

Thanks.

What about eathing the SWA braid or the 3rd core in the cable? Is there a need to use the braid aswell as using the 3rd core? I would have thought this could cause earthing impedance issues?

 

[ban-all-sheds]

Thanks.

What about eathing the SWA braid or the 3rd core in the cable?

Two separate items.

The armour is armour, and it has to be earthed.

The 3rd core may or may not be the cpc.

The armour may be used as the cpc if you don't have one of the cores for that.

Is there a need to use the braid aswell as using the 3rd core?

I can't see why you would ever need to use both the armour and one of the cores in parallel as the cpc.

I would have thought this could cause earthing impedance issues?

¿Que?

 

[Cav43]

I would have thought this could cause earthing impedance issues?

¿Que?[/quote]

Ok, bear with me I might be getting myself in a knot here but:

If the 3rd core is linked to the earth bar as a cpc (ie: low resistance) and then the braid is also connected between the earth bars - but over time the braid becomes a corroded connection (ie: higher resistance) doesn't that equate to a low resistance in parallel with a high resistance??

 

[Adam_151]

What sort of problems to you think that might cause?

P.S. they will be different in resistance even when new

I can sort of see the logic in argueing that the armourings and cpc core form a loop and if anything was to be induced in either then its effectivly a shorted turn... but I don't see that this is valid as the currents in the cable will sum to zero...


However, wandering off topic, if you were to use some SWA, and provide a separate cpc outside of the cable, then in an earth fault condition, then the currents surrounded by the ferious armourings would sum to hundreds or thousands of amps, BS7671 warns not to use single core cables armoured with ferrious material, but I'm not sure whether the effect on the EFLI in this situation is signifcant

 

[ban-all-sheds]

If the 3rd core is linked to the earth bar as a cpc (ie: low resistance) and then the braid is also connected between the earth bars - but over time the braid becomes a corroded connection (ie: higher resistance) doesn't that equate to a low resistance in parallel with a high resistance??

Yes.

So therefore the resistance of the two in parallel is slightly lower than just that of the core...

 

[Cav43]

Thanks lads.

Just a sanity check.

 

[streetlighter]

We use 3-core SWA on streetlighting with the 3rd used as the cpc and the armour. Gets the earth loop down on long runs and also gives a solid earth integrety.
In case a gland becomes loose or if the armour suffers from corrosion.

 

[333rocky333]

We use 3-core SWA with the 3rd used as the cpc and the armour.

An APEA requirement for petrol stations also