How big must the earth conductor be ???

[peteludman]

I have just sent the niceic a email to find out what the score is with swa cpc so will let you know the outcome, they normaly take a couple of days to reply.

 

[scotsparky]

why bother 17th ed GN1 Pages 146,147,148,149 will tell you

 

[peteludman]

they sent alot of info so if u give me a email i will forward it it dose help

 

[mrloy99]

the book is
guide to the wiring regulations
17t edition iee wiring regs
by Darrell locke
ECA

electrical rasearch association (era ) carried out a investigation into the merits or not running additional cpcs externally to armoured cables.

The report included a comparison of fault current with stand of the armour of cables to BS5467, using the k values given in chapter 54 of BS7671. It showed for all cables except 120mm squared and 400mm squared 2-core cables, the fault current-withstand of the armour was greater than the fault current required to operate a BS88 fuse within 5s. The 120mm cable was within a few per cent of passing and should be used unless it is intended to run the cable at full current carrying capacity.

Also, calculation of current sharing between the armour and external cpc show that if a small external cpc was run in parallel with the armour of a large cable there is a risk that the fault current-withstand of the external cpc will be exceeded. Because of this it is recommended that the cross-sectional area of the external cpc should not be less than quarter of the line conductors.

fault current withstand yes but what about earth loop impedence?

 

[Sparkyphil83]

Evening,
Just joined upto this forum after a search online for some info, hopefully someone can help me?
I have taken over a job from another contractor who has run in a 50mm 4 core swa from a substation to a TP&N DB.
My concern is that it appears they have not run a separate earth and are relying on the armouring as the CPC. I believeit should have a 25mm earth ran in alongside the SWA?.
My other problem is that the cable is now burried underground and had a tarmac road above it ect.
What are my options?
Thanks in advance

 

[Pete999]

Problem !!! Hope someone knows where to find this out.

Got a SWA 4-core ( assume that the conductor size is in excess of 100mm2 ). It connects to a metal bus-bar chamber. The armour is acting as the CPC.

The chamber has no earth bar but has a earthing bolt, and relies on the casing to act as the protective conductor from the chamber to associated fused isolators all mounted on a steel framework.

What SIZE earthing conductor should be installed linking the gland of the incoming SWA to the earthing bolt within the chamber ???????

Should a earthing conductor also be installed between busbar chamber and each of the fused isolators - how can I verify that the chmaber and steel framework has a cross-sectional area that is acceptable for the incoming cable size and the fuse rating ?

Does any know where to find out such information ????

Got a feeling that 95mm2 swa is the limit for utilising the Armour as the earth, there is a chart in 7671 somewhere. You need to look it up. Could be wrong though.