How big must the earth conductor be ???

[chocolate]

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 ????

 

[sparks1234]

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 ????

IMO I would install a conductor at least half the size of the line conductors

 

[Guest123]

Install a seperate earth bar adjacent to the chamber.

From there a protective conductor at least half the size of the live conductors to the gland of the SWA, one to the earthing terminal of the chamber (belt and braces), a supp bond of the steel framework and protective conductors to each of the switchfuses.

 

[scotsparky]

As the Main live conductor is over 16mm the SWA isnt big enought o be a CPC so a seperate CPC has to be installed . I will get back to you with the chart its in the new GN1 as well as 16th

Old 16th GN1 Table D10A p154 and Table D10A P155

you have to work out the conductor size but a 100mm i would recon 50mm is ok but prov it with adiobatic.

 

[Darkwood]

As the Main live conductor is over 16mm the SWA isnt big enought o be a CPC so a seperate CPC has to be installed . I will get back to you with the chart its in the new GN1 as well as 16th

Old 16th GN1 Table D10A p154 and Table D10A P155

you have to work out the conductor size but a 100mm i would recon 50mm is ok but prov it with adiobatic.

If i recall i think that this applies to 2core swa single phase but 4core swa starts requiring additioal earth at a higher size but like yourself il have to dig the info out.

 

[scotsparky]

It apllies to a lot of SWaA both Bs 5467 and Bs 6724 It relates to the circumfrence of the cable

It also it liknked in with your K fatctor on your adiobatic.

 

[Guest123]

Hi

The CSA of the armourings of 4core SWA complies with BS7671 requirements for use as a CPC for cables upto 240mm.

Any size above this does not comply as the "k" value is insufficient.

That is not to say however that an additinal CPC should not be run, and I always do but if it was an existing installation that you had been asked to carry out work on, then you couldn't really put an argument forward for installaing one as technically it complies.

Just ensure that all joint/glands are tight and install the appropriate links from gland to earth bar etc.

 

[scotsparky]

not always lenny i have scanned tables fom GN1 17th please note 16mm cables ps i kniow 1 is upside down

pages 148 &149 on Gn1 17th edition

 

[Guest123]

Yes, sorry should have mentioned the 70 degree and 90 degree PVC and XLPE variants.

 

[john.irvine]

Using regulation 543.1.4, page 130 17th edition

The nearest to your 100mm would be either 95mm² or 120mm²
Using www.prysmian.co.uk cable detail are

90mm² 4 core SWA, conductor resistance @ 20C is 0.193Ohms/km
Armour resistance 1.1 Ohms/km nominal area of armour 147mm²

120mm² 4 core SWA, conductor resistance @ 20C is 0.153 Ohms/km
Armour resistance 0.76 Ohms/km nominal area of armour 206mm²

Minimum cross-sectional area of cpc in relation to cross-sectional area of line conductor =
K1 x S
K2 2
Where K1 is the KI value of the line conductor table 43.1 chapter 43 page 77 of 17th edition
|Where K2 is the value of K for the protective conductor tables 54.2 to 54.6 page 129-30 of 17th edition
K1 = 115, K2 = 51, 115/51 = 2.255
S 95/ 2 = 47.5 or 120/2 = 60
For 90 mm² 2.255 x 47.5 = 107.1 mm² actual size 147 mm²
For 120 mm² 2.255 x 60 = 135 mm² actual size 206 mm²

 

[scotsparky]

cheers john very clear!


killed the discustion i was having with lenny though baaaahumbug

 

[peteludman]

One of my books say use the steal wire 0f swa shhould be the only earth to be used except 120mm and 400mm 2 core i will dig it out and tell u more if u want

 

[scotsparky]

sorry mate dont agree with that. Would like to see that as itsgoing agains some of the IEt recomendations.

 

[peteludman]

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.

 

[scotsparky]

peteludman have a look in Guidance notes ! selection and erection there is a list ther which show you the worked sizes of SWA both 70 and 90 degree you will note 16mm 2 core and others dont cover it so suplimentay earths have to be installed.