Sizing bango earth fly lead

[Unswitched]

Hi Guys,

How do you know what size of earth wire to use from the bango to earth terminal in a DB, when using say:

25mm 4-core swa sub main feeding a DB. (L1, l2 l3 and neutral, armour used as cpc).

I would gland, with a bango and choose a size half of the phase size, so 16mm earth lead i would use. Is this ok, or OTT!?

so if 4mm2 armoured is used, I would use a fly lead of 2.5mm earth? Cant be wrong can it?

Cheers guys

 

[Penance]

543.1.4 says when not using adiabatic equation to calculate then use table 54.7, table states where line conductor is equal to or less than 16mm then protective conductor should be the same size.

 

[Penance]

Just realised you said 25mm, same table says to use 16mm protective if line is greater than 16 but less than 35.

 

[Crosswire]

On a related topic, what size fly lead should be used when the swa ISN'T the cpc? i.e. when merely wanting to earth the armouring, and one of the cores is the cpc?

 

[johnboy6083]

IMO the flylead should still be sized as if it was CPC, as it would be relied on to disconnect the circuit in the event of somebody damaging the SWA. I dont know if this a BS7671 requirement, but in my mind its good practice

 

[somersetsparks]

I would use 16mm in this instance

 

[gooner85]

i would use the correct size regardless of whether there is a CPC core there as well

 

[Deleted member 26818]

If you were to not use the armour as the CPC and were to use a separate conductor run along side the SWA, how would you determine the CSA required?

 

[johnboy6083]

adiabatic.

 

[High Tower]

If you are using a core as a cpc, then the fly lead that utilizes the Armour as additional fault protection then surely this fly lead becomes supplementary bonding?

 

[johnboy6083]

supplementary boinding is to extend or maintain the equipotential zone, not take the full current of a L-E fault.IMO, the flylead, if used on an insulated glandplate, should be sized as a CPC.

 

[platty1005]

OP using 25mm 4 core with SWA as CPC
so CPC = S/2 if same material but it isn't it is steel
So you need to check CSa of SWA is compliant IE equivalent to 16mm copper
Assuming SWA CSA is compliant then fly lead should be 25/2 = 12.5 mm
No 12.5 cable so in this case 16mm copper fly lead is BS7671 compliant.
for 4mm with SWA as CPC fly lead 4mm is SWa compliant
for 4mm with core as CPC it is already 4mm so fly lead 4mm

 

[spark 68]

Hi,

I have just been looking through GN8, GN8 9.3.5 and it says where the amouring is not used as a cpc it becomes an exposed conductive part (which requires earthing), but does not give any details as regards the banjo cpc/earth tail sizing.

Looking at GN8 table 8.1 supplementary bonding for exposed conductive parts give the usual 'where not protected mechanically min 4mm' etc.

The situation is not helped by GN8 not up to 17th ed. (this is the latest version).
I hear what other posters are saying re: sizing the same as cpc, but can anybody point to the relevant regs and guidance on this point ?, as I assumed (rightly or wrongly) the banjo tail would be 4mm, again when the armouring is not used as a cpc, for example where glanding the armoured cable into say a metal DB or metal Isolator etc.

TIA

 

[platty1005]

If you are NOT using the Armour as the CPC then you must be using a separate CPC that must be sized correctly
It follows that the Armour is an exposed part of the circuit and must be connected to earth by a CPC so the banjo fly lead = same size as the CPC what ever that is. EG: CPC = 2.5 fly lead 2.5, CPC 4mm fly lead 4mm and so on To size the CPC use Adiabatic formula or table in Regs

 

[spark 68]

Hi Platty,

The situation I had in mind, is when using armoured cable say 3 core for SP+E, or 5 core for TP+N+E, or 4 core for TP+E, it follows your cpc will be the same size as the line conductors as it is one of the cores, so this bit is straight forward.

Are you saying the banjo earth tail in this case has got to be the same size as the cpc ? or S/2 or by adiabatic ?, again this is when the armour is not used as the cpc, but is still required to be earthed, or is the exposed conductive part just extraneous as I cannot see a definite reg relating to this.

I was just asking out of interest, as the information seems a bit vague, I have seen earth tails in 2.5mm and in 4mm etc. , connected to the banjos, although I never took that much notice of what size the cpc/line conductors were, and whenever I have fitted them I have just used the same size as the cpc.

Any info./clarification welcome