95mm 4core SWA, just trying to go definitive on the steel to copper calculation

[andysparkfree]

good afternoon guys
I have a 95mm 4 core swa to this new office build , thats fed via a new exterior mains panel located next to a new sub station.
now all feed cables on this project have been designed by an independent company, and i currently have flagged up that i believe that using just the steel armouring for both the cps and the bonding conductor is undersized.

details of the mains
300mm incoming cables from the substation
its a PME supply
95mm 4 core swa (200 amp) feeding one of the new buildings
new building has main water bond located within

now i have double checked that the smallest earth thats allowed should be 25mm, but when ive done my (very rough calls) ive come up with the steel adding up to 13.37mm

am i correct with the above

thanks guys

 

[Pete999]

good afternoon guys
I have a 95mm 4 core swa to this new office build , thats fed via a new exterior mains panel located next to a new sub station.
now all feed cables on this project have been designed by an independent company, and i currently have flagged up that i believe that using just the steel armouring for both the cps and the bonding conductor is undersized.

details of the mains
300mm incoming cables from the substation
its a PME supply
95mm 4 core swa (200 amp) feeding one of the new buildings
new building has main water bond located within

now i have double checked that the smallest earth thats allowed should be 25mm, but when ive done my (very rough calls) ive come up with the steel adding up to 13.37mm

am i correct with the above

thanks guys

 

[westward10]

What size does the main protective bond need to be, the cable you are using will support main protective bonding up to 16.0. The most obvious thing to have done is run a separate cpc with the cable.

 

[Ian1981]

good afternoon guys
I have a 95mm 4 core swa to this new office build , thats fed via a new exterior mains panel located next to a new sub station.
now all feed cables on this project have been designed by an independent company, and i currently have flagged up that i believe that using just the steel armouring for both the cps and the bonding conductor is undersized.

details of the mains
300mm incoming cables from the substation
its a PME supply
95mm 4 core swa (200 amp) feeding one of the new buildings
new building has main water bond located within

now i have double checked that the smallest earth thats allowed should be 25mm, but when ive done my (very rough calls) ive come up with the steel adding up to 13.37mm

am i correct with the above

thanks guys

For a 95 mm 4 core at 70 degrees operating temperature I get steel is 147mm
The copper equivalent would be around 17mm.
Depending on the DNO incoming neutral supply you need to provide protective bonding to the building sized using table 54.8 of bs7671 if a pme earthing arrangement for the installation.

 

[andysparkfree]

Hi , you might of noticed by now , this area is not on my expertise list(that list is very short lol) , but following multiple conversations with my (sometimes friendly)nic help desk, i am led to believe that it has to be a minimum of 25mm earth up to a max of 35mm due to the neutral being 300mm incoming
and yes i agree about the separate earth, but just need to present it to the cable calcs company in such away that makes it clear if they are actually wrong
or if its me

 

[Ian1981]

Hi , you might of noticed by now , this area is not on my expertise list(that list is very short lol) , but following multiple conversations with my (sometimes friendly)nic help desk, i am led to believe that it has to be a minimum of 25mm earth up to a max of 35mm due to the neutral being 300mm incoming
and yes i agree about the separate earth, but just need to present it to the cable calcs company in such away that makes it clear if they are actually wrong
or if its me

Try 50 mm if your DNO neutral incomer is 150mm or over

 

[andysparkfree]

ok so , the steel adds up to and equivalent to 17mm copper, but should be at least 50mm for both a cpc and bonding conductor,
so basically a new 50mm earth will need to be installed

thanks guys (so much better than the £1000 a year help desk hahaha)

 

[westward10]

When we say copper equivalent that means the conductivity of the steel in relation to copper. The armour has a csa of 147mm but its conductivity when compared to copper is around 17 - 18.0mm which is what you need to know for main protective bonding requirements.

 

[Leesparkykent]

It's undersized for bonding. If a 50mm bonding conductor is needed in copper then the CSA of the steel used as a bonding conductor would need to be 425mm. The resistivity of copper at 20ºC is 17.2 x10-9. The resistivity of steel varies with carbon content from 100 to 1000 x 10-9.GN8 suggests using a factor of 8.5 for the ratio of the resistivity of copper to steel.

 

[andysparkfree]

so the cable calcs company rang me , he said he rang it all through amteck and it worked out at 47mm equivalent to copper, however then he said the pme connection was at my db and not at the main switch feed end? (who would of thought they designed it lol)
so anyway i said ill chuck it all on an email for them to look at in the morning

 

[Dillb]

so the cable calcs company rang me , he said he rang it all through amteck and it worked out at 47mm equivalent to copper, however then he said the pme connection was at my db and not at the main switch feed end? (who would of thought they designed it lol)
so anyway i said ill chuck it all on an email for them to look at in the morning

So the "design" company didnt even do their job correctly, just relied on some information fed into a computer? Nice to see they double check that everything they are giving out is correct information.

 

[westward10]

I suspect the result they got was correct from the inputted information that the armour was adequate with regards to being a cpc. They overlooked the fact it was required to also support main protective bonding.

 

[andysparkfree]

thanks for your reply above , you have all been a great help (thank you)
but can i just run the following past you

so a 150mm 4core swa(pvc), steel has the copper equivalent as 240, which i then divide by 8.5 which then gives me a rough copper value

so applying that to my 95mm 4core is 160 divide 8.5 =18.82mm in copper earth values (assuming that its pvc cable)

i hope im right lol

and thanks again

 

[Leesparkykent]

Using the Steel armour for earthing and using the steel armour for bonding are totally seperate scenarios and shouldn't be confused.

If you choose to use table 54.7 to size your CPC for earthing, rather than the adiabatic equation then you need to use K1 x S
K2 2 as S is greater than 35mm

K1 can be selected from table 43.1 which is 115 if 70~C PVC cable

K2 can be selected from table 54.4 which is 51 If 70~C PVC cable.

115/51=2.25

2.25x95mm=213.75

213.75/2=106.87

So the CSA of the armour has to be a minimum of 106.87mm to comply with table 54.7 and be used as a circuit protective conductor.

Skip to table 54.8....which is the minimum CSA of the main protective bonding conductor in relation to the neutral of the supply. You have said the suppliers neutral if the DNO cable is greater than 150mm so using this table a 50mm copper bonding conductor is needed.

You don't have a copper protective conductor in your cable so you need to read the note under table 54.8.

" The minimum copper equivalent CSA is given by a copper bonding conductor of the tabulated CSA or a bonding conductor of another metal affording equivilant conductance"

So using the guidance notes we have established that the ratio of restivtivity of copper to steel is 8.5 to afford equivilant conductance.

50mm copper x 8.5= 425mm in steel.

The steel armour is adeqate as a CPC but not as a bonding conductor.

 

[davesparks]

The question is about the adequacy of the steel as a main bond, not as a cpc. It’s a different calculation

 

[bigspark17]

A 50mm earth should be pulled in alongside the swa. Alternatively If the water as stated above is the only extraneous part that needs bonding why not remove the metal pipes and install plastic. Are you sure theres no other parts that need bonding, structure or gas?

 

[andysparkfree]

unfortunately there is 6 w.c units, 3 shower units and 3 kitchens all done in copper
so ive had various conversations with the design company (not that they say there wrong as such) but the regs say 50mm separate earth so thats what is going to be ordered
now an interesting question i have regarding the pme separate earth and possible long/ high currents within the earth if there is a fault on the neutral , is that in the main panel that has the 300mm feed tails , it goes into a 300ma rcd pri to feeding any outputs like the building that has the 95mm feeding

has that any relevance in this whole saga ??

thanks guys

knowledge is power and all that

 

[Leesparkykent]

Not unless the far end was made TT.

 

[westward10]

I don't see the benefits of a 300ma rcd on a TN system, what type of site is it.

 

[andysparkfree]

very strange site to be honest , theres the main offices we have just done , followed by an asbestos lab building then 2 x vehicle garages for there vehicles then a massive stores building and a scrap/ recycling plant at the far end with a few portacabin type units for something or other