Splitting main bonding

[johnboy6083]

Hi ladies and Gents.

At work we're having discussions upon the provision of main bonding to remote steel kiosks that have 2 supplies to them. One is for the pump, which is usually the largets of the 2 supplies, the other is to a small DB for lighting and power.

Our job is the installation of the submain and buildimg services to the kiosks.
Ive raised the issue of main bonding, and its been mentioned from above that it should be ok, as the combined CSA of the submain SWA and the inner core (we are using 3 core cable) will be large enough.
My argument is that it is not permitted to split the main bonding across 2 cables. Am i right, as i cant find anything to support this, but i also cant find the opposite?

Also the supply to the pumps is often 3core, and i cant isolate the pumps to use the armour anyway). The problem is that as that is the largest supplty to the kiosk, the main bonding needs to be sized to this anyway. Im aware that if i use the adiabatic to calculate the min size of CPC for the pump cable, then my bonding only has to be half of that, but sometimes i cant get the time/current curves for the protective device of the pumps.
Another relevent question is that most of the pumps are supplied through VSD's, which include sensitive overcurrent and short circuit protection. Can you get time/current curves for VSD's, and is it permissible to use them to calculate CPC,. or do i have to go with the protective device uspstream of the VSD?
Im aware that VSD manufacturers stipulate min CPC/earthing conductors for their VSD's usually for EMC purposes, but im afetr a solution for bonding purposes.



Thanks in advance.

John

 

[Engineer54]

What are the cable sizes involved?? It's not recommended to have two different metals with different K factors being combined, or of having different CSA sizes for obvious reasons. Nothing wrong in having two conductors of the same size and length providing your bonding requirements....

 

[johnboy6083]

the submain for the CU is a 32A supply and ranges from 6mm2 for small runs to 35mm2 for long runs. I disagree on the sizing of these toom, but its not my problem. Thje supply for the pumps can be any size, usually from 16mm2 upwards right upto 185mm2 ect. obviously the pumps arent that big, but its the volt drop.
As for the different sizes, there is a report out which tested the urrent sharing between unequal conductors, and the results sugest that the current is shared failry accurately with regards to the ratio of sizes. Ill search it out.

 

[Engineer54]

I know the report your talking about (i think), the problem with that report, is they were only using 7m of cable, and they were assuming far too much in their calculations. The conductivity of steel and copper, is far too different, to get any form of equalized ratio. Same goes for unequal conductors, which is why parallel conductors must always be of the same material, construction, CSA, and length.