Hi All
On an open air type construction site in west London (Cross Rail) I have a 100A 3 phase Incommer in a small wooden box at the top edge of site. (nowhere near overheads and not at risk of overheads falling close)
It then goes to metering and onto a 100Amp/0.3 Amp 4 pole RCD in a plastic box (GOOD that its plastic) - its good that its 4 pole.
There are no client side fuses (This is Bad - south of the meter or after the RCD) and from the RCD it travels 80m in 25mm SWA to a 3 phase DB in a movable metal box on metal legs (So the box provides weatherproofing - as it sits outside in a flower bed essentially. - Its earthed obviously and I am happy with this. No worries.)
The 3 phase DB then goes onto supply three or four metal site cabins and various other kit (For the 10KVa 3 ph 110V transformer the Sole RCD is the 300mA at the origin 80m away which is not good design, but essentially I'm OK with the safety part of it - interestingly there is an 80m run back to the origin for the Tx - 160m round trip - half in 25m 4 core swa to the DB and half in 4mm three core SWA. from the DB - but with a little leniancy on the allowable loadings even volt drop is OK).
All other design criteria seem OK to me through Zs/Ra backed up by the 300mA RCD, thermal constraints vold drop etc. I am installing a new circuit and using the existing SWA - replacing the 110v Single phase Tx for a 3 phase Tx)
With agreement with the clients consultants I have allowed 30Amp three phase for the whole site and no more than 10A Three phase for the Tx
Now to my question: Do these site cabins need "Bonding" there are two (or 4 perhaps?) ontop of each other and a 5 m gap and then two on top of each other. My feeling is that the B32Amp single phase RCBO supply to each cabin in 3 core SWA will provide earthing via the internal core and another earth path via the SWA armour........What would bonding do? I think - nothing - as there's no zone to keep at equal potential and bonding is not required?
I've gone through Guidance note 8 and cant see anything like this scenario.
I think 544.1.1 would only apply to TNS and exporting from a TNS and TT when its a closed building and theres some sort of equipotential zone to protect?
Interestingly I learned by word of mouth - that on railway sites such as this - earth rods are not allowed and only earth mats layed in the ground can be used.............(in place of an earth rod)
These cabins are +/- 10m away from any train overheads and on the other side of a fence between them and the track.
As you can see I'm tying myself in some knots with this job (But reasonably confident in most things except this bonding/ earthing bit - I'm still definitely learning here)
The metal boxes of the cabin walls will attain internal equal potential anyway hey?
Lastly a shout out to JP - he has bravely fielded my calls recently and answered questions - free of charge - such as - "can you rely on an S type 300mA RCD supplying a remote sub DB that goes onto supply a B20Amp MCB for a bit of kit (Fibre optic incommer), with no sockets involved on a SWA supply......." -------------It takes a bit of explaining and thank you to him for taking the time. It was the S part of the RCD I was not sure about and that at the origin...so again perhaps not good design but OK.
I also asked about disconnection times and the part of the table 41.1 (Page 59) under 400V - how I was to achieve that disconnection time of 0.07 when I could not test the 300mA S Type RCD at x 2 or x5 - I did not have the kit for it......Simple mistakes - the Uo = voltage to earth......so I can ignor that bit of the table as all my voltage to earth on this site is 230V. I knew it was 0.2 and 1 second for final and distribution circuits but checked that table and misread the table wrong.
So Thank you John for taking some time out to chat.
Kind Regards
Tatty
