I'm responsible for the maintenance of a large mobile steel training unit, and I'm looking for any advice or considerations on earthing and bonding to ensure full safety and compliance under BS 7671.
Optional: Walkthrough video –
System Overview:
Supply: 3-phase TNS from a DNO transformer ~5 meters away.
Ze at origin is very low.
A dedicated CPC runs directly from the TNS point to the main steel structure of the unit.
The unit is suspended on two A-frames with swing bearings. One leg of each frame is connected to a local earth rod via earth tape.
Construction & Environment:
The entire unit (interior and exterior) is steel.
It operates in a flooded condition — approx. 300 tonnes of chlorinated water is introduced during training exercises.
All electrical accessories (lighting, sockets, switches) are IP-rated appropriately.
Electrical components including transformers are housed in a dry, segregated viewing/control area.
Electrical Layout:
The 3P supply is split between two transformers:
3P → SP 115V AC @ 50Hz
3P → SP 115V AC @ 60Hz
Protection:
BS88 fuses on the incoming supply.
Internal circuits are protected by 110-115V RCDs.
All MCBs used are double pole.
What I’m Asking: Given the nature of the unit — flooded, mobile, and all-steel — are there any additional bonding or protective considerations you’d recommend beyond what’s already in place?
Specific points I’d welcome thoughts on:
1.If there's value in supplementary bonding of additional internal metalwork (platforms, gratings, pipework) within the flooded zone.
2.Whether there are any unusual failure modes to be aware of in a reduced voltage (115V), RCD-protected environment with submerged conductive structure.
3. Anything above and beyond BS 7671 that others have implemented in similar harsh/training environments.
We’re aiming for maximum safety, fault tolerance, and best practice so I’m open to all insights, even if they’re outside the box.
