Earthing & Bonding Advice Flooded Mobile Steel Training Unit

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 –

youtu.be/yybt94TYOpE

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.

Parents
  • Now that's an interesting project!

    Not something that BS 7671 would cover directly - for the 'sinking ship' interior I suspect the closet approximations would be something between a swimming pool (section 702) and a conducting location with restricted movement (706) - although the combination - more conductive (earthed) parts than a typical swimming pool and wetter than a typical restricted location - is arguably more onerous than either alone. Looking at those sections, LV (230V or 115V) tends to be avoided for the wet areas - with SELV (with a limit as low as 12V) preferred. Earthing/bonding is no panacea - is some cases it can complete the circuit allowing a shock to occur, especially in situations where water or broken insulating parts (e.g. bulkhead light covers) can allow current to by-pass the usual layers of protection.

    I'm not sure from your description which equipment is in what part of the system .. I presume you don't have sockets in the area intended to be flooded...

    I'm curious about the 60Hz circuits too ... normal transformers aren't capable of frequency changes.

      - Andy.

  • Thanks Andy, there is a URL in my main post to a YouTube video showing the unit in action to give some more exsplaination. Also there is 2 Transforms fed of the same circuit and both output 115v but one transformer is 50Hz output and the other transformer issues 60Hz.

Reply
  • Thanks Andy, there is a URL in my main post to a YouTube video showing the unit in action to give some more exsplaination. Also there is 2 Transforms fed of the same circuit and both output 115v but one transformer is 50Hz output and the other transformer issues 60Hz.

Children
No Data