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Earthing & Bonding Advice Flooded Mobile Steel Training Unit

Rob Phillips

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
  • 0

    Slightly unusual I agree !!
    What loads are 'live' during flooding, normally, and under any likely fault conditions ?
    Do you expect trainees to come into contact  with realistic 'live water' as they might if the flood zone included say damaged light fittings?

    I presume most of the high power requirement is providing the shake rattle and roll effects, and water pumping, and is actually outside the wet zone and electricity in the wet zone is low power and could be ELV or very lightly RCD tripped.

    My main concern would be that a lot of the ADS is based on assumptions that may be wrong here, hinging on the principle that disconnection from 200V in half a heartbeat is OK and so on because current is limited as assume the epidermis  is dry and largely insulating until it burns through.

    One could back up the RCDs with something that actually measures and reports the out of balance currents, so that you get the earliest warning of a deterioration - probably water ingress, without actually stopping the exercise.

    It will be a reasonably good  equipotential zone especially when flooded. The chlorination of the water will make it quite conductive, but not as conductive as sea water would be.

    I'd expect your likely electrical problems would mostly involve dissimilar metal corrosion and water ingress where it should not.

    It is quite practical to earth things in a way that allows the connection integrity to be verified in-situ, either with current injection clamps or two wires to different places on each part so a ring-round continuity test is possible.
    In terms of how good the bonding needs to be in milliohms, that all depends on the PSSC at that point.

    Mike

  • 0 in reply to mapj1

    Thanks Mike, there is a URL in my main post to a YouTube video showing the unit in action to give some more exsplaination 

  • 0 in reply to Rob Phillips

    yes, I watched it before posting - it seems the wet areas are lit - is that 230V lighting?

Reply Children
  • 0 in reply to mapj1

    No 110v

  • 0 in reply to Rob Phillips
    Rob Phillips said:
    No 110v

    Surely BS 7671's answer to that is 702.410.3.4.1, or all specified for AD 7 or AD8 (including wiring system)?

    (The choice in itself of AD7 vs AD 8 in this application is not at all certain ... AD7 implies "might get flooded' ... but AD8 is "will be submersed" ... probably more likely interpretation here.)

    Although I have an open mind, not having seen anything other than the video posted, I would take some convincing that BS 7671 was appropriate for all of the system.

    I'm also concerned that the information is not made available by those who did the original design, or the re-design (refurb).

  • 0 in reply to Rob Phillips
    • Hmm. 110v may make it more realistically ship shape, but unless the idea is to have realistically live water for candidates to stay clear of and get tingles from, then lights that are SELV (at about 12V probably) would be the modern choice.

      Motors for pumps etc will be 3 phase, I assume, but the electrons stay in their own earthed enclosures that are not flooded.

      I presume this lighting 110V is earthed on one side or in the middle and is not IT (which perhaps might be an alternative) ?

      I'm in agreement  that this is not an easy fit to BS7671 by  the way, this is not really an installation in a building, and in many ways is closer to a big 'product' like a funfair ride.

      So there are other standards but the design authority has to be able to mix and match the right parts of them to suit the situations arising  within various zones.
      (Take a day at Alton Towers or any similar with big rides where folk get accelerated, wet and swung about and in some cases driven past jets of flame - a quick look about will show it is not really BS7671 territory once it gets beyond the supply distro to the rides.)

      I'd be very nervous of just an EICR-like pass being seen as either necessary or sufficient.


      M

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