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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.

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  • 0
    Rob Phillips said:
    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.

    I think we need to go back to asking what standards the unit was made to originally, and what is recommended by the manufacturer.

    As others have said, there are many safety issues here for which BS 7671 is not necessarily equipped to help.

    There are aspects of "machinery" and probably offshore safety standards, as well as the supply to this being under BS 7671.

    There may well be other issues that have been risk assessed, that persons commenting in a Forum such as are unaware.

    The original supplier and/or manufacturer should have provided the information you are looking for in accordance with Section 6 of the Health & Safety at Work etc. Act 1974. See in particular 6(1)(c).

  • 0 in reply to gkenyon

    Originally designed and constructed by Vosper Thornycroft in 1984 and commissioned in 1986, the unit was built to maritime engineering standards and classified as a ship. However, following a comprehensive overhaul, it no longer falls under maritime classification and is now subject to British Standards regulations.

  • 0 in reply to Rob Phillips
    Rob Phillips said:
    However, following a comprehensive overhaul, it no longer falls under maritime classification and is now subject to British Standards regulations.

    But there are a number of risks that aren't addressed in BS 7671, as others have said. With respect to the 'machinery' elements, these are specifically outside the scope of BS 7671 - see Regulation 110.2 (xi).

    I would need to see more information, because regards the "no longer maritime", if it was, it's also potentially outside the scope of BS 7671, see Regulation 110.2 (iv) and (v).

    During overhaul, any risks to "reclassify" should also have been documented, along with changing maintenance requirements, so again the information should be provided by the person carrying out the "redesign" or "overhaul".

    Rob Phillips said:
    and is now subject to British Standards regulations

    I'm not sure I understand that statement, there are maritime standards that are "British Standards", including BS 8450 Installation of electrical and electronic equipment in ships. Code of practice, but also all the supporting international standards ???

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  • 0 in reply to Rob Phillips
    Rob Phillips said:
    However, following a comprehensive overhaul, it no longer falls under maritime classification and is now subject to British Standards regulations.

    But there are a number of risks that aren't addressed in BS 7671, as others have said. With respect to the 'machinery' elements, these are specifically outside the scope of BS 7671 - see Regulation 110.2 (xi).

    I would need to see more information, because regards the "no longer maritime", if it was, it's also potentially outside the scope of BS 7671, see Regulation 110.2 (iv) and (v).

    During overhaul, any risks to "reclassify" should also have been documented, along with changing maintenance requirements, so again the information should be provided by the person carrying out the "redesign" or "overhaul".

    Rob Phillips said:
    and is now subject to British Standards regulations

    I'm not sure I understand that statement, there are maritime standards that are "British Standards", including BS 8450 Installation of electrical and electronic equipment in ships. Code of practice, but also all the supporting international standards ???

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