Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Suggested Answer
  • Replies 16 replies
  • Helpful Answers 1 Helpful answer
  • Subscribers 37 subscribers
  • Views 432 views
  • Users 0 members are here
Options
You may also be interested in

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.

  • 0

    there are maritime standards that are "British Standards",

    As one saying goes.. "the nice thing about standards is there are so many to choose from".

       - Andy.

  • 0

    Hello Rob:

    This is way out of my past experiences but I will ask only one question - From an "external" perspective how many real grounds currently exist?

    I am thinking of the electrical safety requirements for EV charging stations!

    Peter Brooks

    Palm Bay FL

  • 0 in reply to Peter Brooks
    Peter Brooks said:
    how many real grounds currently exist

    Exactly ... 'clean earth' term for example, no such thing.

    However, when you are inside a large metal box full of water ... well, we have standards that deal with that.

  • 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

    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.

    How are the outputs of the "transformers" configured? Are the secondaries referenced to Earth? Is there a separate secondary winding for each output circuit/curent-using-item? I'm just wondering if we might be looking at a separated system rather than a conventional TN 115V or RLV (55-0-55) system? Are there power electronics involved? (these might not provide isolation) - if there really is 50/60Hz conversion going in it can't be just simple wound transformers. Or has someone used a 60Hz nameplated transformer but it's actually running at 50Hz?

    As for standards,  I suppose BS 7671 does tend to be used as a "back stop" standard - i.e. it applies where no more specific standard exists.

       - Andy.

  • 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

Community Rules & Guidelines