Earthing & Bonding Advice Flooded Mobile Steel Training Unit

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.

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

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.

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

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).

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.

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

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 ???

Given that long extensive usage, there should be copious records of previous maintenance and possibly incidents, which should help in getting a handle on the unusual and unexpected, especially if there were corrosion style estimates of rates that predicted maintenance (replacement) style actions, which have turned out to be correct/incorrect.

I would also throw in possible different rates of degradation on the seals for various parts because of the 'chlorination' which may have 'additives', plus they may have assumed rubber seals which have now been replaced by silicone and similar 'O' ring seals (reminds me of ozone degradation of rubber is aircraft because of their high altitude flying).

And don't let "Standards" (for the wrong scenarios) stop you doing the right thing!

No 110v