Bonding to a decommissioned museum piece

[domoggy]

Hi All, would really appreciate some of your thoughts on this one...

I have been asked to install lighting and power circuits to an old hovercraft (about 130 ft long) that it is constructed of aluminium and is sat on four metal legs on top of the tarmac outside and just across from the main building and is not intended to be moved.
The museum's main db is in the main building and I am taking a 32 amp sub main in SWA from the db in the main building across in some catenary wire and then into the side of the craft. This will be protected by an RCBO and will also then feed an RCD protected db within the craft, to supply the lighting and power circuits.
The circuits will all be wired in double insulated cable and there are some metal clad sockets mounted directly onto the craft's structure.
The supply system is TNS.
The craft's structure is aluminium, there are no gas or water services going into it.

Can anyone tell me what their thoughts are on bonding the structure?
I have contacted the IET and they firstly advised me that it wont need bonding and then once I mentioned I was mounting metal clad sockets on the structure they advised me to think of it as a caravan so the structure should be bonded.

Thanks in advance

 

[Rpa07]

Can you test the structure to your incoming earth of the TNS to ascertain whether it is extraneous or not - BuT - because of the power being installed the metal sockets will not be floating so bonding would be a way to go.
Come back with an ohm value and let the debate begin.

 

[Rpa07]

It does sound like an interesting project though - how did you get that job. Send a pic you of the craft exterior - I do like a good hovercraft.

 

[Wilko]

I fancy converting the hovercraft to TT, or should that be WT?

 

[Paul228]

Yes

 

[LeeH]

Why the 2 RCD's in series?

Can you use an alternative to metal clad?

 

[davesparks]

Test it to confirm whether bonding is required.
The suggestion that it should be treated as a caravan makes sense to me, but I can't remember what that entails in the regs as I've not been near one for years.
Why are you putting rcds in series on a TNS system? That is just bad design practice.

 

[Midwest]

Looking in BYB, section 708, supplies to caravans (IMO) and section 721, electrical installations in caravans; scope of what that applies to is defined, i.e. leisure accommodation vehicle = caravan

I was once told by a DNO engineer, that most TNS supplies, will be at some point most likely to have been converted to PME, when a cable is repaired.

ESQCR prohibits the use of PME supply (to caravan 708.411.4). 721.411.3.1.2 requires structural metallic parts accessible from within a caravan shall be connected to main earthing terminal in caravan.

I had something vaguely similar at a domestic property, where I replaced the CU. The owners have a mobile caravan, which they winter at their house and for years, have plugged the caravan into their (PME) house supply, to run a heater. They wanted me to run a new supply to their garage, so they could 'plug' the caravan in there.

I asked Elecsa technical if 708 applied to this caravan;

'The scope of section 708 applies to caravan/camping parks and similar facilities and not necessarily a domestic situation. However if the conditions are similar there is no reason why the designer cannot take section 708 into consideration.

If we assume 708 does not directly apply then there are no other specific regulation in BS 7671 for the situation described that forbid the exporting of a PME earth, therefore the decision will be down to the designer.

It was my opinion, that 708 does apply to my customers caravan. So therefore, I advised against using the caravans electrical installation, connected to the house PME supply. Unfortunately, I've never seen the caravan to see it's electrical installation, but been told it has standard sockets etc.

So OP, in my opinion, I would treat your hover craft as a leisure accommodation vehicle.

 

[Toneyz]

As above treat the hovercraft with the same principle as a caravan make the hovercraft a TT installation with RCD D.B. etc. bond structure and don't bother with RCBO there is no requirement to do so.