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Advice Please - Mains cartridge fused Isolator switch or MCB and isolator Switch.

Former Community Member
Former Community Member
I manage a residential mobile home park with homes that individually have mains supply TT coming to a meter box recessed into a breeze block shed. The tails then come from the meter into the shed and into an old style bakelite rewireable 40a fused isolator switch. There is then an outgoing 10mm SWA cable running from the isolator switch underground from the shed to the home and then into the consumer unit for the home (total run approx. 8 to 10 metres). Having had an EICR for one of the homes I have been adviced that these isolator switches will need to be replaced as the swa cable is not properly terminated (it is not glanded in to the isolator switch and the wire stranded sheathing is twisted together and connected to the earth block.) My question is can this isolator switch be replaced with an 18th edition 3 way metal enclosure with double pole isolator switch and 40a mcb or should it be replaced with a 60a (or 40a) cartridge fused double poled mains isolator switch. Additionally would it a be a good idea to put a 60a RCD into the shed and either use a five way enclosure (in place of 3 way enclosure) or a 2 way enclosure fed from the cartridge isolator switch. All the homes have a standard consumer unit containing either a 60a or 80a RCD and 3 to 5 suitable MCB's (32a, 16a, 6a).
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    Section 708 of B.S. 7671 ( Reg. 708.1 Note 3) says that for residential park homes they are covered by the general requirements of B.S. 7671 together with the relevant particular requirements of Part 7. Section 708 covers circuits intended to supply leisure accommodation vehicles, tents or residential park homes in caravan parks camping parks and similar locations.  I gather that these are static homes. Is that correct? For isolation double pole switches are required.


    Each residential park home shall have an individual 30mA R.C.D. for its final fixed circuit. The final circuit is from the consumer metering point  to the consumer and not the distribution circuit to the connection/metering point. This shall be double pole. This shall be accessible to the consumer. 


    A fixed connection to supply a residential park home shall be individually protected by an overcurrent protective device in accordance with the requirements of Chapter 43.


    Z.
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    Hi Nick

    It is difficult/impossible to fit an SWA gland to those old switchfuses, so I am not surprised at the way this has been done. Purists may say this is a bit scruffy but provided the armour is correctly terminated to Earth and the sheath continues into the Switchfuse it is not a codable item. In fact, this is the way in which most incoming "modern" split concentric cables are terminated into the distributors fuse. If the switchfuses are in good condition no action is required. Should you wish to replace the switchfuses, any kind is acceptable, a metal enclosure is not mandated as this part of the installation is not domestic. I suggest a cartridge type fuse rather than an MCB, it will cause you less problems with faults inside the home..


    As you will see elsewhere, I am collecting EICRs with incorrect coding and I would like a copy of yours, particularly if the coding is C1 or C2, and a photograph of the inside of a switchfuse if you feel you could provide it. If an estimate for the change is also provided I would like that too if possible. davezawadi (at) yahoo.co.uk.


    Kind regards

    David CEng etc.

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     My question is can this isolator switch be replaced with an 18th edition 3 way metal enclosure with double pole isolator switch and 40a mcb or should it be replaced with a 60a (or 40a) cartridge fused double poled mains isolator switch. 


    Yes, a metal clad garage unit may be used with an M.C.B. This can be easily reset if required as opposed to having to replace a cartridge fuse. The effectiveness of earthing has to be confirmed at this position by a qualified electrician for safety reasons.


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    Basically the answer is yes. I take it you have some form of time delay RCD at the origin being a TT supply? It would be better to keep local 30mA RCD protection inside the static caravan.
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    I expect it’s inner insulation on show which is the problem, but a photo would answer all.
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    I maintain a similar static holiday village park with chalets. Each chalet is protected by a B50 M.C.B. Each chalet has an electric shower, typically 8.5kW as there is no gas allowed on site. The chalets have cookers and electric heating. Rarely, if ever, do the B50s trip off. I dread to think what would happen if the park was fully occupied and we had a bad cold snap in the summer.


    Z.
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    +1 for where are the RCDs?


    The old "bakelite" devices at least had the advantage of lacking any exposed-conductive-parts - a distinct advantage for a TT system upstream of the first tier of RCDs. The armour of an SWA cable, especially if underground, should definitely be considered an exposed-conductive-part so it's essential to have effective ADS before that.


      - Andy.
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    Take care, it is the installation in the mobile home which must be TT, not the supply cable. The situation is rather different from mobile caravans, where the supplies from the socket are TT, and the connecting flexible cable. You will notice BS7671 is quite careful 708.415.1, the socket must be TT, but if there is no socket then the installation must be TT. Park homes have always been a bit tricky, it could probably do with tidying up, mobile units are a good guide.
  • 0
    From the description in the original post it seems this installation has never been safe or compliant installation.


    A couple of photos would indicate if this installation is as dangerous as it sounds from the description, in the meantime you should be advised to consider isolating the installation until remedial work has been carried out.


    In reality if these are occupied homes isolating the supply’s isn’t going to feasible, so you need an electrical contractor in ASAP to replace the intake equipment ensuring that adequate RCD protection is installed to protect the distribution circuits to the park homes, because if there is a fault on the distribution circuits the fuse may not clear the fault and everything that is connected to the the earthing of the installation may be live at 240 volts, as I have seen happen.
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    Pardon Andy, are you suggesting that there is no RCD protection on the distribution circuits? I thought the question, defect, and suggested fix is to the switchfuse(s), not the overall supply. It is true it may be TT, or that the person asking is not sure, but I am feeling that the description is incomplete. Surely if TT an Earth electrode would be mentioned, and the term Earth block, presumably the MET, surely suggests it is TNC-S? No installer would fit a block and no Earth connection to it and probably would have ignored the SWA armour as well, and left it floating. Even if the Earth is missing at the distribution point, each home will have an Earth electrode, otherwise where would the internal Earth system be connected? I suppose it could have been just linked to the SWA, but then unless it is new this would have shown up on any inspection, EICR or new work since. Unless someone is digging up the SWA and damages it there is in reality very little risk whatever, but perhaps the original poster would clarify it for us? The home EICR should have picked this up upon Inspection shouldn't it, they got the switchfuses without any trouble?

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