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Advice Please - Mains cartridge fused Isolator switch or MCB and isolator Switch.
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).
The original description of this installation is not very clear and complete. On the sites near to me I have observed caravan or residential homes connected via a local "hook up point". The hook up point comprises a 30 mA R.C.D. and a meter and M.C.B., plus a 16 Amp blue socket. These points are normally rated at 16 Amps, but recently the site owners have been de-rating them to B10 M.C.B.s at the hook up point. The submain supplying these can be TN-C-S. but at the hook up point this is converted to TT, the C.P.C. not being continuous into the caravan or residential home. With the caravan or residential home there may be a conventional consumer unit with 32 Amp M.C.B.s for ring finals or showers or cookers etc. These are installed by the makers not realizing that reduced current supplies may be available. The owners soon realise just what can and can not be used together. The Consumer unit is fitted with an R.C.D at 30mA sensitivity. The caravan earthing is TT and an earth rod is installed at the hook up point.
I like that idea that two R.C.D.s protect the installation.
Of course this permanent residence mentioned in the O.P. may be considered to be a permanent home and be supplied directly by a submain. In which case P.M.E. earthing may be possible and will comply. 708.411.4
Read the original post through and take note of what it says, in particular "Additionally would it a be a good idea to put a 60a RCD into the shed".
As I read it the original poster states it is a TT installation and there is not any RCD protection to the distribution equipment in the shed or the SWA distribution circuits to the park homes. If that is the case it is potentially dangerous and requires urgent remedial work.
A couple of photos to verify if there is a suppliers earth terminal that is being used will clarify what's what.
1) TT needs a very low Z earth electrode if there is no RCD, it is not clear if you have one.
What is the armour of the SWA connected to? Where there is a pre RCD live wire inside the SWA the armour earth needs to be a good enough earth to operate some ADS of some kind if the cable is ever spiked !!
(but it does not mean you need to have a 30ma RCD at the origin, either 300mA or 100mA and in either case with time delay would do, so long as there are 30mA ones further along before you get to customers sockets)
2) The armour wires of your SWA are not glanded.
Ugly and a bit rough yes, but not really a regs issue so long as the armour wires have been properly marshalled into some sort of earth terminal.
You could make still it look nice and still retain the existing switch if it is in good order.
That could be done by peeling back the SWA outer jacket only, and glanding the SWA armour into any suitable small box somewhere beside the switches and taking a CPC from there (pirahna nut if box is plastic ) along with uncut the cores and inner' bedding insulation of the SWA straight out of the new box, and continue straight into the existing gear as before, either close butted or in plastic conduit if it needs to run in the open for a bit.
(While the inner of an unzipped SWA looks and behaves like a rather willful flex the makers do not specify if it meets the requirements of being insulated and sleeved - like T an E . A bit of conduit or plastic trunking makes up for that.)
Many thanks for all the replies. It seems there is no definitive answer and either a MCB fused Isolator or a cartridge fused isolator can be used providing they are double poled. With regards to the RCD, if we were to put an additional RCD in the shed this would be just protecting the swa cable going from the shed to the consumer unit in the home. There are no swa distributions circuits our side of the meter just a single swa 10mm two core cable to the home. All the homes have there own separate meter direct from the DNO which are in a meter box attached to the shed. The supply from the DNO is via overhead cables which are then distributed underground by the DNO to the meter boxes of the sheds. Recently the DNO's have been replacing the old meters with new smart meters and a double pole isolator switch (in the meter box). They have not mentioned the lack of an RCD in the shed but have left a label saying the tails are undersized. The tails are currently all 10mm and they are requesting that they be changed to 25mm. This leads me to a a second question, as they seem to be replacing their existing cartridge fuses which were 60 amp with cartridge fuses that are 100a. My second question is what size tails are appropriate giving that our side of the meter we were only going to supply a 60 amp fuse max or a 40 amp fuse. I understood that for 10mm swa cable you should have no more then a 60 amp fuse. For this reason wouldn't 10mm tails or 16mm tails be sufficient? Also who is responsible for changing these if the connection to the isolator switch should be done by ourselves but the connection to the meter should be done by the DNO. Do I need to arrange this when both my electrician and the dno's electrician are there together?
I will try and take some photos of the supply when I am next on the park this week.
You need to replace the tails onwards from the new isolators the DNO have installed for you, if as you say the supplies are TT earthed you then need RCDs and fused switches to protect the SWA cables to the park homes and these SWA cables are distribution circuits within your installation.
There are various ways of installing the required protective devices and switching, though generally a cartridge fuse will be used to protect SWA distribution circuit, you need an electrician to sort this out.
I do have a qualified electrician lined up to take on this work. The main reason I was asking for advice, was that I had a couple of independent qualified electricians do an EICR on similar homes with the same set up in the shed, but whilst one passed the existing installation in the shed the other failed it and adviced it must be replaced.
When I had my competence assessment last year my assessor asked me some specific questions about TT installations, in particular regarding the choice of consumer units, he passed comment that there are a lot of electricians who do not understand the requirements for TT installations.
If the installation is TT and there is not any RCD protection upfront of the SWA cable to the park homes and the homes have dual RCD consumer units with a main switch then the consumer units are not adequately protected as the internal tails and busbars are not RCD protected.
So a fault on the SWA cable or within the consumer unit could cause major issues in a TT installation without an upfront RCD.
Andy, it may be foolish to comment until you have full details of the installation. In the general case you are not wrong, but you could be very off beam.
I don't understand why the DNO wants 25mm tails to 60A fuses. This is very strange. Also, they may be referring to the Henly blocks or whatever where the tails split into several pieces to feed each of your isolators, the input from the meter will need 25mm tails to the block, although I would have expected them to have changed these pieces because they will have to re-visit to do it. Everything seems a little strange. Photos please Nick, easy to post here, just size to 1200 or less wide and push the little postcard above to select the picture files on your computer.
As an aside, I would like to see the EICR paperwork you have, both please if you could, as part of a research project into the quality of EICRs, davezawadi (at) yahoo.co.uk, address redacted if you wish.
Andy, that is interesting about your assessment, what did you answer about a TT domestic CU construction? What about any associated "switchgear" and its containment? Did he ask about possible failure modes of the install?
