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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).
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.
Personally I'd go with a cartridge fuse every time - for all practical purposes MCBs don't discriminate with MCBs (or the overcurrent element of RCBOs) regardless of rating - so if you went for an MCB in the shed it's reasonably likely that L-N faults within the home could trip out the shed MCB as well as the one for the final circuit in the home - so losing all power not just the faulty circuit. With a cartridge fuse you have a far better chance of keeping the overall supply on.
If you do go down the MCB route it doesn't necessarily have to be a DP MCB - a single pole MCB and a DP switch disconnector would suffice.
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.
Yes - every part of the installation has to have protection from electric shock - not just final circuits after the home's consumer unit. Where the supplier's earth facility is used, then MCBs/fuses can provide that protection, but if it really is a TT system with earthing connections connected to a local electrode rather than the DNO's earth terminal, then you need an RCD to provide automatic disconnection.
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 suspect it's the electricity supplier (the folks who bill you for electricity) or their metering agent rather than the DNO proper that have been replacing meters - DNOs these days have little to do with anything beyond the cut-out. Hence you're probably dealing with a "meter fitter" rather than a fully fledged electrician. Generally they like to see 25mm² tails on new supplies, but will accept 16mm² on existing systems provided the fuse in the DNO's cut-out is downrated to 60A (or sometimes 80A). It's not just a matter of the load, but the ability to withstand faults (short circuits) before your switchfuse - a 25mm² or even 16mm² will be fine on a 100A fuse for faults, but smaller conductors will need a calculation to prove they're OK - which they are disinclined to so - they much prefer just reading standard requirements from their script. Besides they know full well that people often increase their loads after they've gone - so won't want to rely on your downstream fuse to limit the load - as you're free to uprate that.
As they've been very nice and installed an isolator (many don't), that can be switched off and your new tails can be connected into its outgoing terminals by your electrician - no need for any more supplier involvement on that count.
