Our old Wylex CU has got plug in MCBs, however I'd like to put in an RCD onto the shower circuit and I don't want to be replacing the CU just yet. So - can you get plug in RCBOs (MCB and RCD in one) - or am I hoping for one step too far and just be grateful that I've got plug-in MCBs?
Thanks
D
No such luck, I usually fit the appropriate fuse/MCB in the CU and an inline RCD and enclosure to the shower cable, such as this one.
SJW A.C.S. Ltd.
No chance: an RCBO needs a feed from both the live and the neutral rail, while a plug-in would only get at the live side. Either fit a dedicated RCB downstream, or split the feed to the CU with a Henley block and take your shower and other RCB-needing circuits through a new CU with an 80A or 63A RCD as its incomer.
Stefek
You couldn't actually do that, unless your main service fuse was rated at 80A or 63A. Equipment has to be rated to the next downstream CPD, in the case of a CU that would be the service fuse. Easiest way of wiring in and RCD to the shower is the way I said originally. If you did it your way and put another CU in you may as well change the one thats there and have only one CU.
And it's RCCB, (residual current circuit breaker, (RCD with mechanical switch, usually used as main switch on CU's)), or RCD, (residual current device, (to just be operated under fault conditions)), in this instance we only need RCD.
SJW A.C.S. Ltd.
Incorrect, the main fuse is NOT your property and may be changed at the wim of the supply company. Either all the fuses / CBs must add up to less than the rateing of the consumer unit or fit YOUR own switch fuse. The rateing of this fuse to match your consumer unit.
Surely he is talking about changing the CU here, not the main fuse.
Surely this is not true either. The word is 'diversity'.
I think we're getting a little too involved for what we needed here. As a rule I always fit CU's with ALL RCD's etc rated at 100A. Unless you specifically ask for it you wont get a bigger service fuse than that. If the REC did decide to change the size of the service fuse they wouldn't just turn up, pull it out and change it, you would be informed so you could add a switch fuse before say a 63A was upgraded to a 100A. It also depends on who you get from the REC to do any work on the meter, some will check on the size of tails etc before changing a service fuse or before upgrading their own length of tails, as I have experienced in the past when the REC have been to upgrade a supply from 63/80A to 100A and I have not had the new CU and tails in place. It is rather difficult to keep the rating of CPD's below the rating of the DB as even the most basic of installations would have cooker, ring main, immersion heater and lights. Take this as 2 x 30A + 10A + 5A and you're already over 63A. Possibly a CU feeding a garage or shed would manage to come in under this threshold but not one feeding the entire house, especially if you are going to put a shower on it as the OP was so this would put it well over the 80A mark then. So the OP's CU would really want to be rated at 100A.
SJW A.C.S. Ltd.
Surely not. Almost every house in the country has more MCBs/fuses than incoming capacity. It isn't likely to all be in use at once.
I've got 4x32A + 2x16A + 3x6A = 178A. My incoming is only 60A. I'm unlikely to use more than 32A at a time, as the house is gas heated and uses gas cooking. I have no electric shower.
Even when the gas fails, the most I'd use is:
5kW electric heating (3kW fan + 2kW convector) 3kW immersion 2kW washing machine 2kW dishwasher 2kW tumble dryer 1kW microwave 1kW everything elseWhich is a fraction over 65A. Even then, the chances of all the thermostats in all those appliances coming on simultaneously for long enough the blow the fuse is tiny. At 65A, it would probably never blow anyway, even if left indefinitely.
Christian.
You'd think so, wouldn't you. However, in practice, not only do you frequently see shower circuits and similar dedicated feeds Henley-blocked off from the main feed - where you could make a sensible Nature Of The Load argument which says that there's no way for the one shower to pull a consistent overload of, say, 60A through switchgear rated only for 45A, making a 100A mainfuse an adequate protection for short-circuits somewhere in the switchgear upstream of the MCB/RCBO which provides overload protection; it's also standard for split-load CUs to have the RCD/RCCB rated for a continual current of only 63A or 80A, while the main incomer is rated for 100A, and for these boxes to be regularly and unremarkably connected to installations where the service fuse is 100A. It's Diversity which makes such practice reasonable - even with the RCD side of split-load CUs being populated with a couple of 32A MCBs for rings and a 40A for an electrick shower, the likelihood of the notional 104A "total" (or the indefinitely-sustainable
120A you could pull if you put enough fanheaters on the rings and kept the leccy shower going at full pelt) would be unlikely enough in practice to make it sound engineering practice to use an RCD across all these circuits whose continuous rating is a "mere" 63A or 80A. Like all uses of diversity, though, it depends on actual occupancy/usage patterns: as counter-example, in the case of the youth hostel warden who asked here about installing three electric showers, it's quite foreseeable there that a group of hostellers will all turn up soggy, cold, and anxious to dive into the showerroom pausing only to have their boots cut off their feet by the warden to save the communal carpet ;-) and it would not be good practice to apply diversity to those three showers in such a case.Stefek
Diversity should be applied by the selection of the loads connected to the diss board, in fact if you examine a wylex consumer unit it states load must not exceed N amps. The main fuse up stream of the meter is to provide over load protection to the meter and the suppliers works, overload protection for your instalation includeing the main swith is up to YOU. Either fit
100A rated consumer uints or fit a suitable switch fuse to provide overload protection. All power systems must be designed so that no single fault may cause danger.
Of course you cannot add up the rating of all the MCBs/fuses to give a total rating.
thermostats
And the chances are you would want to heat the house, heat water, wash up (via a machine), wash clothes, dry clothes, heat food and stick all your other appliances on at the same time is very small.
At 65A, it would probably never blow anyway, even if left
I agree. It probably would not blow.
I have only seen a couple of blown main fuses. One involved a drill and a long (now melted) drill bit, and the other as far as I can tell just blew. The house where the fuse just blew had been recently rewired and had an NIC installation certificate and the 80Amp fuse blew. The LEB swapped the fuse and could find no fault on the house. I suspect the fuse was well over 20 years old. So what is the lifespan of these fuses?
-- Adam
adamwadsworth@(REMOVETHIS)blueyonder.co.uk
The 60A fuse in my house blew recently (before I replaced the CU!). I wasn't even drawing much power. No faults could be found. No MCB tripped. It probably just died of old age. Rather naughtily, I just replaced it myself. I try to keep spares around, rather than relying on the REC to bother turning up.
Christian.
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