consumer unit


Postby slnd » Tue Jul 21, 2009 2:27 pm

I have recently had a new consumer unit installed. It is a MK Sentry split load type. One group of circuits is protected by an 30 mA RCD rated at current of 80A. Adding up the MCBs there are 112 amps worth on this one RCD.

My understanding is that this is incorrect. The maximum possible current should be less than 80. Would be grateful if anyone out there could comment, confirm or otherwise.

Thanks
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Postby kbrownie » Tue Jul 21, 2009 5:29 pm

[quote="slnd"]

My understanding is that this is incorrect. The maximum possible current should be less than 80. Would be grateful if anyone out there could comment, confirm or otherwise.

Thanks[/quote]
From where did you get your information?
The fuse in your supply cut out will determine the maximum current being supplied.
KB
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Postby moggy1968 » Tue Jul 21, 2009 5:39 pm

it depends what circuits they are supplying. when designing circuits you are allowed to apply something called diversity. this allows for the fact that not everything will be running at the same time.
of more concern is that you only have one RCD by the sounds of it. If this board was fitted after July 2008 it should be a split load, dual RCD board. ie 2 RCDs taking care of a roughly equally loaded sets of circuits. Could it be, that in an attempt to meet the regs for RCD protection of circuits your sparky has fitted all or most of the circuits on the RCD side of an old 16th edition board. If so this is very naughty as it doesn't meet the regs for minimum disruption.
If you can supply more info about the nature of the board we can advise you better.
did he also register the work with building control, i.e. is he a member of a competent persons scheme? he should also have left you an installation certificate and a schedule of results.
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Postby sparx » Tue Jul 21, 2009 6:50 pm

Hi, you are overlooking something called 'diversity'.
If you added up the total load on a cooker say fan oven 13A plus 4 elements on hob at around 8Amps each it should not run on a standard 32A circuit but it does!
take another circuit ; upstairs ring circuit, MCB 32A total load a couple of bedside lights a clock/radio alarm , a couple of electric blankets in winter ,
total load 10A if all on together.
don't worry clever people have worked this out, also no danger since main fuse in company service head cut out never more than 100A usually 60/80A anyway,
regards Sparx
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Postby slnd » Tue Jul 21, 2009 9:11 pm

Thanks sparx

Agree taking into account diversity there is unlikely to be a problem. Main fuse is 100A in this case.
The part I was unsure about was whether it was acceptable to use diversity in this case. It would (just) be possible to avoid any (remote) possibility of overload by redistribution of the circuits between the two RCDs
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Postby moggy1968 » Wed Jul 22, 2009 7:20 am

it is better to share the load more or less equally, also if you run an RCD consistently at more than it'srating you will eventuallycause it to fail prematurely, but I think thats unlikely in this case.

Sparx, et al
with regard to diversity can you help me out, I noticed on one of your other posts you talked about diversity when calculating a cooker load. My understanding was that diversity is used to calculate total load on an installation, and that on a single circuit (cooker for example ) it was not used. my rationale was that it is entirely possible that everypart of a cooker could be on at once (cooking sunday roast for example) is this assumption incorrect? I would love to use a bit less 10mm!
thoughts all you sparkies out there!
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Postby kbrownie » Wed Jul 22, 2009 9:30 am

[quote="slnd"]Thanks sparx

Agree taking into account diversity there is unlikely to be a problem. Main fuse is 100A in this case.
The part I was unsure about was whether it was acceptable to use diversity in this case. It would (just) be possible to avoid any (remote) possibility of overload by redistribution of the circuits between the two RCDs[/quote]

Sind, original post quoted one RCD not two. If you may two RCD split you are wise to split circuits ie downstairs lights and upstairs sockects on one side and upstairs lights and downstairs sockets on other side.
This will help if one RCD trips as you will still have power for lighting either through a lighting circuit or plug to socket circuit.
KB
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Postby sparx » Wed Jul 22, 2009 8:19 pm

Moggy, most short run cooker circuits can be on 6mm2 can't remember wiring standard cooker -oven/hob on more than that, unless very long run or buried in insulation.
Page 96 On-site Guide, household cooking appliance, diversity:
First 10Amp of rated load plus 30% of remainder (plus 5A if socket outlet on panel).
so if total load of oven & hob came to 45A (not unusual) then allow 10A plus 30% of remaining 35A or 10.5A so load = 10+10.5=20.5Amps,
so could theoretically go on a 4mm2 cable!!
Reason is every part of cooker is on thermostats, no way can they all be on together except if all turned on at once from cold,unlikely even at major holiday celebrations, then only untill first ring hits temp setting within a minute or so then randomly on & off out of step.
When area boards are designing supplies for new housing developments although each house is fused at 80/100A they allow 15A/house when calculating cable/substation sizing! without diversity sub would be the size oh several houses!
best regards SPARX
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Postby slnd » Wed Jul 22, 2009 9:49 pm

[quote="moggy1968"]it is better to share the load more or less equally, also if you run an RCD consistently at more than it'srating you will eventuallycause it to fail prematurely, but I think thats unlikely in this case.

Thanks for the comments Moggy

I also think some redistribution would probably be sensible here. The second RCD is lightly loaded. This would then remove all possibility of an overload. I am still unsure if the current configuration would be regarded as compliant with the 17th edition. Looking at manufacturers literature I suspect not.
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Postby kbrownie » Thu Jul 23, 2009 9:25 am

When dividing the circuits between dual RSDs its earth leakage that would be my concerned about. Your MCBs should cope with any overload and operate when needed.
Most appliances have an amount of earth leakage some greater than others so to restrict nuisiance tripping. I'd then consider which circuits to install either side of the split.
KB
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Postby moggy1968 » Thu Jul 23, 2009 6:09 pm

thanks sparx, appologies for going off topic a bit, an interesting insight. when working on your own it can be difficult to get the opinions of others so you go down a path that may not be the right one sometimes.
15A!! so thats why the lights dim during the advert break for corras!!
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Postby sparx » Fri Jul 24, 2009 8:23 pm

Yup, Moggy , it's a fact that grid control and power stations look at TV mags to gauge likely load peaks and 'tap-up' transformer outputs to suit,
so over running large events can give problems,
not really of topic, all about diversity,
regards lonely sparx LOL
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