Consumer Unit


Postby simbad » Mon Sep 06, 2010 6:35 pm

Please note i love reading articles on this board, my following question is not intended to help me with any work, as i dont have any just curious to know.

i have been reading the following http://www.diydoctor.org.uk/projects/consumer_unit.htm

ok all seems straight forward, my only question is

ref 1st picture

i can see wire "G" coming out from the Main double pole isolating switch to supply the RCD.

my question is how does the buss bar connect to the Main double pole isolating switch?

does it fit in the same place as where wire "G" is coming out, or is there a second place to fit?

hope it makes some sence, again don't worry am not intending on doing any such work.
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Postby ericmark » Tue Sep 07, 2010 10:15 am

Yes likely same place as G wire. However normally the buzz bars and link cables are pre-fitted and there are many ways in which the manufactures lay out the internal wiring including the use of neutral buzz bars and the pictures are only an example.

The "Tails" J and K would be a report point on a PIR as too much copper is showing although in real terms not a danger and better a little copper showing than to clamp on insulation instead of copper.

Consumer units are type tested distribution boards. And to be classed as a consumer unit it must follow the manufactures layout including types on MCB/RCBO fitted you are not permitted to fit different makes. The writers of the Part P regulations do not seem to know the meanings of some words and call a circuit everything that comes from one MCB/RCBO of a consumer unit so a house with an 8 way consumer unit will have 8 circuits. However the IET/BSI define a circuit as everything coming after a protective device so a fused connection unit (FCU) forms a new circuit. This deviation in use of language makes some grey areas as to what needs registering. The Kitchen rules seem somewhat silly where you can do work in a utility area with high powered washing machines, large stone sinks etc. without registering the work but need to register in the kitchen because it has a food preparation area!

However in the real world there are some important points to remember where working on items like consumer units.
1) Any power if not terminated correctly can cause heat. All connections need to be tight.
2) Consumer units are often supplied to warehouses unpopulated (No MCB's fitted) and they put the required size items in the units prior to sale. It is very important to check all terminals are tight as sometimes they have not been tightened in warehouse.
3) There is no way to check a MCB other than the weight and feel of switch and there have been some counterfeit items found so buying from Ebay etc is really not a good idea.
4) RCD's are not tested by pressing the test button. That only tests the mechanical operation making sure nothing has got stuck. There are proper meters to test the electrics loosely called 17th Edition Test Sets these are far more complicated than a multi-meter.
5) In an ideal world from your meter you will have an isolator fitted by the DNO to which you connect the consumer unit. However in the real world the only way to isolate is to remove the DNO's fuse and even then only the line is isolated and really the neutral also should be isolated. The is just one of the many dangers changing a consumer unit.
6) When adding RCD protection to a previously unprotected circuit there are a host of problems likely to arise with shared neutrals, damp in socket boxes, overheated and degraded cables, and faulty appliances. It is highly likely the job will not stop at changing a consumer unit.
7) Although a good PIR will identify most of the problems before a consumer unit is changed it will not high light them all. And some can be rather expensive to correct.

Once a problem has been identified one can't ignore it. And there are many ways to get around problems with RCD tripping both correctly and erroneously and when you change a consumer unit one can't really say "Oh I'll look at that problem tomorrow after getting some advise" as likely you will have fridges and freezers plus a real need for lighting. I know many people do change their own consumer units but to do it correctly you need test equipment, disconnections, inspections and reconnection which will likely mean can't be done on a weekend, you will be without power for a day, and the test gear and inspection fees will mean it will cost more than not DIY'ing the work.

What I would say is look at the difference between twin RCD and twin RCD with some RCBO's and all RCBO's and work out what the cost against convenience means to you. Look also at the use of Ali-tube cable or SWA for freezer and cooker feeds and the problems with non ceramic hobs and hygroscopic water ingress with mineral insulated cables. Plus the use of emergency lighting to counter the problems when a RCD trips. Each house and each house holder will see the problems in a different light and there is no right or wrong solution. And do get an inspection report before any change to RCD's to identify problems. I would get a 10ma plug in RCD and move it around the house. Then any freezer, washing machine, or other items that are faulty will be identified before the change. And the 10ma RCD can after the change be used for garden stuff so it is less likely to trip main 30ma RCD.

And forget the methods to fit a consumer unit and leave that to the experts. (Ex spurt = drip under pressure?)
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