Split Load Consumer Unit.


Postby Emulated » Fri Jun 11, 2010 3:19 pm

I had a new consumer unit fitted about 5 years ago with just a d/p mains switch and mcb's. My outside sockets of around the same age are wired into a seperate mcb but with no rcd trip protection. This means I have to use a plug in rcd everytime I use an outside socket. I have broken 2 from dropping them and was wondering if I should have the consumer unit replaced with a split load or can I fit a rcbo in place of the mcb?
Thanks
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Postby ericmark » Sat Jun 12, 2010 3:42 am

I will try and go through pros and cons.
Some consumer units just will not take RCBO length and they will not physically fit but likely such a new unit will take them. However they would be of single unit width so only switch the line the neutral is only monitored which with a TN-C-S supply is not a problem. However if the supply is TT then it would be safer to switch neutral. To switch neutral either double width RCBO or a twin RCD system is required. Since with a TT you should already have a RCD although 100ma not 30ma I would assume you have a TN-C-S supply?
The biggest problem with RCD's is the combination of many small leakages adding up to cause them to trip so the more individual units the better so where the supply allows using all RCBO's is far the better method. However they are not cheap at around £35 each changing all the MCB's in a 10 way board will cost £350 where a complete 10 way consumer unit populated is around the £80 mark.

However the labour in changing a MCB for a RCBO is far less than that for changing a consumer unit so likely will be in total around the same price. So if I was doing job I would want to fit RCBO's as a far better job where permitted.
DIY wise I am uncertain of the Part P implications. Changing a whole board does without question require fee paying and LABC involvement. Changing like for like i.e. new MCB would not required LABC involvement however it's not like for like and you would need to enquire as to if LABC involvement is required.

Personally I would not consider it to be a DIY job. You need to identify each neutral and it would be easy to swap polarity plus you could unearth existing faults which also would need correcting. If I was changing to RCBO's I would be using the insulation tester first to ensure no existing faults and testing the earth fault loop impedance as I as last person working on the system would be responsible for system safety and easy to disturb an earth wire. Once completed then I would do 6 tests on each RCBO 3 tests each on both positive and negative half cycle. So 1/2 rated ma i.e. 15ma to ensure they will not trip when they are in normal use. Then rated ma i.e. 30ma to ensure they do trip when 30ma is leaking and final 5 x ratted ma i.e. 150ma and check they trip within the required 40ms unless of course they tripped within this time on 30ma test. To complete these tests one needs a full test set which costs around £750 to buy. Not tried to hire but I think about £100 for the week. And you of course need to know how to use it and be able to fill in the installation certificate.

The other option is to use RCD sockets. These at just over £20 each and screwfix prices double is cheaper than single are not cheap. There are two types active and passive the active is most common and this means if the power fails they need manual resetting. The reason for this is should the volts drop below a threshold many RCD's will fail to work and in a consumer unit you are unlikely to get this volt drop but under fault conditions you could if at a distance from consumer unit get a volt drop causing them not to trip. Although unlikely in a house. Also second safety should you be using a hand held item which can be left switched on if you get a power cut you may leave it unaware and it could start running again when power returns without you being there. However for fridge and freezer one would want this to happen.

Before 17th Edition required cables buried in walls under 50mm to be protected with RCD or be special cable it was common to use the RCD sockets so the fridge / freezer would not be on the RCD circuit. However new build this is now rare. It is permitted to have non RCD protected sockets marked up as fridge / freezer but rare. There are also auto re-setting RCD's but about the only time I would use these expensive units would be in a disabled persons house where they could not access the consumer unit and at £350 each they are not cheap.

One final point if you use renewable power some inverters produce what is referred to as a "Modified Sine wave" and these need a "A Type" RCD not a "AC Type" RCD unlikely this will worry you but any boat/caravan users may need to be careful selecting right type.

So to re-cap likely best is RCBO and easiest is RCB sockets or fuse connection units. It is common to take power to an extension/conservatory from a single socket in a house using a fused connection unit these are available with RCD built in which would then protect all sockets after it.

I use an RCD socket for garden rated at 10ma so any faults in garden will not trip the main 30ma which protects the house. To use 10ma sockets does mean they will not be redundant if consumer unit is changed later. I would always run pond pumps through a 10ma RCD the same with garden lights. The lawn mower is not a problem as if it trips you will know but non attended items could trip out house while you are out.

Does that answer your question?
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Postby Emulated » Sat Jun 12, 2010 1:11 pm

Does that answer your question? you asked. More than answer it and I will need to read it several times to grasp it all. Therefore many thanks for your input and I will see if the electrician who fitted the CU is still available to sort it out for me.
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Postby ericmark » Sun Jun 13, 2010 9:06 am

One thing I did really talk about is the unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation. Until 2008 this was not clarified in the regulations they said every installation shall be divided into circuits to avoid hazards and minimize inconvenience in the event of a fault. However most Electricians considered this to only refer to the MCB's and in 2008 it was made clear this also included the RCD's in an installation.

What it says is one must take account of danger that may arise from the failure of a single circuit such as a lighting circuit. And it has been considered by many electricians that to use just two RCD's for whole house will comply with these requirements.

By using two RCD's it does reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation. But the question must be asked does it reduce it enough? Maybe in a household with no computers it would but where each member of house hold has their own PC and you could have five computers running then likely no.

The safety is another point. With street lights illuminating stair wells with light weight curtains maybe with a power cut one could exit the house in an emergency safely. However heavy curtains and central staircases can mean one relies on the house lights. Having these on any RCD means they are more likely to trip and being on same RCD as sockets means very likely in an emergency the lights will have tripped.

What we should do is a risk assessment however it seems that's alien to domestic sparks and rarely is each house considered on it's own merits. In my house I considered the problem and installed two battery backed (Emergency) lights. One above stair well and the other in the garage where the consumer unit is so if a RCD trips I can navigate my way to the consumer unit safely.

Using RCBO's does reduce the risks of course and although on a new build one has to have lighting on RCD protection or use special cables and even then the bathroom lights have to be on RCD. With existing properties one is not required to up-grade and you could leave the lights without RCD protection.

The same applies to items like fridge/freezer and even in a new build as long as the socket is marked as for fridge/freezer you can have non RCD protected sockets for them. Although since one would have to use Ali-tube cable it is not common.

However although I have RCD protection in my house on all circuits I would consider if this is what you want and you do need to consider pro's and con's carefully and remember old equipment may trip the RCD and you could well need a new oven after changing to RCD's on all circuits.

RCD sockets may be a better option for where problems may accrue. Anyway I will leave you to think about it. However my son who decided to become a radio ham at 14 my well not have been with us today if I had not installed RCD protection on all circuits. He is now an electrician in his own right and I hope since he works on 11000 volts will never get a shock as at that voltage you don't get second chance. But when he was in his 20's and working on low voltage (230v) he had his fair share and RCD's are a god send.
ericmark
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