Extra Ring / radial circuits


Postby PeterG65 » Sun Jun 13, 2010 6:28 pm

Hi
Would the following be legal. (after the quick description of the current set-up)

A new rewire has been under taken by qual Elec.

A New Consumer Unit has been instaled, split load, there are two non-rcd covered circuits.

But the 2ring set up (up / down stairs) has a small problem when I repair computers which casuse the RCD to fail I have to go to the celar to reset it and as it also has the TV and other units with clocks which have to be reset and Sky boxes .

Can I install a ring main 32A 2.5mm cables to two radial curcits with their own 16a/rcd seprate curcit in a small room over a desk length of about up to 4 meters in length..

each "16a/RCD" unit in would have 3 or more double sockets in 100x50 Tower trunking. Marked with warnings about max current for the set of sockets.

Many Thanks for your comments.
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Postby ericmark » Sun Jun 13, 2010 11:40 pm

“Hi
Would the following be legal. (after the quick description of the current set-up)”

It could be but at a cost you may not want to pay

”A new rewire has been under taken by qual Elec.

A New Consumer Unit has been instaled, split load, there are two non-rcd covered circuits.

But the 2ring set up (up / down stairs) has a small problem when I repair computers which casuse the RCD to fail I have to go to the celar to reset it and as it also has the TV and other units with clocks which have to be reset and Sky boxes .”

314 DIVISION OF INSTALLATION is the regulation covering this. It is a problem for electricians as it says “reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation” but it does not define normal.

”Can I install a ring main 32A 2.5mm cables to two radial curcits with their own 16a/rcd seprate curcit in a small room over a desk length of about up to 4 meters in length..

each "16a/RCD" unit in would have 3 or more double sockets in 100x50 Tower trunking. Marked with warnings about max current for the set of sockets.

Many Thanks for your comments.”

This could be one method. However there are some problems with it not least you need to run cables from the consumer unit and if these are buried in the wall you will need to use Ali-tube cable, plus the problems in complying with Part P.

Computers do not draw much power and so isolation transformers can likely do the job with a lot less outlay. You will likely be able to get a 750kva transformer for under the £100 you would need to pay the local council building control for permission to do the work which will deliver 3A which is likely enough to power a PC.

The UPS (Uninterruptable power supply) come in two flavours. Those, which isolate, and those, which during normal operation have a direct connection. The former will do what you require.

As to the safety of that supply you have a number of options. Using an isolation transformer for a single item you may consider no earth is best option, or centre tapped earth, or RCD. I would personally use an RCD. I have had problems in the past with centre tap because the fuse is often only fitted in line not neutral. In fact our benches had 10ma RCD’s fitted to sockets our thoughts were if it trips a 10ma RCD then the power supply is destined for the bin as likely to cause problems in the field.

Earths in a workshop are of course considered as part of the risk assessment, and there is a big question mark about the use of anti-static mats, and wrist bands, because they can give the user lethal shocks. I know in the university very regular checks were made as to the earth loop impedance, and you, will as a place of work, come under the Electricity at work act. Even if self employed. It is considered that the strap should be connected via a 1M ohm resistor which is large enough to stop lethal shock but small enough to leak away static.

However this is not a DIY question and really you should be posting on something like the IET forum which is designed for the more technical questions.
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Postby PeterG65 » Wed Jun 16, 2010 11:35 pm

ericmark wrote:

314 DIVISION OF INSTALLATION is the regulation covering this. It is a problem for electricians as it says “reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation” but it does not define normal.

”Can I install a ring main 32A 2.5mm cables to two radial curcits with their own 16a/rcd seprate curcit in a small room over a desk length of about up to 4 meters in length..

each "16a/RCD" unit in would have 3 or more double sockets in 100x50 Tower trunking. Marked with warnings about max current for the set of sockets.

Many Thanks for your comments.”

This could be one method. However there are some problems with it not least you need to run cables from the consumer unit and if these are buried in the wall you will need to use Ali-tube cable, plus the problems in complying with Part P.

However this is not a DIY question and really you should be posting on something like the IET forum which is designed for the more technical questions.



(Under old Part P I could have done the work but since 17th ed, I should not as I do not have more than a working knollege from my school days as a elec's mate)

The cables would have been in trunking except for a length from the Consumer unit to the trunking in the celler.

Just thought it would work but will have to look in to other options.
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Postby ericmark » Thu Jun 17, 2010 11:47 am

Your post was headed would this be legal.
As a result I tried to answer with that in mind.
Very few installations will comply with all the regulations and there is a problem when work and domestic are done in the same premises as of course in a commercial premises you don't have to consider children and you can have an area for authorised personal only.

Computers should not trip a RCD but we all know they do. Thinking legal this is a problem as if the leakage exceeds 3.5ma it should not be using a 13A Plug.

543.7.1.1 Equipment having a protective conductor current exceeding 3.5 mA but not exceeding 10 mA, shall be either permanently connected to the fixed wiring of the installation without the use of a plug and socket-outlet or connected by means of a plug and socket-outlet complying with BS EN 60309-2.

So if a computer is to comply and we can have 15ma leakage without tripping a 30ma RCD then we should be able to supply 4 computers and it would never trip the RCD. However in practice this is not the case. It is all down to the spike filters which considering all power is converted to DC and feed in to capacitors and then turned back to AC transformed to required voltage and then back to DC with yet more capacitors I fail to see how any spike will end up at the PC's board and if it does then far more likely generated by the power supply. i.e. Not mains born.

But we can not do anything about that and we have to realise that even one desk top from time to time if switched on when sine wave is at wrong place in it's cycle will trip a RCD.

So to repair PC's likely one will need to break the rules. I use a UPS and I know it does not comply. It will not trip the RCD if the computer is faulty and there is no RCD built into the UPS and if I was asked to set up the same in a school or college I would refuse.

And we are back to risk assessment. As a radio ham I was trained to consider the risk with my hobby and I had to take an exam to prove I had enough knowledge to continue learning on my own. However very few hobbies require the precipitants to take exams. And when I did my course access to HE with IT the only health and safety issues were using seats set correct and not looking at screen too long. Even though as part of the course we wired up rooters.

So it must be up to you to decide if you feel you have sufficient technical knowledge, relevant practical skills and experience for the nature of the electrical work undertaken and is able at all times to prevent danger and, where appropriate, injury to yourself and others. If so by definition you are "Competent". Oddly a "Skilled person" Is a person with technical knowledge or sufficient experience to enable him/her to avoid dangers which electricity may create. So skilled is not as good as competent!!!

But if you are competent then you should be able to make it safe even if it does not follow to the letter the requirements for electrical installations as laid out in BS7671:2008 and at the end of the day you just need to use some common sense.

Part P is something else. That does not seem to follow common sense in first place seems one can fit all the sockets you want in a utility rooms with high powered washing machines, tumble driers and sinks. But you can't in a kitchen as it has a food preparation area. And I can see nothing about a food preparation area which makes it more dangerous than a utility room? OK bathroom but even that fitting a socket in my dad's bedroom is no more dangerous than my mother's bedroom but because my dad's has a shower in it comes under Part P. And BS7671:2008 defines a circuit as an assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s). So fitting a fused protection unit will create a new circuit but under Part P seems they have a different definition and only if taken from a consumer unit is it considered a new circuit. There are two documents one called "Approved Document P" and the other "BC Consolidated Bldg Regs" and they conflict with each other. Schedule 2B is of interest where it states you don't need to notify replacing any fixed electrical equipment which does not include the provision of (i) any new fixed cabling; or (ii) a consumer unit; This does not seem to agree with the first document which says you can fit sockets in rooms other than kitchens and bathrooms and I fail to see how anyone can fit a new socket (not replace) without having some "fixed cabling" and the consumer unit bit is daft as a consumer unit which is a type tested distribution unit must be safer than a non type tested distribution unit?

My mother has a saying "First catch yu monkey" and in reality for most DIY work in owner occupied housing will never be seen by anyone able to complain until there is a change of occupant at which point a PIR should be carried out anyway. It does mean if some one was injured in your house the authorities could prosecute easier.
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