Improving my knoweldge..

[Shaneyboy]

In an effort to educate myself about my house’s circuitry and wiring, I have been busy reading up on some basic home electrics and investigating all my fixtures and light fittings. My consumer unit is no longer the daunting box of wires that it used to be. I now understand the basics of a ring main and a radial circuit. However could someone explain the following to me:

1. I have two MCB’s for upstairs and downstairs lights. Both MCB’s have two sets of twin and earth cables attached. Two red live wires are coming out the top, black and earth are connected to the relevant neutral and earth bars. As lighting circuits are usually of the radial type where the cable leaves the CU and runs to each outlet before terminating at the last fitting, why are there two cables instead of one?

2. What is the difference between a fused connection unit and a switched fuse unit, or are they both the same thing?

3. I am in the very early planning stages of sorting out some power to the garage. Where a radial circuit wired in 2.5mm from a 20-amp MCB must not run across an area greater than 20m, does this include the cable run from the house to the garage or just the area of the garage interior?

Kind regards.

 

[flameport]

1 - Either there are 2 circuits connected to each MCB, or whoever installed it made it a ring because they didn't know what they were doing.

2 - The difference is one has a switch, the other does not.

3 - Where have you got this 20m limit from?

 

[sparkticus]

In an effort to educate myself about my house’s circuitry and wiring, I have been busy reading up on some basic home electrics and investigating all my fixtures and light fittings. My consumer unit is no longer the daunting box of wires that it used to be. I now understand the basics of a ring main and a radial circuit. However could someone explain the following to me:

With safety always in mind I hope.

1. I have two MCB’s for upstairs and downstairs lights. Both MCB’s have two sets of twin and earth cables attached. Two red live wires are coming out the top, black and earth are connected to the relevant neutral and earth bars. As lighting circuits are usually of the radial type where the cable leaves the CU and runs to each outlet before terminating at the last fitting, why are there two cables instead of one?

This may be a lighting ring, if so then it is not required to be a ring.
Or (and more likely) you have another lighting or door bell circuit "bunched" with the original lighting radial. This is not good practice.

You could safely remove one red at a time and see what does not turn on anymore?

2. What is the difference between a fused connection unit and a switched fuse unit, or are they both the same thing?

3. I am in the very early planning stages of sorting out some power to the garage. Where a radial circuit wired in 2.5mm from a 20-amp MCB must not run across an area greater than 20m, does this include the cable run from the house to the garage or just the area of the garage interior?

No idea what you mean by the 20m area. Please provide detail.


Please be aware that if you do this work yourself then you must:

1) Notify your local area building control first
2) Do the work in full compliance with wiring regulations
3) certify the work you have done including completion of an electrical installation certificate showing detailed test results.
4) Have building control inspect the work and the certificate.

 

[Paul_C]

Or (and more likely) you have another lighting or door bell circuit "bunched" with the original lighting radial. This is not good practice.

It depends. If somebody just combined two original circuits to make one without any thought as to the load or whether all the lights would end up on one circuit, then it's certainly not very good practice.

But it could be that the circuit started out that way for convenience of wiring. So long as the loading is satisfactory, there's no reason why one should not take two cables from the distribution board to feed two different sections of the same circuit. Sometimes it works out more convenient that way.

Another common occurrence is where a light has been added at some point (e.g. an outside light), and it's just easier to run the cable direct from the board than to tap into that circuit elsewhere. Again, so long as loading etc. is satisfactory, that's fine and perfect acceptable practice.

No idea what you mean by the 20m area. Please provide detail.

The floor area specified for the various "standard" BS1363 radial circuits? These have changed quite regularly over the years.

Please be aware that if you do this work yourself then you must:

1) Notify your local area building control first
2) Do the work in full compliance with wiring regulations
3) certify the work you have done including completion of an electrical installation certificate showing detailed test results.
4) Have building control inspect the work and the certificate.

Not all strictly true. The Wiring Regs. (BS7671) are not mandatory. And if you notify the LABC, the fee you pay is to cover the authority inspecting and testing.

 

[Shaneyboy]

Thanks for the swift replies. Safety in mind always. I switch off the circuit and test that it is dead with my multimeter before I do anything.

The 20m restriction I got from a book called 'The Complete Handbook of Home Electrics' by Julian Bridgewater, it is also in the B & Q DIY manual, sad, I know, sorry.

As and when I do the work, a friend of mine who is a domestic installer will be doing the work and I will be assisting him, which I am greatly looking forward to.

 

[sparkticus]

Thanks for the swift replies. Safety in mind always. I switch off the circuit and test that it is dead with my multimeter before I do anything.

Yes, never work live.

The 20m restriction I got from a book called 'The Complete Handbook of Home Electrics' by Julian Bridgewater, it is also in the B & Q DIY manual, sad, I know, sorry.

Yes, since you have a registered domestic installer on the case then speak to them about any restrictions.

As and when I do the work, a friend of mine who is a domestic installer will be doing the work and I will be assisting him, which I am greatly looking forward to.

Good, there are many requirements. A registered domestic installer will know the ropes.

 

[sparkticus]

Not all strictly true. The Wiring Regs. (BS7671) are not mandatory. And if you notify the LABC, the fee you pay is to cover the authority inspecting and testing.

Not strictly true but sound advice for a DIYer which I though the OP was from the original post.


True that BS7671 is not mandatory but what would you propose a DIYer comply with?

The LABC in my area expect a completed electrical installation certificate.

 

[PrenticeBoyofDerry]

The LABC in my area expect a completed electrical installation certificate.

and so do mine! plus schedules of inspection and tests.

True that BS7671 is not mandatory but what would you propose a DIYer comply with?

or take to court with them as evidence that the system that was installed, was compliant to relevant requirements.