Adding RCB to lighting circuit at consumer unit

24 Oct 2004
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United Kingdom
Installed outside lamps using 1.5 SWA. The outside wiring is connected to a light switch located inside the house. The 1.5mm T&E from the switch is connected to SWA cable within the house using appropriate jointing box and cable glands. I have only just realised that the downstairs lighting circuit that I am connecting the outside lighting to is not connected to the rcb in the consumer unit. The RCB in the consumer unit only protects the socket circuits. Can I add an RCB to the downstairs lighting circuit to offer the required regulation protection to the outside lighting circuit I have just installed
maldin :?:
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When you say RCB, do you mean RCD?

If so you could change the downstairs lighting MCB to an RCBO, but this is notifiable work.

A photo of your consumer unit would help.
Using one of these
as the light switch would provide RCD protection as would the RCBO route.
The latter would protect all the circuit which needs consideration as any faults could switch off all the lights on that circuit. To comply with:-

314.1 Every installation shall be divided into circuits, as necessary, to:
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.

Lighting circuits are normally either split into two or have emergency lights so if there is a fault one is not plunged into total darkness. It is down to a risk assessment it could be that street lamps mean you don't need any emergency lights.

Adding to circuits is not just a case of sticking in a few extra cables one has to measure the impedance of the existing circuits to ensure with a fault condition the protective device will open within the required time.

As already said the work is notifiable and so you need the readings so with LABC charges and hire charges for the test equipment it is likely to cost over £200 which is likely more than a registered electrician will charge to do whole job so not worth doing as a DIY job.
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Thanks for all your input. The consumer unit is manufactured by MEM and has individual breaker protected circuits for:
Downstairs sockets, Upstairs sockets, Smoke Detectors, Cooker, Heater, Upstairs Lighting and Downstairs lighting. There are 2 spare breakers marked sockets which are not used.
Yes I did mean RCD not RCB in terms of protection. :oops:
Ideally I would have liked to install a separate circuit with RCD protection just for the outside lights but this is not possible without major re-decoration. The original outside lighting circuit was wired to the downstairs lighting circuit, I have so far only replaced the existing T&E that was ran in PVC conduit with SWA and suitable glands for outside protection.
The ciruit needs some form of protection as the lighting is 240v. It may be possible to remove the spare socket breakers and install a separate RCD for the downstairs lighting and outside lighting ciruit. I'm not concerned with failure of the complete downstairs lighting circuit should their be a fault that causes the RCD to trip. Thats what it is designed to do. I will locate an electrician that can size and install the RCD or RCBO for me.
I do not have an emergency battery back power system or standby generator or a street lamp of suitable intensity should there be a failure. I do however have a torch.
On the MEM, if it is a Memera 2000m or 2000AD range, the MCBs can be replaced by RCBOs fairly easily, the RCBO is a combined unit which has an MCB and an RCD, they are widely available. Similar to ericmarks second picture. It is notifiable work.

It is not uncommon for an outside lamp to cause an RCD or RCBO to trip, as they are prone to rainwater getting inside.
Hi AdJohn,

It is a Memera, although it was installed before 17th Edition, it's a model AD10HED. I have looked at MEM's RCBO's and will check with the manufacturer to confirm that they will fit my consumer unit. They appear to offer a Type B and C the difference being the sensitivity. I'm attempting to locate the required formulae / calculations that would enable me to select the correct sensitivity, just to satisfy my curiosity. thanks once more for all the help from you guys.
The type B and type C are referring to the magnetic setting of the MCB part, what type of MCB is there now?

As flameport asks, why do you think you need them on an RCD?
In a domestic installation you need a "B" type. The reasons for a overload-tolerant one do not apply domestically, and your circuit has to be capable of withstanding the longer fault duration that might occur.

Be careful as there are a lot of "C" type around from industrial installations where MEM is widely used.
Hi Spark123,
I only assemed that I would need an RCD because the outside lights come directly off the downstairs lighting circuit and will be 240volt. I have previously installed an outside socket using SWA, IP66 enclosure with inbuilt RCD. This was installed according to regs. I understood the RCD was required to protect me from electric shock should I come into contact with a live cable or equipment that has become live due to a fault. I believed the same would apply to outside lighting. Or am I being over cautious.
Hi JohnD,
If required I will purchase a type "B". The part number on the RCBO should be sufficient to differentiate between the type "B" and type "C". At the moment the downstairs lighting circuit just uses a standard MCB, I assume it will be 5A or 6A but will need to remove the CU cover to confirm this
you can order by description from a competent supplier.

You need a 6A "B" type 30mA RCBO. There are several MEM and Eaton part no's that will fit (same company using different brand names), you can put an industrial Memshield 2 part in a domestic Memera (but not the other way round). Price and availability may vary as the industrial ones are very widely used.

The part no's beginning "AD" are a bit cheaper, but will be adequate for domestic use. The Memera 2000 range devices are fully interchangable with yours, which is a Memera 2000 AD.

The part no has an embedded "B" for B type and an "H" for High Sensitivity (i.e. 30 mA) and a "06" for 6Amp, and a "1" for single phase but I have forgotten the numbering key for their part no's. It is easier to understand when you are looking at a rack full of them. I have in my hand now an MBH106 which may not be a current part no. but you need someting similar.
Hi JohnD,
Thanks again for all the useful information. I will be ordering the required part and will have it installed over the weekend.

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