change of mcb to rcd

[thirdtimelucky]

I have a Proteus split load c/u. There is a circuit to an external garage which is terminated on the c/u with a mcb. Is it possible to change the mcb for a rcd?
It would be possible to change over circuits to put the garage onto the rcd load and change one of the rcd's over to the mcb load. However, I would prefer to simply replace the mcb with a rcd if that is viable?

 

[ban-all-sheds]

I have a Proteus split load c/u. There is a circuit to an external garage which is terminated on the c/u with a mcb. Is it possible to change the mcb for a rcd?

You MUST NOT replace an MCB with an RCD (even if you physically could, which is unlikely given their width and that they are 2-pole devices), because an RCD does not provide any overcurrent protection.

It would be possible to change over circuits to put the garage onto the rcd load and change one of the rcd's over to the mcb load.

What does that mean?

 

[rjm2k]

I'm definatly no expert but I believe an RCBO combines the function of an RCD and MCB into a single device taking 1 slot in the CU. Whether or not this is what you should be using I have no idea and I don't think what you have posted is clear enough for anyone to base advice on, really you need someone qualified on site.

Perhaps a photo of your main and garage CU along with details of the cable used to connect them, distance etc?

 

[Taylortwocities]

This circuit to the garage:

Why do you want to RCD-protect this cable?

What sort of cable runs from the consumer unit to the garage?

 

[JohnW2]

This circuit to the garage: Why do you want to RCD-protect this cable?

I presumed that the OP wanted to protect the garage circuit, rather than the cable to the garage (implying that there is no RCD in the garage).

To thirdtimelucky: although, as you have been told, you must not replace the MCB with an RCD, what you should be discussing with an electrician is adding an RCD to the garage circuit (either in the house or the garage), whilst also keeping the existing MCB.

Kind Regards, John

 

[Taylortwocities]

I presumed that the OP wanted to protect the garage circuit, rather than the cable to the garage (implying that there is no RCD in the garage).

Me too, but i wanted him to tell us that.

Personally I would prefer to see the RCD in the garage itself.

Let's see what his input is.....

 

[rjm2k]

Re-reading, my interpretation is that the main CU is split into RCD protected and non-RCD protected.

Currently the garage is non-RCD protected so OP wants to swap this with one of the currently RCD protected circuits (thus making that circuit non-RCD protected!).

Are you sure the garage is not RCD protected? Just because it's on an MCB on the CU doesn't mean it's not also protected by the RCCB(RCD) on there too.

Can you post photos?

 

[thirdtimelucky]

1. The cable to the garage was in situ before we moved to the property
2. The run is about 30mtrs.
3. The cable is underground and appears to be basic 2.5mm
4. I realise that it doesn't meet standards but I would prefer not to have to excavate the patio to re-run the alternative higher grade and armoured cable
5. It is terminated in the garage on a bog standard double socket.
6. I don't have a problem with fitting a rcd c/u in the garage but I thought I should protect the cable just in case anyone in the future dug up the ground and hit it.
7. I am not an electrician, however I do have C&G finals in Telecomms which includes electrical engineering and have fitted a few ring mains in my time prior to part p.

 

[thirdtimelucky]

post script. When I said 'protect the cable' I meant by terminating it in the house with an rcd BEFORE it reaches the garage

 

[rjm2k]

As another layperson, I would probably do the same as you, ie protect the non-regs cable in case it gets dug up. I wouldn't unprotect another circuit to do that though, so I would look into an RCBO (about the same cost as an RCCB) to swap with the MCB in your CU (assuming again that the garage MCB isn't also RCD protected through an RCCB without you realising it) and checking the technical details of the RCBO re compatibility etc.

Of course, since it's all non-regs and basically illegal, the world will probably end if you do any such thing.

 

[Taylortwocities]

1. The cable to the garage was in situ before we moved to the property

So, its wrong.

2. The run is about 30mtrs.

I hope you don't want to run too much off this circuit. Because of volt drop

3. The cable is underground and appears to be basic 2.5mm

Twing and earth PVC cable is NOT suitable for anything than fixed wiring inside buildings

4. I realise that it doesn't meet standards but I would prefer not to have to excavate the patio to re-run the alternative higher grade and armoured cable

Well, tough doo doo. Its wrong, its dangerous and you are now taking responsibility for it.

5. It is terminated in the garage on a bog standard double socket.

Handy

6. I don't have a problem with fitting a rcd c/u in the garage but I thought I should protect the cable just in case anyone in the future dug up the ground and hit it.

Over current protection such as you have is all that the cable will require. I would not want it connected to teh RCD side of my house, as an external circuit is very likely to trip your critical circuits in the house (eg freezer, wine cooler, etc)

7. I am not an electrician, however I do have C&G finals in Telecomms which includes electrical engineering and have fitted a few ring mains in my time prior to part p.

Yes, well, me too. and I did 10 years with BT Telecom then another 10 years in data comms before deciding to take up being an electrician. I did 3 years at night school before considering myself competent to risk other people's lives with fatal voltages rather than 50v DC.

And PS. The term is ring final circuit. Ring mains are to do with water distribution systems.

If you are doing it yourself:
You will need to rip out that T&E as its not suitable and install an appropriate sized length of SWA cable. This will need terminating and testing in accordance with BS7671. And you'll need to notify the local authority before starting work.

Or find a local electrician who can do teh tricky bit. I expect he would welcome you digging a half meter trench up your garden.

Sorry chap. That's the deal.

 

[JohnW2]

I presumed that the OP wanted to protect the garage circuit, rather than the cable to the garage (implying that there is no RCD in the garage).

Me too, but i wanted him to tell us that. Personally I would prefer to see the RCD in the garage itself.

If the job were done properly ('acceptably'), I would agree - for the reason you give (although changing current MCB to RCBO would avoid that problem). However, given the OP's subsequent revelations, if he insisted on sticking with the buried T+E, despite advice and despite the fact that it is probably (we're not told if it is protected in any way) unacceptable per current regulations, then it would probably be better to have RCD protection at the house end (probably by changing MCB to RCBO, thereby again also removing the problems of using a 'house RCD'). The OP's problem is that I imagine no reputable electrician would have anything to do with a circuit involving the current cable, so his only option would be to do it himself, illegally without notification.

Kind Regards, John.

 

[Taylortwocities]

agreed. if the T&E is staying in the ground, any additional protection would be better than not.

It goes against the grain though. RCDs are meant to providing Additional Protection. In this case the RCD would be there to provide basic protection.

And you don't learn that in C&G Telephony.

 

[thirdtimelucky]

The cable has been there since we bought the house, 13 years ago.
The T&E is buried somewhere under 4" of concrete and I have no desire to dig that lot up at 73 y.o. Running a new cable overground wouldn't be an option as I would still have to dig up across either my drive or patio.
As I am the only person using the power in the garage no one else is at risk even if there was a fault.
I have 4 options
1. cut off the power (That would cause some inconvenience)
2. leave it as it is
3. have it all renewed (not cost effective) it would be expensive
4. make it as safe as possible by other means. Other means being an rcd or rcbo (if an rcbo would do the job)

 

[JohnW2]

agreed. if the T&E is staying in the ground, any additional protection would be better than not. It goes against the grain though. RCDs are meant to providing Additional Protection. In this case the RCD would be there to provide basic protection.

I agree (for a TN system) with the spirit of what you're saying, but I don't think you're using quite the same language as does BS7671. 'Basic protection' is defined in BS7671 as "protection against electric shock under fault-free conditions", and essentially consists of things like insulation, barriers, enclosures and 'out of reach'. Chapter 41 (specifically 410.3.2) requires such 'basic protection' plus "independent provision for fault protection" (for TN systems, usually an MCB and satisfactory EFLI) and, in some (seemingly most, these days!) situations, also 'Additional Protection' (usually an RCD), per 411.3.3. Assuming TN (and satisfactory EFLI), the circuit with T+E cable we're discussing appears to satisfy both the first two of those requirements and, with an RCD/RCBO would satisfy all three. The only thing it doesn't appear to satisfy is the requirement for buried cables to be adequately protected (by armour or equivalent).

Of course, if it's a TT installation, then your argument probably falls flat on it's face, since the RCD is then the only means of providing acceptable fault protection.

And you don't learn that in C&G Telephony.

No personal experience, but I don't imagine that one does!

Kind Regards, John