UK DIYer Newbie - adding additional sockets

[Pete999]

Would there be any benefit in putting a fused connector unit before the socket? if so, what size should the fuse be?
Thanks

13 Amp. . .

 

[pc1966]

There comes a time when you have to step back, look at something, and take a difficult decision that might cost more money now but is going to save you a lot in the future.

If you had a car that was 10 or so years old and the garage told you it needs a grands worth of work to get it through its MOT in most cases you would call it quits and replace it, as you can see it is downhill from that point onwards.

Here you have a fuse board that is at least 30 years old, has an identifed serious risk in how it is currently connected, and quite possibly other non-obvious faults that thankfully have not yet caused an obvious problem, fatal shock, or fire.

I am advising you to get a professional electrician it to replace the board and sort out that issue at the same time. Do your homework in finding a couple of local sparks who have a good reputation, are members of a professional body, and clearly it is their main job. Get a couple of quotes and if you have any queries folk on here will be helpful.

I can't speak for any others, but I am reluctant to offer any more advice because I don't think you can safely resolve all of the possible issues here. I have nothing to gain from telling you to get someone in - I don't even do that sort of work as a commercial service - and others here are retired or not in your area, so this advice is based on a genuine concern for your safety.

 

[alljohn]

There comes a time when you have to step back, look at something, and take a difficult decision that might cost more money now but is going to save you a lot in the future.

If you had a car that was 10 or so years old and the garage told you it needs a grands worth of work to get it through its MOT in most cases you would call it quits and replace it, as you can see it is downhill from that point onwards.

Here you have a fuse board that is at least 30 years old, has an identifed serious risk in how it is currently connected, and quite possibly other non-obvious faults that thankfully have not yet caused an obvious problem, fatal shock, or fire.

I am advising you to get a professional electrician it to replace the board and sort out that issue at the same time. Do your homework in finding a couple of local sparks who have a good reputation, are members of a professional body, and clearly it is their main job. Get a couple of quotes and if you have any queries folk on here will be helpful.

I can't speak for any others, but I am reluctant to offer any more advice because I don't think you can safely resolve all of the possible issues here. I have nothing to gain from telling you to get someone in - I don't even do that sort of work as a commercial service - and others here are retired or not in your area, so this advice is based on a genuine concern for your safety.

No, that's fine - just looking for a short-term solution until finances allow for a proper resolution. Many thanks for all your help.

 

[pc1966]

No, that's fine - just looking for a short-term solution until finances allow for a proper resolution. Many thanks for all your help.

I know that finding ~£500 is not always easy or possible, but really don't touch that circuit at all - disturbing it could trigger a fault which it is not adequately protected against. Better still pull the 45A fuse and wait until it can be safely sorted.

If the circuit protection was good then no problems with advising you how to add a socket, but not when it is the electrical equivalent of an unexploded bomb.

Out of curiosity I tried to calculate what would happen if there was a L-E fault at the remote socket, making some broad assumptions on the way:

• Your supply is 230V with an impedance of 0.3 ohms, roughly split between the service live and earth (a little below the assumed worst case for TN-C-S supplies).
• You have about 30m of 2.5mm T&E with a 1.5mm earth conductor
• The fuse is a 45A one following the curves in Figure 3A1 of the 18th edition wiring regulations

This leads to the following equivalent circuit:

In the event of a fault you have a supply impedance to the end point R1 of 0.15+0.222 = 0.372 ohms, and an earth/CPC impedance R2 of 0.15+0.363 = 0.513 ohms

As a result around 260A flows and anyone touching the end "earth" would find it at 133V above the true earth potential. Assuming they have an approximate impedance of 1.875k ohm to Earth (the 50% 100V case from Wikipedia) they would get a 71mA shock, this is above the "let go threshold" so they would probably suffer it until the fuse cleared the fault.

The fuse would take around 2.25 seconds to disconnect, that puts a 71mA shock in the 5-50% probability region of causing heart fibrillation and likely resulting in death. If someone was unlucky and a bit damp, the current could be easily 50% higher and at this sort of exposure time solidly in the > 50% chance of fibrillation.

During that fault time the conductors in the T&E cable would reach about 266C, so around 100C above the short-term damage level. So even if no one is electrocuted, and no bad connections to create higher temperature hot spots, the whole circuit (and possibly any bundled with it on route) would have to be rewired.

If you re-run the calculations with 0.8 ohm supply impedance, the upper 5% or so of a TN-S supply (which yours looks a bit like from the photo), then you get around 166A flowing and a resulting disconnection time of around 35 seconds. As a result the 2.5mm T&E's CPC conductor would reach a temperature of around 623C, most likely starting a fire.