Bathroom wiring and missing CPC

[powernun]

Hi again, everyone,

I am now looking into wiring the lights, a couple of shaver sockets and an extractor fan for my bathroom.

My plan is to use the 6A light circuit for this, but there are a couple of complications.

1. The light circuit CPC is not continuous. There is definitely a CPC coming out with the twin and earth from the Consumer Unit for the circuit, and there is definitely a CPC on some (but not all!) of the light switches and pendants. But there is no continuity. The reason is almost definitely a 2-core (no earth) cable somewhere in the middle, as I have found them in some other switches and pendants.
2. The lights are mainly wired using the "junction box" method. Basically, junction boxes are littered everywhere, making tracing quite a pain, and providing no neutral at the switches.
3. I want the extractor fan to be a dumb on/off one (for now)

To solve (1) and comply with the regulations, I figured the safest thing to do temporarily would be to borrow the CPC of a nearby socket, by using a 4.0mm² single-core cable in green/yellow. That would reinstate the CPC at least in the bathroom wiring and potentially in other accessories as well. After I finish in the bathroom, I will trace the broken CPC and properly reinstate continuity by rewiring that section with a 2-core and earth.

For (2), to make my life easier and provide me with a neutral at the switch, I thought I would wire the bathroom lights and shaver sockets with the 2-plate method. I will first take the feed from the previous junction box ("Previous Circuit 3 Accessory" in the diagram), feed the switch, then feed the shaver sockets, then go back out from the bathroom to the next junction box ("Next Circuit 3 Accessory" in the diagram). That way, I will at least not have a hidden junction box for the new bathroom wiring, and I will also have a neutral at the switch.

As for the fan, I don't want it to come on with the lights. For now, I only want manual control of it, which I will achieve with a second gang in the light switch, and a 3-core and earth to a 3-pole isolator, then finally the inline fan. The reason for doing it this way is that, in the future, I plan to fit a shelly after the 3-pole isolator to control the fan using humidity and occupancy sensors. That would, of course, make the 3rd core redundant, but I will wire it with 3-core and earth anyway, just in case I decide to take the shelly out later.

Does the above make sense? Any improvements or alterations I should consider? I have also included a diagram describing the above a bit better.

Diagram (please open the PDF instead if the PNG is not high enough resolution)

 

[Murdochcat]

How old is the property / wiring?

I recommend you fix the faults BEFORE making any changes

And what makes you think the CPC isn’t continuous? Have you tested for this?

 

[powernun]

How old is the property / wiring?

I recommend you fix the faults BEFORE making any changes

And what makes you think the CPC isn’t continuous? Have you tested for this?

It's a 1930's property, but the wiring itself is a mix. Most of it post-2006, some pre-2006.

I have checked CPC continuity for all circuits between the CPC in the CU and the CPC at the accessories. The lights circuit is the only one where there was no continuity in the accessories with a CPC. I am pretty sure of the reason. I have partially traced the light circuit:

From the CU for the lights circuit, a twin and earth comes out
-> that then goes to a junction box
-> from the JB a 2-core (no earth) comes out and goes into the wall
-> within the wall somewhere, the 2-core (no earth) is connected to a twin and earth
-> then that twin and earth is connected at the JB for a room's lights
-> from that JB, a twin and earth goes to the pendant
-> then from the pendant to the switch

So that 2-core cable is the problem, as it is sitting between the twin and earth from the CU and the twin and earth of the lights JB. It is buried in the wall, going through multiple rooms.

I agree with fixing any faults first, which is why I thought that supplying CPC from the sockets should fix the fault. From my understanding, this is a valid way to fix it, with the only risk being that if the socket is maintained in the future, or the circuit is removed, the CPC could be removed from the lights. I plan to only keep this in place for a month or two and label the 4mm in the socket accordingly. Also any circuit or accessory alteration I would be aware of anyway. This should eliminate all risk of the CPC being disconnected again until I am finished with the bathroom. After which, I will dig the walls and replace the 2-core with a twin and earth.

 

[Murdochcat]

When you say you have checked continuity - what are the readings? Or did you just follow the cables

 

[powernun]

When you say you have checked continuity - what are the readings? Or did you just follow the cables

I measured the resistance between the CU's earth bar and the cable's CPC in the accessory, using a calibrated multimeter.

For example, on the sockets, the resistance between the CU earth bar and a socket's earth point was about 0.05 ohms (dead short), but on the lighting circuit, the resistance between the CU's earth bar and a switch's twin and earth CPC conductor was 0L (open).