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- 13 May 2026
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Hi
Something weird appears to be going on here in that when i switch off the lighting circuit mcb, the lights do not go out. However, if I also switch off the kitchen heater mcb, there is no power to the lights.
I have confirmed this by switching off the mcbs the opposite way round i.e. kitchen heater mcb off - lighting circuit on, and I still have power to the heater. I had a look at the kitchen heater fcu, and the wires are as expected i.e. black and red to heater, and black and red from the CU, plus the earths, therefore nothing amiss there. Further basic diagnosis is as follows:
Lighting circuit and kitchen heater circuit
1) Both off - no power.
2) Lighting circuit off - lights still on.
3) Kitchen heater circuit off - still on.
4) Fuse removed from bathroom fan fcu with both circuits off - no power.
5) Fuse removed from bathroom fan fcu with lighting circuit on - all lights except bathroom on, and no power to kitchen heater.
6) Fuse removed from bathroom fan fcu with kitchen heater circuit on - kitchen heater has power, also bathroom light and extractor fan with timed overrun.
I had a look at the bathroom extractor fan (with timer overrun - Greenwood vac with a pull cord for manual operation without switching on the light) fcu and not mincing my words here, it is totally spaghetti. Also, there is a blue+brown+earth cable running directly from the CU to the bathroom pull cord switch, which I find weird.
I would like to add that the flat was built by a New Town development corporation circa 1985, and going by the wall switches for the non bathroom lights, the reds are looped through the switches, and the blacks (to/from the light fittings either side - not the black that becomes the switched live) are looped at the fitting/rose. As the single wires for the lighting circuit runs in metal conduits, there is a fourth earth wire (2 looped to/from the fittings either side, 1 for the fitting, and 1 for the conduit) rather than the expected 3 that would be found.
When the person who was here before me had a very bad road traffic accident that affected their mobility, the bathtub was ripped out and a shower cubicle was installed. I am thinking that somebody had asked if it was possible for the bathroom extractor fan to run independently of the bathroom light, seeing the pull cord on the fan and thought it would be ok without fully understanding how timer overrun extractor fans work, hence a partial explanation for some of the wiring mess.
I have attached images below.
Could somebody help me as to what all this mess is please, before I call in the professionals (the fuse carrier to the fcu is cracked, and the light fitting is showing signs of corrosion so would need replaced asap)?
Something weird appears to be going on here in that when i switch off the lighting circuit mcb, the lights do not go out. However, if I also switch off the kitchen heater mcb, there is no power to the lights.
I have confirmed this by switching off the mcbs the opposite way round i.e. kitchen heater mcb off - lighting circuit on, and I still have power to the heater. I had a look at the kitchen heater fcu, and the wires are as expected i.e. black and red to heater, and black and red from the CU, plus the earths, therefore nothing amiss there. Further basic diagnosis is as follows:
Lighting circuit and kitchen heater circuit
1) Both off - no power.
2) Lighting circuit off - lights still on.
3) Kitchen heater circuit off - still on.
4) Fuse removed from bathroom fan fcu with both circuits off - no power.
5) Fuse removed from bathroom fan fcu with lighting circuit on - all lights except bathroom on, and no power to kitchen heater.
6) Fuse removed from bathroom fan fcu with kitchen heater circuit on - kitchen heater has power, also bathroom light and extractor fan with timed overrun.
I had a look at the bathroom extractor fan (with timer overrun - Greenwood vac with a pull cord for manual operation without switching on the light) fcu and not mincing my words here, it is totally spaghetti. Also, there is a blue+brown+earth cable running directly from the CU to the bathroom pull cord switch, which I find weird.
I would like to add that the flat was built by a New Town development corporation circa 1985, and going by the wall switches for the non bathroom lights, the reds are looped through the switches, and the blacks (to/from the light fittings either side - not the black that becomes the switched live) are looped at the fitting/rose. As the single wires for the lighting circuit runs in metal conduits, there is a fourth earth wire (2 looped to/from the fittings either side, 1 for the fitting, and 1 for the conduit) rather than the expected 3 that would be found.
When the person who was here before me had a very bad road traffic accident that affected their mobility, the bathtub was ripped out and a shower cubicle was installed. I am thinking that somebody had asked if it was possible for the bathroom extractor fan to run independently of the bathroom light, seeing the pull cord on the fan and thought it would be ok without fully understanding how timer overrun extractor fans work, hence a partial explanation for some of the wiring mess.
I have attached images below.
Could somebody help me as to what all this mess is please, before I call in the professionals (the fuse carrier to the fcu is cracked, and the light fitting is showing signs of corrosion so would need replaced asap)?
