Does this one need an electrician?

I am not a bright spark, but I am a very cynical one. Given the oddities here do not assume that the black wire is a neutral. I once; long ago, found a three phase system where somebody had used a red black and green three core cable for the three phase conductors. The explanation was basically that "Old George did that and he picked up bad habits during the war". John

This will be very common - in fact in many older installs it would be rare to see a grommet. It is more an issue during installation than use unless it is on a circuit that is fully loaded to near capacity relatively frequently (where thermal expansion might result in regular cable movement).

You would normally only sleeve a wire when it is being used for a purpose that conflicts with its colour. So for example, in a single "one way" light switch you will typically have two wires *both* of which are live - one permanent, one switched. Since this is commonly supplied with a normal T&E cable, one should over sleeve the black/blue in Red / Brown (depending on the colour scheme being used), to indicate the change of use. Again however is is very common for this to be missing. For an electrician, they may disregard the convention since it is "obvious" that is what is going on. Arguably if you are poking about in the back of switches, then it should ideally also be obvious to you as well.

(The sleeving is still very handy, when there are multiple cables - say in a ceiling rose, since it makes identifying the switch cable easy when the rose has been disconnected. As evidenced by the number of posts we have had here over the years along the lines of "I changed the ceiling rose, and put all the reds together and all the backs together, and now the circuit trips when I turn the light on")

Do you know if it *is* actually neutral though?

Depends on if it is still connected to anything. The "by the book" way of dealing with unused wires is they should be made off in an unused terminal, and connected to earth at at least one end. In the latter case you could sleeve it in green/yellow.

In reality, taped over wires are common. Folded in some sleeving is also common ( and arguably better than tape where the glue could fail over time and it comes unwrapped).

Is it broken? Is there an immediate safety risk to users?

If not it might be better left alone.

Surprisingly there can be quite a big disparity between distribution side, and consumer side as to what is normal and accepted.

Time for tea :-)

Likely it was originally a multiple switch plate, possibly three switches which might explain the two taped off red wires which would likely be switched lives going to multiple lights originally.

It looks like the loop in is being done at the switch instead of at the rosette, hence the two reds in COM which will be the live feed going from switch to switch. L1 will be the switched live going to the rosette/light fitting. Check the rosette you will likely find only two wires probably done in singles the black neutral being the only wire connecting all the rosettes and the red being the switched live from the switch. Our lighting in our 1975 bungalow was done looping in at the switches in our case all the wires were done in singles (kind off). Red sheathed wires with a green stripe on one side entered the back box. Stripping the outer sheath revealed a second red sheathed conductor and a bare CPC. At the rosettes the single and earth switched live and the black neutral were present. Are you sure the black wire is actually black and not just discoloured.

As for grometting the back box some lighting 16mm deep boxes do not need a grommet as the cable entry is peined over forming a rounded edge, running your finger over it should verify that.

Richard

Indeed. Worse, I encountered a disco where a three coloured light unit was wired up with for colour three phase + earth cable, with the earth being used as a common neutral, There was no real earth

Yup, you sometimes find "loop through switch" configurations, like in:

Have a plaster handy in case it is not :-)

I'm not. I said neutral 'could be' connected to the earth terminal. I think it's a 'known unknown'. Is there an easy way to find out whether the black wire is neutral or earth without tracing it back?

Thanks. I get that for the red ones. We discussed this with my previous switch. My bigger concern is the black one, which is ambiguous. I cannot responsibly sleeve it as earth if I don't know.

Agreed, but to be fair I did identify the live cables.

Well, no. This is my question, how to find out without taking the house apart!

Well, yes. The adhesive seems to be deteriorating here.

It's more of a theoretical risk if the black wire is neutral, in a fault situation could the two screws not become live? Also, for a future inspection I would like to regularise the position.

I am reluctant to ask my friend but I'm sure he would fix it for a curry.

:-)

Indeed and I was not intending that comment to sound derogatory. More a case of doing a bit of research first to make sure you are aware of some of the more common options that might be in play.

Do you know which circuit it is on? If so turn it off. Now measure resistance between the unknown wire and earth. You would expect a low resistance in a house with TN earthing. Then disconnect the circuit neutral in the CU, and test again. You would expect to read open circuit if it is actually a neutral. If it is actually an earth, the you would still see a low resistance.

If it turns out to be a neutral, then disconnect from the backbox and stick a wago or similar on it to keep it out of contact with anything.

The biggest risk I suppose would be on TN-C-S installation that has lost connection to the incoming Combined Neutral and Earth (CNE) conductor. That would make all earthed metalwork in the house (as well as anything connected to neutral) float at mains voltage. However that is a rare circumstance (partly due to the Multiple of PME - making sure that the CNE is earthed in multiple locations).

In that fault scenario I expect you have bigger worries than the screws on the light switch!

Even if live they ought not create a significant shock risk - since it would be hard to stay in contact with them for any amount of time.