switched live is also neutral?

[skuller19]

Replacing a ceiling rose that was taken off some time ago and not replaced in an old house. 2 reds and 2 blacks, I have identified which one of the blacks is switched live using a multimeter. But it appears the 2 blacks also are connected when tested with the meter, indicating it is both a neutral and a switched live? How is this possible and how can I rectify this?
Thanks

 

[GMES]

Hi and welcome to the forum, I see in your profile it says you're a Trainee are you currently on a course.

 

[Adam W]

:welcome2:

 

[skuller19]

Hi and welcome to the forum, I see in your profile it says you're a Trainee are you currently on a course.

First year apprentice, done this job before but didn't encounter this problem

 

[davesparks]

It's a good job GMES pointed out that you are a trainee, I was very near going off on a rant as any qualified electrician should know this!

The reading you are getting will be via the other lamps on the circuit. With the switch closed the S/L is connected to the permanent live, which in turn is connected to neutral via the other loads on the circuit.
Removing all of the lamps on the circuit, and disconnecting any other loads should get rid of this odd reading.

If you had gone a bit further with the testing you might have found that the switched live also appears to be connected to earth. But I'll leave it to you to work out why.

 

[skuller19]

Thanks learning everyday,there is no earths on this circuit may not have been required or just omitted way back when this house was built?
Wiring it up with the switched live in same block as lamp live, 2 reds together, and remaining black with blue going to the lamp correct?
Light does not turn on when power is switched back on? Owner says they changed ceiling rose on the same circuit before I'm going to have a look at this to make sure it's done correctly

 

[davesparks]

An earth was not required many many years ago, but obviously it is now.
So BEFORE you start work on any circuit like this you need to make a proper assessment of the safety of the circuit for continued use.
Generally replacing a single light fitting or switch due to it having been damaged etc will be fine as long as the new item is double insulated/has no exposed metal parts etc. But you should also make the owner aware of the situation and explain to them why it is sensible to think about and plan for rewiring the circuit(s).

There is a dodgy bit of advice floating around that suggests it is acceptable to fit an RCD and stick a little sticker on the CU as a method of resolving this issue, but this is not referenced in, nor compliant with, the regulations.
The suggestion originated with the NIC and even they stayed that it was only to be considered as a last resort if fixing it properly is impossible for whatever reason.