Todays "how did it ever work" puzzle

[ProDave]

Call to a rental property.  "Landing light is tripping"

Get there and find whenever the landing light is turned on, the rcd trips.

It's an old style wylex board, split load half on rcd half not.

Almost certainly original wiring of a 1970's house. Didn't take long to establish it's the usual landing light borrowed neutral (L from downstairs lights, N from upstairs lights) the trouble is one lighting circuit is on the rcd side, the other is on the non rcd side.

But looking at the board, no obvious signs of any alterations the lablels of the two lighting circuits are old faded and worn like they have been there for donkeys, so I can't see one of the circuits has been moved recently.

My puzzle is how has it ever worked?
 

 

[bluetobits]

6w lamp fitted for night light?  

 

[ProDave]

the next installment of the puzzle, is just a few weeks ago, before the present tenant moved in, the property had a satisfactory EICR (not done by me) So whoever did that failed to spot the borrowed neutral or even the fact you can't use the landing light.

And how he got to the CU to do any testing is a mystery. The CU is in a corner, and somebody has built a corner cupboard around it all. With the doors open, it was as much as I could do to get the front off the CU and reach in with some probes. There is insufficient access to do any proper work on the board without ripping off that cupboard.

 

 

[SLIPSHOD & SLAPDASH]

I once found  the strappers mixed between the landing 2 way and the hall 2 way. Most of the time it was fine but with all switches in a particular   order the up & downstairs lighting circuits were joined together. I had tested the house a year earlier and missed this. Could it be the tenants have changed and the old ones knew not to turn the light on/off from one of the switches

 

[ProDave]

That's an interesting point S&S  The hall lights are on 2 way switching between the ends of the hall, and at one end share a switch plate with the landing light switch, so that's a distinct possibility.  I know the hall lights work, but I never actually checked they work correctly from both ends ion all combinations. I also know it's a new looking light switch at the foot of the stairs.

I was on limited time this morning so had to leave it to go to another job. That's something i will be checking when I go back.

There were a number of odd things about this install that again I didn't have time to check, like the strappers all appear to be blue (it's wired in singles in conduit) and the blue that was sleeved green/yellow and connected to the earth terminal was another strange one (disconnecting it temporarily made no difference)
 

 

[steptoe]

I was going to ask if it was new tenants,

It never ceases to amaze and dumbfound me the faults and problems some tenants seen happy to live with...

 

[Evans Electric]

I think thats the answer ....original tenants just got used to it ...never bothered to get it put right.

I pointed out a smashed socket with exposed live parts to a customer once ....." Oh don't worry about that , been like it for years "      Small children in the house but not bothered to get it done.