Whats with these old single core lighting cables! Both RCD's tripped together

[KDT Projects]

So im replacing an board for a new 17th dual and ive decided to put the upstairs lights on one RCD and the downstairs on the other, you know how i was taught to to do it so it minimises inconvenience if a lighting circuit trips, you know.

But then i come across these single core wires making for identifying each lighting circuit all that more harder. So anyway after a little trial and error i eventually wire the upstairs circuit onto one RCD and the downstairs onto another (well thats what i thought i did)

Feeling pretty happy with my work i turned on the breakers one by one and everything is good, well, it was right up until the lighting breaker MCB. I switched this on and both RCD's trip. Well now im seriously confused. I reset them and tried the other lighting MCB, same thing happened, both RCD's tripped.

So, maybe ive wired the neutrals to the wrong neutral bars for each respective RCD. OK so ill switch them around and try it again. No joy, both RCD's still tripped. Now i spend a long time staring at the melee of wires and getting a headache trying to calculate a solution.

I decided to turn to the internet for a solution, and low and behold a bright spark informs me that the two way lighting in the hall may be the problem. A borrowed neutral perhaps. Anyway i ended up wiring both lighting circuits into the same MCB on the same RCD and hey presto, problem solved.

Thanks for pointing that one out BS7671! Oh and my tutor for explaining this to me. I guess its like anything, exeperience counts for everything.

Anyone else come across this?

 

[High Tower]

All the time, its really common!

 

[nickblake]

Its a very common problem and happens all the time with older houses normall a shared neutral where they use the live from the ground and take the neutral from the first floor normall found on the landing lights in the 2 way switching ,also the single double insulated wires were also very common during the 70's and is still complient due to the double insulation , you'r not the first and wont be the last to come accross this problem

Dont forget replacing the CDU is notifiable under Part P of the building regulations

 

[Hawk81]

So common and has been discussed many times on this forum. I will say I am not a fan of the stick the 2 lighting circuits on the one RCD and leave the borrowed neutral. I prefer to fix this problem (as borrowed nuetrals are dangerous) and have the 2 lighting circuits on separate RCDs.

 

[poirot]

Quite common, just rewire the 2 way lighting and feed the landing light of the upstairs lighting only.

 

[gpz900jim]

ideally the 2 circuits should be on separate rcds, but if the customer wont let you lift the carpets, then i suppose the only option is to leave as is. You cant force people to have work done. If I get into this situation, and the customer wont have the work done, I comment on the eic that the 2 circuits are on the same rcd due to shared neutral on 2 way lighting circuit

 

[BluSparx]

I spent 2 hours chasing a fault like this the other day is very common but really annoys you like mad.

 

[ezzzekiel]

unfortunately a lot of people use this as an escuse not to do it correctly, rewire the affected lights or at least run a neutral in

 

[KDT Projects]

ideally the 2 circuits should be on separate rcds, but if the customer wont let you lift the carpets, then i suppose the only option is to leave as is. You cant force people to have work done. If I get into this situation, and the customer wont have the work done, I comment on the eic that the 2 circuits are on the same rcd due to shared neutral on 2 way lighting circuit

I should of added that i did note this as a departure on the eic and explained the issue to the client who was happy to not have thier new carpets lifted. Its not ideal but i did all i could do in the given situation and with the given clients circumstances.

 

[telectrix]

so, now you.ve found the problem at one house, it won't be such a pain next time. how i do it, if customer will allow, is to disconnect the live feed from downstairs cct. to landing light and feed live in from upstairs. (assuming the actual fitting is upstairs). some weird and wonderful ways of achieving 2 way switching were done in the past, involving singles and JBs. pants. i always run a 3core from one switch to the other. (via intermediate switch if used)

 

[ezzzekiel]

unless its gonna be difficult to draw then i pull 2x 3 cores and wire at light to avoid damaging walls

 

[piz]

before you change the con unit a simple IR rest test or continuity test between neturals should tell you if you are goimg to experience problems