Electrium consumer unit and RCBO trip why?

Not really relevant. You need to be registered with a scheme or involve the local council building control to replace / install a consumer unit in a domestic property, especially a rented property. I am guessing you don’t have insurance to cover this work either.

In terms of testing, it should have been tested before energizing, as per the current edition of BS7671. The “bang test” where you assume all is good if it doesn’t go bang was obsolete many years ago, and for good reason. This is a simple fault, and should not take more than a few minutes to identify, although it may take longer to fix.

As has already been said, you need to go back to doing the “dead tests” you should have already done - continuity, polarity, insulation resistance. These need to be done on individual circuits, as well as testing between circuits to prove they are really separate.

 

No the new one sorry.

 

Now change the fitting for a standard 60 watt incandescent lamp and see what happens. That will confirm if its the luminaire or the wiring. Borrowed neutrals were specifically prohibited in the 15th edition, and ever since by the requirement to start and end P & N of a circuit in corresponding ways in the CCU or DB, so use CB no 1 use neutral No 1 (and if you are vigilant, use earth No 1).
As for being a member of the partially qualified electricians club (Part P and domestic installer), that's going to upset a few on here , I find myself in a situation where I trained apprentices and taught qualified electricians 16Th Edition, and this scheme has the gall to tell me I can't install a two bit domestic circuit, but I can still install a supply from a 1600 KVA transformer to the busbar chamber. The world is crazy, run by idiots who have the political power but do not understand the industry or those who operate within it. As for Part P, all it has achieved is flooding the market with 'Electricians' who have been on a Part 'P' course, who think they can do the job. The fees charged for enrolment into the scheme are added on to the customers bill, making it too expensive, so they still go an get the guy from down the pub to do the job. Rant over, back to sanity.

 

We see on here so many times where we don't get the whole story, going back to your original post, you seem to say disconnect the one lamp at ceiling rose and all is OK.

So if it were my job, the question is overload or earth leakage, so for a couple of quid you can get an MCB, so you can rules out earth leakage if it trips, and put the clamp on around the feed cables and see what leakage if any there is, I assume you have already tried with the insulation tester and shown no DC leakage, so there is a possibility that the leakage is only there with AC due to some capacitive or inductive linking. You could not have fitted the consumer unit without the test equipment so I am assuming you have it, however it seems common not to test for AC leakage as there is no provision on the test certificate to enter readings for AC leakage, and I know I tend to use the installation certificate as a check list for what needs testing.

If you did not fit or test the consumer unit, you will still one hopes have the test results, however I note when Part P first came in it said the LABC did not have to pass on their results to the owner, only the completion certificate is required, but still seems odd it would not be passed on.

If you don't have test equipment then the temp fitting of a tungsten lamp as already suggested seems good. I have only once gone down the inform the LABC I am doing the work, and I know they were keen to view my test equipment, however they did not seem to want to see the calibration certificates, lucky, as I did not have any.

In the past I have tested items for earth leakage with the PAT tester, mine did have a meter showing the leakage, but it was rather old, I have not a clue how it tests it, but clearly a standard multi-meter is useless.

 

If the landing light is tripping the RCBO:

1. Carry out safe isolation of the entire installation and remove the consumer unit cover, then remove the circuit from the RCBO.
2. Insulation test the lighting circuit with the live and neutral joined together to earth with the CPC connected along with all earthing and bonding in place and the lamps in place, operating the two way switching controlling the landing light and any other two way switching to ensure all the live conductors are tested to check for an earth fault.
3. Insulation test the lighting circuit with the live and neutral joined together to the neutral bar in the consumer unit, operating the two way switching controlling the landing light and any other two way switching to ensure all the live conductors are tested with the lamps in, to check for a borrowed neutral.
   
4. Reevaluate your choice of RCBO, presumably it a 6 amp Type A RCD and Type B MCB, does it need to be a 6 amp Type A RCD and a Type C MCB or possibly a 10 amp Type A RCD and a Type B MCB or even a 10 amp Type A RCD and a Type C MCB? Zs permitting of course, to accommodate any inrush current.
5. Reassemble the circuit with the correct circuit protection having completed the dead tests then complete the live tests and all should be well.

 

There is no such thing as a time delayed 30 mA RCD or RCBO that will comply with the requirements of BS7671 for providing 30 mA RCD protection for a domestic lighting circuit and for cables concealed in a wall.

So that is not an option.

 

If you do find a borrowed neutral then there's two options, rewire the landing light to remove the borrowed neutral or combine the two circuits into one at the consumer unit and supply them from the same RCBO.

 

There is another possibility, the light fitting may be miswired with the neutral actually connected to the circuit protective conductor.

 

After 32 posts, I am not sure we have discovered whether the rcbo is tripping on an earth fault or overcurrent yet.

 

A privately rented home requires a satisfactory Energy Performance Certificate, the landing light is LED and it would be a fair assumption that most or all the others are as well to get the required EPC rating, the current drawn by all the lights in the house is probably only around one amp.

 

Indeed, but we don’t know whether turning the switch on for the fitting is giving an earth fault or shorting phase to neutral.

 

You'll need to take a neutral from the downstairs circuit to the landing light and remove the neutral which probably comes from a bedroom light, it wont be easy.

 

If you have a seperate smoke detector circuit you could take a neutral from that and double that with the downstairs lights.

Usually theres a smoke on the landing so could be simple.