Installing fluorescent lights in loft - junction box problem

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Help!! I am installing fluorescent lights in loft. I tried pulling spur from junction box in loft and leading via 5 amp fuse to new light switch then on to my loft lights. Problem - this is interfering with lights in landing - when landing lights off and I turn on loft lights, landing lights flicker on and off as fluorescent motor starts - when landing lights on, no power to fluorescent light circuit. I tested power and find that when landing lights off there is power between + and - in junction box but when landing lights on there is just power between + and earth. What is going on, where am I going wrong and how do I resolve it????
 
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sounds like there is a neutral being switched somewhere, possible in the landing light circuit.
check the connections in the switch, light fitting and any associated joint boxes and make sure they are correct.
 
Checked that and what appears to be happening is that whoever installed the wiring has run 3 core down to switch with red going in, blue coming out and earth running alongside! The 3 core goes down to switch from junction box but also goes out in about 3 or 4 other directions to other areas (don't know where). How can that be if switching on light knocks out current across + and neutral but give scurrent across + and earth? Is it normal to have current across + and earth?
 
Checked that and what appears to be happening is that whoever installed the wiring has run 3 core down to switch with red going in, blue coming out and earth running alongside! The 3 core goes down to switch from junction box but also goes out in about 3 or 4 other directions to other areas (don't know where). How can that be if switching on light knocks out current across + and neutral but give scurrent across + and earth? Is it normal to have current across + and earth?
 
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you have a switch upstairs and a switch downstairs for the landing light dont you?

What you are checking with your multimeter is 240v but no current. if you take the lamp out from the landing light you will find it will not work at all.

You have connected your new light to a live and switched live, so when the landing light is off your meter will show 240v which is true because there is a difference of 240 volts because it is getting the neutral through the landing light lamp.

What you need to do is to find the last light on this circuit (it will have 2 cables, one mains in the other to the switch) you can then get a permanet live and neutral from there.

As for should ther be 240 v between live and earth yes there should, but it is best left alone, as i said find the last light.
 
OK, I will get on my knees and crawl around the loft in the hope that I find the last light in the circuit! Thanks for your help - you may yet hear back from me!
 
OK, job done and working! Thanks a lot for your help guys.
 

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