I'm trying to install 3 new wall lights. There were lights there previously. When I check to make sure the wires are live before installing the fixture, they are live. Once the fixture is installed and I check again, the wires are no longer live. Any help would be greatly appreciated.

KPS1:
What are you using to test for voltage?
What kind of fixtures are you installing?

Using a circuit testing screwdriver to test the voltage from the live wires.
Trying to install 3 x wall uplighters with R7s linear halogen bulbs, bought from John Lewis.
Before the lights are fitted the brown live wire at each of the 3 points causes the light in the screwdriver to glow, confirming they are live. The light fittings are pretty simple, with a connection box into which I connected brown to L, blue to N and the bare wire to the earth connection.
After fitting one of the lights, and having reconnected the fuse in the fuse box and checked the dimmer light switch none of the 3 live wires cause the screwdriver to glow. The fuse in the fuse box did not flip back either.
I've tried fitting all 3 lights, still nothing. When I unfit the lights, the live wires make the screwdriver glow again. Puzzled.

KPS1, it would be good to let folks know that you are in the UK as most of our members are North America based. We do have a few UK knowledgeable members though.

Sorry. I didn't realize that this was a North American site. Thanks for letting me know.

Sorry. I didn't realize that this was a North American site. Thanks for letting me know.Don't worry, there are still folks that can help. I was making the point so others would know.