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I'm so bad at DIY that I can't even change a light bulb properly.

The porch lamp outside my house isn't work. I tried changing the bulb, but it tripped the RCD. I double checked the fixture (ES, max 60W) and the original bulb (edison screw, 40W), and bought a new bulb (edison screw, 42W). Again, it tripped the RCD.

Given that it's an outside lamp and two bulbs (including a brand new one today) have broken, is it likely to be the fixture? Or is it likely to be a problem with the bulb (is it possible I need a special kind of bulb)?

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If the bulb fits in the base, it is the right kind. It could be a problem with the fixture, but it could be anything else along the line as well, a bad switch, bad connection somewhere, faulty insulation, chewed by rodents, etc. RCDs are very sensitive, it doesn't take much current leakage somewhere to set them off.

If a careful visual inspection doesn't reveal anything, about all you can do is take various components out of the system until you find the one that was tripping the RCD. For instance, test the fixture by removing the wiring and connecting the wires to a simple keyless base with a light bulb installed. If the RCD does not trip and the bulb illuminates, then you know the fixture is at fault.

If the RCD still trips, then the fixture is not at fault. Next try taking the switch out of the system by wiring the switch wires together so that you have an unswitched circuit. Test again. If the RCD still trips, identify intervening junctions and connect your test light at each one, starting at the switch and working back towards the distribution panel. You will eventually make a connection that does not trip the RCD. Then you know the bad section is immediately upstream from the successful connection.

I've left out many safety details here in order to outline basic trouble shooting. If you are at all uncomfortable working on building electrics, do not do any of this, find someone that knows what they are doing. Also, if you have an older UK ring system, these confuse most people, you need to find someone familiar with this rather unusual arrangement.

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