This is begining to sound like '20 Questions' or 'Ask me Another' isn't it
Sorry if this is repeating what others have said but from everything you've described the problem can only be one of voltage drop caused either by a bad connection, or connections, or perhaps the whole thing being wired with very thin wire incapable of passing the current required for a 500w bulb. I'm sure we've all seen horrific examples of amateur electrical installations in France! If this were the case however I think your nose or you smoke detector might have discoverd it before now
Another possibility is that you've got live and earth but no neutral. In this case the amount of current you could pass down the earth stake might be enough to light a small incandescent bulb but would not be enough to get the barest glimmer out of a 500w one.
What size (wattage) was the original light and did it work and why are you changing it, was it too dim
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