I've encountered the following phrase in a Jeeves and Wooster book, The code of the Woosters:
He had been beaming like a searchlight until I mentioned this aspect of the matter, and the radiance suddenly disappeared as if it had been switched off at the main.
I'm trying to guess the meaning of the phrase "at the main" here and I suppose that it metaphorically refers to switching the searchlight off by pressing the main power button or something like this. Am I right?