Main switch - Interruptor principal em um equipamento. Mas nesse caso fala da instalação elétrica residencial, portanto a chave geral, o disjuntor geral do quadro elétrico que alimenta (fornece energia) a residência toda.
It is very important to remember to turn off the main switch when you have to change the light bulbs,...
Install or make maintenance of electric shower or some other equipment that is permanently connected. Ideally, you should always consult an electrician not to run the risk of suffer electric shocks.
"Permanently connected" assumes that they are permanently connected to the A.C. Power supply/supply network, etc.
"suffer" is usually said about the consequences, as burns or injuries from shocks. Other way to say that would be "run the risk of eletrical hazards".