Jim,
I am making an educated guess that the problem doe not lie within the machine!
You say it happened once in the past but not until today it happened again, a cuople of questions.
Were the houses you were cleaning new builds or recently built ? if so the circuit breaker in the house are extra sensitive as opossed to older properties and a surge will trip them .
Have you tried pluging into to sockets on different circuits within the house ? This will prevent to much strain placed on one circuit, therfore preventing the tripping .
My suggestion would be to look at this first.
geoff