I'm replacing the capacitor for a start motor that had previously burnt up. I'm new at this. I previously referenced the following questions about replacing caps with higher ratings than spec'd:
High Voltage Capacitor, in a low voltage system?
Can I swap an electrolytic capacitor with one with a higher voltage?
selecting voltage rate for capacitors
The general consensus from the answers above, is that it's best to run a bit higher, typically 1/3-2/3 over. My motor's cap is spec'd at 250vac. I'll be running 120vac, where my motor is spec'd at. I can only find 440vac's readily available.
My question is, with LARGE voltage differences above a spec'd capacitor/equipment rating, is there not risk of damaging equipment not rated for those potential maximum overages? For instance, a 440vac cap may let 400vac by, whereas a 250vac cap would in theory have already failed. In essence, do capacitors act as potential fuses during surges and should one take this into consideration when trying to protect equipment behind them?
Best Answer
Relying on a capacitor to fail in an overvoltage condition in order to protect some other equipment is a bad design practice. Capacitors may fail open, short, or somewhere in between; unpredictability makes for poor protection. And I've personally seen overvoltaged caps emit flames.
You should spec all your components such that its voltage rating is at least the highest voltage you expect them to ever see, plus some safety margin. If its unexpected voltage spikes you're concerned about, you can use more capacitance on a DC link to limit the voltage rise, or MOVs on an AC line connection.