# Can a capacitor be used for overvoltage protection?

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?

• It's very unusual to see a capacitor rated for 440 VAC. Possible you mean VDC? – Stephen Collings Jun 14 '12 at 18:14
• @Remiel At 30uf, 440/450vac (running motor cap) seem to be the norm in the electronics catalogs vs 250s (starting motor cap). – Jim Jun 14 '12 at 18:23
• I should have looked more into motor caps before presuming to correct you. My bad. On the plus side, I've learned something! – Stephen Collings Jun 14 '12 at 20:10

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.

A capacitor is not a fuse. Unlike a fuse it is undocumented how a capacitor fails. What I mean to say is: - when a fuse fails, it breaks the circuit and the circuit it safe. - when a capacitor fails, it may either break the circuit or short it.

To prevent a capacitor from failing you shouldn't exceed its rating. If the voltage applied is lower than the rated voltage, then you don't have to worry. Higher voltage rating is always better, but it increases price too. In your case a 440 $V_{AC}$ capacitor will do just fine. A 250 $V_{AC}$ capacitor would do just as wall at a lower price.

In essence, do capacitors act as potential fuses during surges and should one take this into consideration when trying to protect equipment behind them?

That's a novel question and a reasonable one. I don't think that I've heard it asked before.

While it is conceivable that a capacitor could be used in that way, it would be such a "soft" protection system that it would be liable to be of very little use.

Devices which are intended to provide protection are very well developed and usually reasonably well priced for what they do.

Fuses have exceedingly well defined current-time characteristics when used as intended.

Contactors, circuit breakers, earth leak / residual current devices all are generally designed and built to good tolerances.

Zeners are some what "soft kneed" but this is accepted as part of the overall low cost. MOVs varistors Tranzorbs gas-discharge tubes and the like stand in the noise spike gap with well defined accuracy and capability. Polyfuses are a very blunt and poorly defined tool but still serve a valuable role.

Lightning fast (literally if needed) clamps and interrupters are available, crowbars are as fast as you want to pay for.

Capacitors in power equipment are usually not a low cost part and already form part of a larger provide protection system by surge and spike suppressing and blocking and removing noise and sometimes providing a clamp that each a current trip may work against. Given all this it is better to let custom designed parts with guaranteed performance meet this requirement in a close to optimal manner, leaving the capacitors to also do their intended job well.

FWIW - my advice re electrolytic caps in large DC systems is that they last best when run very close to their maximum rated voltage. Unlike eg tantalum caps, they are highly resistance to occasional modest over voltage exposure.

• "... electrolytic caps in large DC systems ... last best when run very close to their maximum rated voltage." Do you have any reference or explanation for this claim? – Kitana Dec 5 '14 at 17:15
• @Kitana Yes. Reference available but not to hand. I have cited specific manufacturer recommendations in other answers re caps on this site. | Guesstimate only: The reason is probably to do with the forming of the oxide layer which acts as the dielectric barrier between the two electrodes. This is of increased thickness with increased voltage and presumably a given voltage spike or delta V excursion is less liable t overstress thicker oxide. – Russell McMahon Dec 6 '14 at 16:13

As the other answers have mentioned, capacitors can fail in a multitude of ways. There are fused capacitors that incorporate an internal fuse so that the fail mode for a short circuit is known to be open. These are typically used in aerospace applications where failure mode analysis is serious business. But I've never seen one at the ratings you require.