Capacitor Fault Protection

[Gunther]

Hello to all. I am planning my electrical system and noticed that most filter capacitors are installed without any fault protection. Shouldn't we be connecting our capacitors with fuses? There was a discussion on the TeamKitfox forum a while back where someone's capacitor internally shorted and caught fire in flight. If a capacitor shorts internally, a properly sized fuse should be able to interrupt the current before the case ruptures. An old rule of thumb says the fuse should rated for twice the max capacitor current.

Any other thoughts on this subject?


Gunther

[Bryan Cotton]

Gunther,
I don't think I have seen a fused capacitor. If it shorts internally, no fuse on the outside will affect what happens inside. If something on the line shorts there should be a fuse or breaker upstream to open up the circuit before the wire fries. Capacitors have lower energy stored than batteries, or an alternator driven by an engine. If you are truly worried about a capacitor supplying enough energy to fry things then a resistor in line with the capacitor is a good idea. It will help the filtering and in case of a short on the bus it will reduce current.

Edit: when I re-read your post it occured to me you were probably talking about the capacitor becoming a short to the bus. There should be a fuse or breaker upstream somewhere. Even the classic master switch is a fuse, though a manual one which relies on the pilot to smell smoke!

[Gunther]

Bryan, thanks for the comments. I was referring to an internal capacitor fault between the positive and negative plates, which acts like a short circuit on whatever the capacitor is connected to. The internal energy stored in the capacitor will contribute to the fault but is typically much smaller than the fault current available from the battery. The alternator will also contribute fault current but it's contribution will also be much smaller than the battery. I think the probability of a capacitor failure is quite low but the consequences can be high. The high temps under the cowl will likely lower the life expectancy of the capacitor as well. Perhaps I will try fusing my capacitor at 5A if I can find a fast enough fuse.

Gunther

[wlarson861]

I mounted my capacitor on the inside of the fire wall as in B and C electric's diagram. The capacitor is isolated from the main buss by a crowbar over voltage circuit and the other path to the bus is separated with a 30 amp breaker.

http://www.bandc.biz/pdfs/505-500_rev_j.pdf

[marsolgp]

You've got a 20 amp alternator and you're going to put a 5 amp breaker on it's output? I'm missing something here…...

[radfordc]

You've got a 20 amp alternator and you're going to put a 5 amp breaker on it's output? I'm missing something here…...

The capacitor should never see 20 amps of current. As long as the cap is good there is no current flowing through it. If the cap shorts the fuse prevents high amperage current from flowing.

[Gunther]

Charlie, thanks for posting the diagram. Yes, the capacitor connects to the DC output of the voltage regulator, which you have shown as being together with the alternator. Fuses are typically used for capacitor protection because they can open quickly enough to prevent the capacitor from rupturing with an internal fault. The capacitor will normally see ripple current from the regulator and harmonic currents from electronic devices such as a radio, efis, ipad, anything with a switched mode power supply.

Thanks again for the great comments.

Gunther

[marsolgp]

I got ya.... See, there was something I was missing. Thanks for clearing that up. I may go ahead and add a C.B. now. Gotta love this forum, always something new to learn.

[Rynoth]

The b and c diagram (aeroelectric connection) for over voltage protection has a 10awg wire running from the cap to ground. I'd think putting a 5a fuse on a 10awg wire is a bit small? What kind of current would be considered abnormal for the capacitor to see?

[Gunther]

I expect the capacitor to see a maximum of 3 amps AC, which corresponds to 15% of the max alternator current, which is just a first order guess. I agree with you Ryan, the #10 wire to ground on the B and C diagram could be reduced in size if the capacitor is supplied with a 5A fuse.

I noticed that the B and C diagram shows a three phase regulator. Since the ripple is much less with a three phase regulator and alternator, one doesn't need as large a capacitor as with a single phase system.

The permanent magnet alternators in the AeroVee and Jabiru are interesting in that the output voltage starts to drop as the output current goes beyond rated capacity. So, if the regulator fails to a high output voltage state, one can help lower the DC voltage simply by switching on more DC load.

Gunther