ECN Forum Forums General Topics of the Trade General Discussion Area PF correction capacitor blowing fuses

I have a customer that has a Sprague 300KVAR PF correction capacitor that is blowing 2 of the internal fuses. I have not had much interaction with this product. Can anyone tell me what would cause the fuses to continuously blow?

A shorted capacitor would be my guess.

That was in fact the case. One capacitor was shorted.

@Merlin, Sometimes we forget the simplest of reasons for something happening. Mostly we’re too focused on a particular component that we skip the nearby components. However, I was wondering how your capacitor might have gotten short. Did it get blown up or was it short because of some connection issues? It is a great help if you can post how your capacitor was getting shorted and what might’ve been the reasons that kept you distracted from this issue. This information can then later be used by anyone else who is in a similar situation. It gets easy to diagnose for others this way.

Afyon,
Capacitors that are constantly subjected to full mains voltage are prone to failure, it's just something that happens.
When the dielectric insulation between the "plates" of the capacitor fail, you then end up with a dead short between the plates, causing short circuit current to flow and the circuit protection to operate.

If you've ever seen inside a large capacitor, it's like a large roll, that is how you can have such a large capacitance in a physically small component.

I see a lot of capacitor failures with things like single phase air compressors, where the centrifugal switch can jam and leave the start winding and the capacitor in circuit, instead of switch over to the run winding.

Many failures of electronic equipment are due to large electrolytic capacitors shorting or, occasionally, going OC.
Often the fault can be visually identified by bulging of the case or leakage of electrolyte.

Thank you Trumpy, for responding to the question from Afyon. Another thing that I found during this situation that I should have thought of. This capacitor bank has 5 capacitors per phase (3 phases). Trying to conserve on a costly capacitor for the customer, I only replaced one. I should have put more thought into this, as I was called back a month later with more fuses blown. I have always understood capacitors to be like batteries, never change only one but change all that are linked together. Therefore, I had to go back and replace the other four on that phase to eliminate anymore problems.

Thanks for the great discussion gentlemen. Its always great to find people that are willing help others.