Rifa X2 cap from 2006, should I replace it? - Page 1
Forum section: electronics / repair

author: acrophobic
author_rank: Contributor
timestamp: August 04, 2019, 12:20:33 pm
content: Like subject title. I'm replacing some electrolytic caps in a UPS from 2006 and noticed a Rifa filter cap. Should I replace it, or could it be considered "safe" since it's not a really old one? Is there a point in time after which these caps are "fixed" by the manufacturer and no longer have the risk of spontaneously blowing up?

author: cbutlera
author_rank: Regular Contributor
timestamp: August 04, 2019, 05:48:00 pm
content: If you have an insulation tester, just measure the capacitor's insulation resistance at about working voltage. If it's above 1G then it should be fine, below 100M I would definitely discard it, in-between, it depends on the operating conditions. Sometimes you can do this test in circuit, if you can isolate the capacitor by pulling the fuse, or switching off the power switch if it has one.I have tested a number of these Rifa capacitors. My conclusion was that their failure is quite predictable, the mechanism being thermal runaway due to the insulation resistance having a high negative temperature coefficient.A new Rifa from Kemet is specified to have an insulation resistance above 12G. I have some unused 10 year old 100nF Rifas with an insulation resistance of around 1 to 2G at 25 degrees Celsius, measured at 250 volts. Old cracked ones that I have tested come in at around 10M or less. These are the ones that are now primed to self destruct.Regardless of age, the insulation resistance of all those that I have tested approximately halves, for each 3.5 to 4 degree Celsius rise in temperature. I don't know if the new ones from Kemet exhibit the same characteristic, or if my 10 year old unused Rifas ever met the specification of the Kemet ones.If the ambient temperature inside the equipment is very high, above perhaps 60 degrees Celsius, even 1G of room temperature insulation resistance may not be enough to prevent the capacitor reaching the tipping point for thermal runaway.

author: magic
author_rank: Super Contributor
timestamp: August 05, 2019, 04:31:43 am
content: The story of how one youtube video ruined a brand forever.But who knows, maybe the new ones still suck.

author: floobydust
author_rank: Super Contributor
timestamp: August 05, 2019, 06:05:22 am
content: Yes, just change them out, for the dollar or two.The Evox/Rifa's have never been fixed, nor any recall or admission of a problem. They could be liable for the fires and product damage.Kemet bought Rifa and the PME271 series is still for saleThe capacitor is odd, using apaper dielectricthat is metallized.The failure mode seems to be the clear epoxy resin shrinking and cracking with age. It might squish the wound portion, or the paper fails somehow with age.Some people think it's due to moisture getting inside once the epoxy cracks. Hard to know because they burn up and mains transients can vary.SEM Lab capacitor failure analysishas some film caps if you can nav their crappy website.

author: james_s
author_rank: Super Contributor
timestamp: August 05, 2019, 06:40:24 am
content: I'd probably replace it if I was already in there. That said, I've only ever had one fail and it was some 30 years old. It made a lot of stinky smoke but didn't cause any damage to anything but itself. I still have a bunch of them lurking in gear I regularly use and I don't plan to do anything about it until a problem occurs.

author: floobydust
author_rank: Super Contributor
timestamp: August 05, 2019, 07:41:54 am
content: I found a drawer full of new old stock Rifa's sitting in inventory for many years. They had never experienced wave soldering, yet were bulged or cracked. I tossed them in the garbage.PME294 is another Evox/Rifa paper dielectric safety cap but in a white plastic case. I have not checked any to see what lurks inside, or seen any fail.https://www.eevblog.com/forum/chat/old-rifa-capacitors-and-a-disaster-story/https://www.eevblog.com/forum/chat/iec-mains-filter-failures/Old Schaffner AC line entry filters use Rifa X-caps and there is no fuse, so when they fail it's quite bad.The original research project on AC mains transient overvoltages was published 1983. 24,000 sites were monitored in USA and Germany. It took years before the values made it into the safety standards. Even then, lightning strikes can vary greatly with the voltage spikes equipment experiences.My point is an old component might be under designed compared to what modern standards now dictate. But the housings should not crack with age.

author: acrophobic
author_rank: Contributor
timestamp: August 07, 2019, 02:37:43 pm
content: Thank you all for much useful information! It's pretty tight and hard to see, but I THINK it looks in pretty good shape, but I'll probably replace it anyway.Looking at the below page, does it matter which one I choose (apart from lead spacing)?https://www.elfa.se/en/passive-components/capacitors/interference-suppression-capacitors/x2-capacitors/c/cat-DNAV_PL_03020502?q=*&filter_Buyable=1&filter_Category4=X2+Capacitors&filter_Category3=Interference+Suppression+Capacitors&filter_AC+Rated+Voltage+X2~~V=275&filter_Capacitance~~nF=220&pageSize=100&sort=Price%3Aasc

author: acrophobic
author_rank: Contributor
timestamp: August 07, 2019, 02:47:07 pm
content: I tried to attach the image in the previous post, but apparently I was too stupid...

author: Bud
author_rank: Super Contributor
timestamp: August 07, 2019, 02:53:56 pm
content: Holy sh!t, just remembered i had a dream last night that my Tek 2430 released magic smoke ftom its Rifa caps. Just remembered it when reading this topic

author: acrophobic
author_rank: Contributor
timestamp: August 07, 2019, 05:08:03 pm
content: Hahaha, lucky it was just a bad dream