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Old 28th Dec 2016, 2:53 am   #161
Argus25
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Default Re: Beware Rifa Class X Capacitors!

Probably, there is no substitute for satisfactory insulation, as Samsung recently found out with their phone batteries.

To allow for peak mains voltages and spike voltages riding on those, it would be hard to imagine that a 1500V or 2000V DC rated high quality (low esr welded lead to foil type) poly capacitor would fail or cause any trouble at all in a "X2" application (but they are bigger) Despite not meeting the official X2 ratings.

Many early mains filter caps were PIO types in sealed tins that seemed very reliable compared to their modern Rifa X2 counterparts. All of these Rifa X2 caps have required replacing in my three 2465B scopes, cracked, caught fire/smoked despite the supposed flame retardant properties. I also wonder about the oval shape too and like vidjoman wonder if they get compressed, stressing the insulation.
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Old 28th Dec 2016, 5:22 pm   #162
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Default Re: Beware Rifa Class X Capacitors!

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Originally Posted by Pat Pending View Post
I don't now believe the Rifa cap failures are age related as I did before, I installed a new input card in a Mitsubishi modular PLC at work on Christmas eve, the cap that I could see through the vent slots into the plastic case deconstructed itself immediately with a loud bang.
How long and under what circumstances had the card been stocked?
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Old 28th Dec 2016, 5:29 pm   #163
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Default Re: Beware Rifa Class X Capacitors!

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Originally Posted by Philips210 View Post
One thing that puzzles me is the formation of cracks in unused components. I speculated on possible corrosion on the leads causing failure of the seal.
It seems then it is mainly due to the composition of the resin having an incompatible coefficient of expansion compared to that of the dielectric. So if the capacitors were stored in variable temperature extremes then this could accelerate their failure.

If anyone has some of these NOS capacitors stored in a sealed bag in a low humidity and fairly constant temperature environment then it would be interesting to see if they have developed cracks.

My limited experience on these caps seems to suggest that poorly stored examples ie in damp conditions, are far more likely to crack which is what would be expected.
The defective examples from NOS lots that I have seen come from lots that were almost certainly stored in unheated environments so there must have been some thermal cycling involved though probably no large shocks.
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Old 28th Dec 2016, 6:29 pm   #164
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Default Re: Beware Rifa Class X Capacitors!

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How long and under what circumstances had the card been stocked?
Delivered new only two days previously, sealed in an antistatic bag. I did note that it had a 2014 date code but that doesn't excuse the pyrotechnics in my book.

Andy.
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Old 28th Dec 2016, 8:05 pm   #165
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Default Re: Beware Rifa Class X Capacitors!

Hi Andy.

In view of how recent that capacitor is (2014) then I'd be inclined to write to Rifa explaining the situation. Incidentally, did you notice any cracks in the case of that cap before you installed it in your equipment?
There's no way that it should have failed so soon. I wonder if Rifa capacitors are being faked in view of their widespread use.

Regards
Symon.
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Old 28th Dec 2016, 8:21 pm   #166
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Default Re: Beware Rifa Class X Capacitors!

Was 2014 the date-of-assembly of the module, the date-of-issue-from-stores, or what?

In the past I've had to deal with 'legacy' gear that was designed in the late-1960s, manufactured in the early-1970s and then sat on a store-shelf until 2005 when it finally got issued to an unfortunate end-user. [OK, in the time between manufacture and issue it had been unpackaged, *visually* inspected, the desiccators replaced and the thing repackaged to NATO standard several times].

Put into service it lasted about 30 minutes.

This is why I flinch, make the sign-of-the-cross and run away when someone describes a part or assembly as "New, Old Stock" [NOS].
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Old 28th Dec 2016, 9:01 pm   #167
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Default Re: Beware Rifa Class X Capacitors!

I actually thought "Oh look what's in there" as I switched on! The unit is a Mitsubishi PLC input module, I don't know what the Rifa caps function is, the thing has 110vac inputs with capacitive lossless droppers driving opto couplers, the row of input capacitors are a different make, there is a single terminal on each module that references neutral to enable the opto's to work, it may have something to do with that. The power to run the whole PLC is supplied from the base unit which is now about 14 years old, the input card was its first failure, having one of its inputs locked on. There were no Rifa caps in that. The failed unit will be returned to the suppliers, the 2014 date code is on the module, I'm assuming the cap would be the same age.
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Old 30th Dec 2016, 5:45 pm   #168
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Default Re: Beware Rifa Class X Capacitors!

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There's no way that it should have failed so soon. I wonder if Rifa capacitors are being faked in view of their widespread use.
That would be the day that fakes (cheap polypropylene capacitors probably) last longer than originals...
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Old 30th Dec 2016, 10:35 pm   #169
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Default Re: Beware Rifa Class X Capacitors!

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That would be the day that fakes (cheap polypropylene capacitors probably) last longer than originals...
That's rather ironic but entirely possible.

Symon.
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Old 30th Dec 2016, 10:54 pm   #170
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Default Re: Beware Rifa Class X Capacitors!

I doubt they're faking the little gold timebombs.
I've seen many bits of kit (mainly in the industrial sector) where rifas are happily ticking away every day as snubbers in 415V thyristor drives etc....but equally seen as many that have filled the room with smoke.

Years and years ago, whilst working for another company, I replaced a dubilier mixed-dielectric suppressor capacitor (s/c) on a DC motor speed controller, with a rifa shiny gold X-capacitor from farnell. It was a quarter of the price of a philips polypropylene.

10 years later the controller appeared on my (own company) bench from the same client with black treacle oozing from it. I fitted a polypropylene cap in its place.
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Old 31st Dec 2016, 3:46 am   #171
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Default Re: Beware Rifa Class X Capacitors!

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Have you any idea how old that one is?
It looks like it has quietly exploded while in storage just like the two WYMAs that i have found.
Since the capacitor's volume has expanded (cracking its own encapsulation) while it was simply in storage we have to wonder how that is possible. Paper contains a number of chemicals and may not even be completely free of water molecules at the time of encapsulation. It has been pointed out that the plastic encapsulation is also permeable to an extent. Then presumably the paper has reacted with the metalization to produce metal salts which; 1) can cause electrical leakage and heating from leakage current and 2) expand in physical volume and therefore make the capacitor, even in storage, explode in slow motion. Just a theory. Most agree, paper as an insulation product has no place inside a non electrolytic capacitor, that is if you want it to be reliable and long lasting.

I had a very interesting capacitor failure, wax paper style. The capacitor was a 0.05uF or thereabouts. Electrically it behaved as a capacitor which was a 0.05uF with a series resistor of about 1 meg and capacitor of about 1uF in parallel with it. It even had two time constants easily seen in a pulse circuit. It got nicknamed "the capacitor with two exponentials" The theory was that the paper had absorbed water and due to salts in the paper, it created a lossy electrolytic capacitor with a very high ESR in parallel with the original value. It also had some leakage or parallel resistance. So one capacitor became a component equivalent to a network of 4 components.
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Old 3rd Jan 2017, 10:11 am   #172
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Default Re: Beware Rifa Class X Capacitors!

Just a thought, what do old filtered or 'surge protected' mains extension leads/sockets have inside them? I have never heard of one going up in smoke so maybe not Rifa style filtering capacitors.
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Old 3rd Jan 2017, 12:09 pm   #173
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Default Re: Beware Rifa Class X Capacitors!

I had one of them go up in about 2002 in the office I was working in at the time. It was on the "PC build" bench. Didn't investigate what it was at the time but it went up violently enough to set fire to the plastic around the surge protection end of things. May have been one of them. It was an expensive enough block to contain one
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Old 23rd Feb 2017, 10:59 pm   #174
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Default Re: Beware Rifa Class X Capacitors!

Good evening,
This one let go this evening in our 30 year old Thorn microwave smelling out the kitchen.
Nasty one this as it appears to be the snubber across the magnetron transformer relay contacts causing the transformer to be permanently energised and getting very hot!!

Luckily it happened now and not a couple of hours later when we would have been in bed !!.

New one to be found to get the oven going again.
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Old 23rd Feb 2017, 11:54 pm   #175
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Default Re: Beware Rifa Class X Capacitors!

Happened a week ago in my HP/Agilent 4275A component bridge. Piles of acrid smoke and a cracking sound. Yup - RIFA X cap.

I didn't take a photo - no need, there are so many pics of the perp on this thread already!

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Old 24th Feb 2017, 7:43 am   #176
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Default Re: Beware Rifa Class X Capacitors!

Urf. Going to take my HP3312A to bits tonight and make sure there isn't one in it! Didn't know HP used them.
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Old 24th Feb 2017, 8:41 am   #177
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Default Re: Beware Rifa Class X Capacitors!

HP used lots.

They were seen as the best, most trusted part for the job and both X and Y rated ones were heavily used.

Some of the parts coming to light aren't visible. I've had an X capacitor go off inside one of these filter cans/mains cable connectors. It burned a hole in the can in order to stink out the building.

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Old 24th Feb 2017, 9:30 am   #178
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Default Re: Beware Rifa Class X Capacitors!

I had one go in my Philips scope earlier in this thread. Indeed it does make a mess!
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Old 24th Feb 2017, 11:19 am   #179
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Default Re: Beware Rifa Class X Capacitors!

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Urf. Going to take my HP3312A to bits tonight and make sure there isn't one in it! Didn't know HP used them.
Looking at the manual (from Keysight's site) C601 is the part, defined at a Z5U dielectric ceramic, 0.02uF +80-20%, 600V. So not RIFA. But certainly not a rated part for connection across L and N either.

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Old 24th Feb 2017, 12:06 pm   #180
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Default Re: Beware Rifa Class X Capacitors!

Thanks for the info - appreciated. I'm going to do a full refurb on the unit so will replace at the time. The sweep function is also non functional at the moment. Finishing off this infernal Fluke 8024B first which turned out to be far more complicated than I was expecting
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