Beware Rifa Class X Capacitors!
 

Hi.
I had one of my Ekco TV's running this afternoon, I Had nipped inside for two or three minutes only to see smoke pouring out of the shed. The set was merrily playing away fine.
It seemed to be very hot. After a good half hour I discovered that the nice new old stock X type cap across the mains had blown it is a Rifa rated at 250v ac

Re: Beware!
 

That's very naughty of the capacitor. If I were you, I'd send the photo to Rifa (with a close up showing full markings, date codes etc).

I do have misgivings about this style of capacitor - the moulding (thermosetting epoxy I believe, although I woundn't argue if somebody said polyurethane) is prone to cracking in prolonged sunlight. And the capacitor itself is made from zinc metallised paper, which Rifa claimed was better than aluminised plastic but I tend to trust less.

Re: Beware!
 

I made a component evaluation of these capacitors for a former employer about 30 years ago.

One of our favourite tests was to thermally cycle components with their leads under axial stress for several hundred cycles - sometimes with added humidity. My findings at that time with Rifa capacitors were that they had used a hard epoxy encapsulent which was not thermally matched to the capacitor element, thus stressing the element and its connections. Failure of the encapsulent under these conditions allowed moisture into the unit, usually resulting in catastrophic failure.

I was also unhappy about the flame retardancy properties of their chosen encapsulent and had a few bonfires during my bunsen burner tests.

We found a lot of absolute junk by testing in this way - it took time and money but enhanced our product reliability. If only other UK electronics producers were as discerning (Quad please note), they might have produced goods which were fit for purpose.

At that time, only Philips and the Japanese had any notion of capacitor construction for reliable operation.

Leon.

Re: Beware!
 

Yikes!!! I have a few hundred of those and use them in that position.

Chris

Re: Beware!
 

I've also seen that particular brand of cap fail with lots of brown goo and a nasty stink. The polypropylene type in the blue case from BC Components (philips) or the red Wima ones seem less prone to this behaviour.

Mmm..my TV12 has two of these, thankfully well under-run, one in each timebase oscillator.

Re: Beware!
 

I think that's quite an old cap and it probably falls well short of what is required now. A 'proper' x cap will have conformity markings on it and a European EN standard number (can't remember what it is off hand). Personally I wouldn't trust it anywhere near the mains. Probably fine elswhere though.


SB

Re: Beware!
 

Yes, I've known several of these do exactly that. Thanks for sharing the results of yours tests, Leon. I'm even more wary of them now :)

Mark

Re: Beware!
 

I'll bet that this capacitor did have conformity markings on it - certainly Rifa were making X and Y capacitors in the mid 1980's in this style. The stresses of across-the-line applications were appreciated, and capacitors specifically designed for this have been around since then. Whether they long-term reliable, of course, is another matter.

Must have a look tonight at my own stock, if I have any left.

Re: Beware!
 

Here's a view of mine, both sides info shown they are classed as X2

Although similar to Trevor's however the RIFA logo is different and the code. In Trevor's the logo RIFA, is in a circle, whereas mine have large text RIFA. Also Trevor's is a "PME 274 M 610" and my lot are "PME 271 M 610".

Maybe these types might be ok, I think one of the properties of X2 means the body will not catch fire.

Chris

Re: Beware!
 

That's the boys Chris!
I have been sceptical of some X types before, I have seen blown ones in microwave ovens and in SMPS.
This though was the most spectacular I have seen, the trouble was the acrid smell and by the time I found what was the problem the cap was quite cool. Good job it was clipped lightly to the chassis and not near the cabinet!


I have replaced it with a nice new one from Valveman49

Re: Beware!
 

I wonder what the subtle code differences indicate between ours. I have a NOS box of 250 with a stock code and date stamp of June 1984. I will have to keep an eye on them as whenever I replace the mains filter I put one of these in it's place, I feel a nervous twitch coming on ;)

Glad you got to it before anything happened.

Chris

Re: Beware!
 

I would be inclined to wire a low value safety resistor in series with these caps now. If for instance the cap failed and started passing any large amounts of current but not enough to blow the fuse it "should" take out the resistor!
I am not too sure of the value to use but I would go for 10 ohm at 0.1w fusible, I know some of the suppression type X rated caps incorporate 150 ohm resistors and perhaps this is what we should be using as mains filter types!

Re: Beware!
 

The trouble is, if you put a resistor in series you clobber the effectiveness at higher frequencies.

As a spark suppressor, a resistor in series to form an RC snubber is used (indeed, is often necessary). But as an RFI filter, you want as low an impedance across-the-lines as possible.

An added complication is, if the capacitor fails but the little series resistor blows open as intended, the user would probably never know but the equipment would have lost its filtering. If the equipment is a noise source, and the filter was to stop the interference getting out, it would be anti-social to say the least.

Re: Beware!
 

Two good points Kalee, and both I know well, I suppose I was looking at the safety aspect of minimising a possible fire hazard!

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I'm probably asking a silly question here but how come these explode or catch fire when the are classed as X2. Is it due to their age? I've randomly tested on a bridge at rated voltage and all passed leakage

Chris

Re: Beware!
 

I don't think it's a silly question at all Chris. But I don't know the answer - if they had been leaky I could have understood overheating and burning, but not otherwise.

I'm all for encouraging murphyv310 and twocvbloke to send failure reports to Rifa (part of Kemet these days) - the PME271 range is still in production and field failures do provide reputable companies the opportunity to improve their products, and design-out the failure modes in future designs.

Here is some information on these capacitors. Interestingly they reckon that metallised paper is superior to metallised plastic for repetitive transients.

Re: Beware!
 

Kalee, thanks for the link, it shows these have been passed X2, I'm quite interested in this as in all my sets I've been putting the cap in post #9 in that position which is a .1uF X2. I've tested each one before doing so and it passed so it's a concern to find this happening on an X2 rated cap. can only hope Trevor's was a duffer.

Chris

Re: Beware!
 

I think in my case, the capacitor was old, and the vacuum itself was hauled out of a skip, I just stripped, cleaned and rebuilt the thing, never thought to replace the capacitor cos it looked fine when I pulled it all out... ???

If anyone does contact Rifa, feel free to include the picture of my blown one, I'm too lazy to contact the manufacturer (especially as it looked old, and probably out of any kind of warranty and was replaced last year before I sold the vac on), I wouldn't have thought they would still be manufacturing them, which does kind of make me wonder, but, it also makes me wonder why the likes of Nichicon keep piling out those cheapy leakies too, but that's another topic altogether... :)

Re: Beware!
 

I had the same problem only three weeks back with a Rifa capacitor failing in the Revox B760 tuner. Went out of the room for only five minutes and came back to find copious amounts of smoke filling the place. Here the 'power switch' only switches the secondary so switching off had no effect! On this tuner the mains fuse is 250mA but it did not blow.

Checked capacitor on a megger and the leakage at 500v measured around 2M ohms. After a while it failed permanent short. This one was an original component so would date it to early 1980's

Cheers

Rich

Re: Beware!
 

I am beginning to think X caps are not what they are all cracked up to be!
In the old days when a mains filter cap required replacement, engineers would fit a radiospares 0.1uf 1000v or a similar type they would last a good few years then go short often with a slight split in the case. It blew the fuse so was rendered safe! Progress Eh!!