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Film capacitor failed short circuited! :confused:
Film capacitor failed short circuited! :confused:
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Old 23rd January 2019, 09:24 PM   #1
filthyone is offline filthyone
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Default Film capacitor failed short circuited! :confused:

Hi everyone,
here is a story that i would like to share.

I was using the capacitor shown in the pictures as a capacitive dropper for a decorating lightbulb (a 18Watt, 230v incadescent filamet bulb with a big filament). It was of course connected in series with the bulb, and has been serving it's purpose for a very long time, always on.

Today, when i got home i noticed that the bulb was blasting full brightness, like never before! I got scared, i thought, WTF has the mains gone up to 500v or something???? Then, after discharging the capacitor (not needed as i found out later) i checked with a multimeter and the cap was measuring a dead short! I thought "maybe some single point on the metal film of the cap is shorted, let's see if i can burn it by passing a significant amount of current through it, and maybe loose some capacitance in the process".

So i connected it to my diy ghetto power supply and this little cap was able to pass 30 Amperes of current for several seconds, without breaking a sweat!!!!!!! Not even a milivolt of voltage drop across it! And of course it remained dead shorted!

Now my question is, is this a normal failure mode for a capacitor of this type? It might be very old (probably i removed it from some old device) but as i recall it was measuring ok in terms of capacitance. Has anyone seen a failure like that before??

PS. Disregard the burn marks on the plastic case and the stains. That's the careless me and my soldering iron and flux.
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File Type: jpg IMG_20190123_225241.jpg (672.2 KB, 152 views)

Last edited by filthyone; 23rd January 2019 at 09:27 PM.
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Old 23rd January 2019, 09:28 PM   #2
Enzo is offline Enzo  United States
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It doesn't matter how common or rare this failure might be, it has happened to YOUR cap.

What voltage rating was the cap?
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Old 23rd January 2019, 09:38 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by filthyone View Post
the cap was measuring a dead short!
I would use one of the WIMA types that are two series capacitors in one case.
Less chance of a dead short meltdown. Technically though, this should be a
safety rated X capacitor, as it is used on the AC line.

Last edited by rayma; 23rd January 2019 at 09:41 PM.
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Old 23rd January 2019, 09:57 PM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by filthyone View Post
Now my question is, is this a normal failure mode for a capacitor of this type?
Yes, completely normal: it is a non X-variety to begin with, as Rayma remarked, it is plain-foil, not metallized, and it is rated at 250VDC when the theoretical DC blocking voltage in a (very) ideal world should be 400V (to cope with the nominal 230V AC peak).

You have been given a free lesson (no house fire started) in electrical safety, so let us hope you will do the right thing next time: use exclusively X-rated capacitors for mains-related applications: these caps withstand several kV DC voltage (not specified, but actually measured), and are always of the shallow met-film type variety, and normally fail in a safe way.
You have been lucky this time.....
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Old 23rd January 2019, 10:12 PM   #5
filthyone is offline filthyone
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Quote:
You have been lucky this time.....
I sure was! The bulb was in the way, acting as an immortal fuse!

I will replace it with an X type, and i will add a safety fuse... Not that it's really needed, but i usually place fuses everywhere.


The cap, as per the first picture is a 560nF / 250v.

Last edited by filthyone; 23rd January 2019 at 10:17 PM.
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Old 23rd January 2019, 10:18 PM   #6
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by Enzo View Post
It doesn't matter how common or rare this failure might be, it has happened to YOUR cap.

What voltage rating was the cap?
It says 250 volts which could mean 250VDC which is possibly why it failed.
It should be rated at 250VAC as an absolute minimum.
The bulb is basically a short when cold when cold so puts full mains across the capacitor for a short while.
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Old 23rd January 2019, 11:10 PM   #7
jneutron is offline jneutron  United States
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Incandescents typically have an 8 to 1 resistance ratio between hot and cold.

Line transients in the USA 120 volt lines are limited to six kilo volts. That is the flashover rating of the outlets.

Back in the 80's, General Instruments would only recommend diodes with a minimum of 800 volts reverse rating for use directly on the 120 volt line as a result of transients. Any lower than that, and there would always be failures.

Jn
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Old 24th January 2019, 02:31 AM   #8
Refugee1 is offline Refugee1  United Kingdom
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I have found those blue box capacitors to be reliable if you do not abuse them. In my experience they usually fail open circuit with a little black burn mark on one end. The 1700V ones are fine on mains. I have got a box of NOS 1700V low value ones and they work as an L/N filter on 240VAC without any problems.
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Old 24th January 2019, 10:38 AM   #9
filthyone is offline filthyone
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OK. Thanks everyone.
It didn't cross my mind that the part was 250v DC. I was convinced it was a 250 AC cap. I will replace it with a proper one, 275v AC / X2 rated one.
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