Radial X2-AC Safety Capacitors (Question)

Across the line use a type X cap, and 250V is fine. You could (should) add a fuse in between the cap and the line for an extra margin of safety.

John-Del wrote on 8/24/2017 1:38 PM:

I thought the X and Y capacitors were used when a short would be a safety problem as in a shock hazard. Why would there be a shock hazard if a cap across the power line shorts? If you use a fuse any concern about a fire is eliminated. Then why would you need the X cap?

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Y is for shock risk, X is for fire risk only.

NT

add a fuse in between the cap and the line for an extra margin of safety.

is

I agree with you, but the company line says to use X caps across the line..

These vintage radios rarely were equipped with fuses and generally used sta ndard caps across the line. In all my years in consumer electronics I only saw a few of them short (spectacularly) and they were in RCA tube TVs in t he 60s. They all made a bloody mess under the chassis but none caused fires . RCA issued a recall that included new ceramic tubular caps and we changed them whenever we serviced these chassis.

I've replaced some in my own radios with mylar caps but always fused them. Nowadays I've got tons of late model power supplies that are donors for al l kinds of parts, X and Y caps included.

You can't count on a fuse to eliminate all of the risk of a fire.

Some types of cap can fail with a "near short circuit" - they get leaky enough to start drawing a good fraction of an ampere, but aren't a dead-short. Imagine what happens if such a cap is "protected" by a

1-ampere fuse, but is drawing 100 mA at 120 volts... that's more than 10 watts, heating up the capacitor. If the cap doesn't either short itself well enough to blow the fuse, or go "open", it can definitely heat up enough to smoke and burn.

I've seen this happen... a non-X/Y-rated film cap was used "across the line", and it overheated and nearly started a fire.

"X" and "Y" caps are intended to be at least somewhat self-healing... if they develop a pinhole and start to short, the localized heating burns away the metallized film in the area of the short, and it opens. If I recall correctly they're also required to use an insulating resin which is at least somewhat flame-resistant.

If you're going to the trouble of replacing an across-the-line cap in some equipment you're refurbishing, I'd suggest going right to a suitable "Y" cap. The additional cost is modest and the labor to install is the same.

Ok, I stand corrected. But the question remains. Fuse and X cap is like wearing a belt and suspenders.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Ok, so there is a failure mode where the cap won't draw enough current to blow the fuse, but can locally heat up enough to smoke and burn. Isn't that true for other parts in the device?

What is the purpose of this cap anyway? I assume noise filtering. Wouldn't that be just as effective on the secondary of the transformer?

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

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It's a possibility, but in my opinion it's highly unlikely for that to happ en. A line capacitor that becomes leaky *and* with at least 20 amps of sou rce current available to it will avalanche as it builds heat and short hard - in any case pulling more current than the small fuse installed in front of it.

X & Y caps are designed to reduce the risk of shock in one case, and fire i n another. Eschewing needless complexity, let them do their jobs and instal l a fuse - if desired - to cover then entire appliance, designed specifical ly for the load involved, starting surge and so forth.

Peter Wieck Melrose Park, PA

You need belt & suspenders since both fail. Just don't put the pics online. :)

NT

All my Y caps are of far lower values than the Xs. Ys are intended to be for line to chassis use, hence far lower values than used for X caps.

NT

in another. Eschewing needless complexity, let them do their jobs and inst all a fuse - if desired - to cover then entire appliance, designed specific ally for the load involved, starting surge and so forth.

Fires & shocks used to happen due to dc rated cap failure. Hence X&Y were d eveloped.

Old radios lacking any fusing don't meet modern safety standards. The mains plug or distribution unit fuse can't be relied on to always provide satisf actory cover. You can repair them as is or can add a fuse, the latter is sa fer of course.

NT

Besides, using an X2 capacitor does call for an additional means of protection. Capacitors rated for being connected across the line "X" come in different sub-categories: "X1" and "X2".

"X1" capacitors are rated for a 4 kV transient overvoltage and they are often found where no other means of protection (fuse) is installed in the equipment "upstream" of the capacitor (that is directly across the power line).

"X2" capacitors are only rated for a 2.5 kV transient overvoltage and the circuits they are installed in need to be fused as an additional means of protection (additional besides the dielectric in the cap).

Both X1 (rated to 4 kV pulse) and X2 (rated to 2.5 kV pulse), when used within their specifications and unless their safety certifications are fake, tend to be better protected than "plain" 600 V type capacitors.

The voltage ratings indicate the maximum AC line voltage that the X or Y capacitors may safely be connected to (not their peak pulse ratings).

So, it's correct (and would be required in a repair) to replace the old

600 V capacitor with a 250 V safety rated "X" capacitor (assuming the nominal mains voltage in your country is not higher than 250 V), but if the capacitor is an "X2" (rather than "X1") type, the device also needs to be fused "upstream" (on the mains side) of the X capacitor.

Regards Dimitrij

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** The use of 600V DC rated caps across a 120VAC supply line is time honour ed and normally perfectly OK - long as you are not in a high lightning area .

The issue that arose was in 240VA counties when wax impregnated paper caps were replaced with plastic film types. Some of these would last only weeks before developing internal shorts and exploding.

The problem was soon found to be due to tiny pockets of air trapped inside the cap when being wound - corona discharge developed across these pockets and destroyed the cap. However, such corona currents did not normally occur with 120VAC so US makers kept using plastic film types.

The fix for 240VAC places was to use two 600V film caps is series, thereby limiting the effective voltage to 120VA for each. Cap makers in the UK and elsewhere developed a method of winding "two in series" film caps as a sing le part and these became standard for class X caps.

.... Phil

People, learn to do the math.

600VDC/2.828=212VAC, so the 275 volt rated capacitor would be equaL to a 777VDC capacitor. 2.828 is the peak to peak factor on a RMS sine wave. That is 1.414 volts peak, on each side of zero

--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)

Erm... no. Firstly 275v ac is 389v dc peak, so a 389v dc rating. Secondly the 2 ratings are not comparable, even after conversion to dc. The 600v cap has no fusing and most likely no double layer safety feature, the 275v ac one has both.

NT

Think again. You have a negative and a positive peak, not just a single peak. You have to add them together. That is why the original DC cap was 600V, not 300V.

--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)

While there is a positive and negative peak, the capacitor only charges to the peak and not p to p. The charge reverses at each half cycle.