Radial X2-AC Safety Capacitors (Question)

[olds...@tubes.com]

I want to replace the original capacitor which is located across the AC
power line in a Zenith Trans Oceanic (tube radio). The original cap is a
.047 at 600V (standard capacitor). I was told that these days, standard
caps are no longer used across the AC power line. Instead, they use
these Radial X2-AC Safety Capacitors.

What I'm finding is that these caps are available, but I am not finding
any of them rated at 600V. All I can find are rated at 250 or 275VAC,
and I did find some .1uf on ebay for 300VAC.
Sal's Capacitors http://www.tuberadios.com/capacitors
has a .047uf at 275VAC on their webpage.

One rule I never violate is the voltage rating of parts. I'll go OVER
but never UNDER the original voltage rating.

However, the old caps were rated at DC voltages, whereas these Safety
Caps are rated at AC voltages. I would think that 275V is sufficient to
use across a 120V AC power line (which is what they are made for).

Therefore, is is safe to use these to replace the original cap, since I
can not find any rated at 600V?

I know this will not be the first radio needing a replacement cap across
the power line, so if I order one of these caps, I'd rather order
several so I have them on hand. That leads me to a second question:
How critical is the uf rating on these line caps? In other words, if I
use a .1uf instead of a .047uf, will that cause any problems, or is the
.1uf offering more protection against power line spikes?

Thanks

[John-Del]

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.

[rickman]

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

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

[tabb...@gmail.com]

275v ac X-rated is safer than a vanilla 600v DC type.


NT

[tabb...@gmail.com]

On Thursday, 24 August 2017 18:53:09 UTC+1, rickman wrote:
> John-Del wrote on 8/24/2017 1:38 PM:

> > 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.
>
> 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?

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


NT

[John-Del]

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 standard 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 the 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 all kinds of parts, X and Y caps included.

[rickman]

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

[Dave Platt]

In article <onn3hu$ml3$1...@dont-email.me>, rickman <gnu...@gmail.com> wrote:

>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?

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.

[rickman]

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?

[John-Del]

It's a possibility, but in my opinion it's highly unlikely for that to happen. A line capacitor that becomes leaky *and* with at least 20 amps of source 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.

[pf...@aol.com]

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

Peter Wieck
Melrose Park, PA

[tabb...@gmail.com]

On Thursday, 24 August 2017 19:37:11 UTC+1, rickman wrote:

> tabbypurr wrote on 8/24/2017 2:22 PM:
> > On Thursday, 24 August 2017 18:53:09 UTC+1, rickman wrote:
> >> John-Del wrote on 8/24/2017 1:38 PM:
> >
> >>> 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.
> >>
> >> 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?
> >
> > Y is for shock risk, X is for fire risk only.
>
> Ok, I stand corrected. But the question remains. Fuse and X cap is like
> wearing a belt and suspenders.

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


NT

[tabb...@gmail.com]

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

[tabb...@gmail.com]

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

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 satisfactory cover. You can repair them as is or can add a fuse, the latter is safer of course.


NT

[Dimitrij Klingbeil]

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

[Phil Allison]

olds...@tubes.com wrote:

----------------------------

>
> I want to replace the original capacitor which is located across the AC
> power line in a Zenith Trans Oceanic (tube radio). The original cap is a
> .047 at 600V (standard capacitor). I was told that these days, standard
> caps are no longer used across the AC power line. Instead, they use
> these Radial X2-AC Safety Capacitors.
>
>

** The use of 600V DC rated caps across a 120VAC supply line is time honoured 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 single part and these became standard for class X caps.



.... Phil

[Michael A. Terrell]

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! ;-)

[tabb...@gmail.com]

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

[Michael A. Terrell]

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.

[Ralph Mowery]

In article <_oydnUQN6YdVaTjE...@earthlink.com>,
mike.t...@earthlink.net says...

>
>> 1.414 volts peak, on each side of zero
> >
> > 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.
>
> 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.

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.

[tabb...@gmail.com]

On Wednesday, 30 August 2017 00:12:17 UTC+1, Michael Terrell wrote:

275v ac = 389v peak, which can be withstood by a 389v rated capacitor. The reason they used 600v was to improve safety & reliability. However time showed that it wasn't good enough. X & Y caps are much better attempts to address the failure/safety issues.


NT

[rickman]

You are very good at math, but not so good at electronics. You don't need
to consider the peak to peak voltage because the cap doesn't see them both
at the same time. It sees one peak, then it sees the other peak. The fact
that they are opposite polarity doesn't mean you need to add them to
consider the capacitor voltage rating.

[tabb...@gmail.com]

On Wednesday, 30 August 2017 00:12:17 UTC+1, Michael Terrell wrote:

Sorry but that ain't so.


NT

[Phil Allison]

Michael Terrell wrote:

------------------------

>
> Think again. You have a negative and a positive peak, not just a
> single peak. You have to add them together.
>

** Nonsense, the DC rating is not exceeded by a reversing voltage.

> That is why the original DC cap was 600V, not 300V.

** False.

It's simply that 600V rated caps survive much better.

AC supply and load bridging involves spike voltages that can break down the insulation.



..... Phil