How useful are these?
I am ordering X1 Y2 1KV 0.01 and will experiment with them.

In amongst the vagaries one needs to elaborate. These are normally used for line filtering.

I have rural power that RF riding on it and is also subject to lightning surges. The spikes were added to by a solar panel weed inverter, going feral. I have had filtering in one form or another, against that lot for probably three, or more decades.

You will note many American radios with Line Caps. Principally transformer-less (Hot Chassis) however, with two wire cables the neutral is the RF earth return. Unfortunately they use them on transformer sets and their two wire utility (mains) power plugs are reversable, meaning that the single ones can be on active and put a charge on non grounded chasses, and make them alive if it fails.

The each way system of two, is a voltage divider. That means a charge of theoretically half the mains appears on the chassis an can / will create hum (does in some).

I have found that the inductive filter first in this application, is superior when followed by caps & MOV's. I did ages a go publish the modified circuit & I note that many commercial power boards use MOV's exactly as I did when I added a mod decades ago.

On Power Boards: I do Tag & Test mainly because of what I do. I have seen several crap ones of these, that beggar belief that they got into the country; yet alone approved. Several had melted down at 8A, when rated at 10A and the 10A circuit breaker of course never tripped.

I had an X type capacitor in a circa 1990 Telecom-branded power board take to smoking recently. I didn't go short; it bubbled away until I cut the power.

X & Y are to service two different purposes on the mains but some are actually both X and , so you can just use the one type. If you use them on the mains, they have been known to explode. More likely caused to explode, so a containment that cannot support combustion is desirable, e.g metal.

Here is a good link. Marcc I would like yours or anyone's opinion on this please..

https://youtu.be/mLfDdbzYpa4?si=xAh0bd53XCPDdUj5

I have not had time to look at all of it:

450V on the #80 is not a value I use. Not good enough. I will not go below 500V with an #80/ 5Y3 or anything that behaves like one. The surge voltage on startup is what kills them. Take note that the original Red Ducon GP cap on tubes like that were 525 Surge / Peak voltage (They knew). Modern cap rarely give surge volts.

Here the two wire plug like the NEMA one does not exist in domestic mains power. Ours cannot be inverted at the socket and there is only one way to wire a socket and they can be checked by non contact devices.

Fuses usually prove useless.

"X" caps go across the line "Y' line to Ground and as far as I am concerned 0.01mfd is the absolute maximum line to ground. Better effect will be achieved by also adding 0.1mfd neutral to active, also after the switch.

Any addition of line caps to an ungrounded chassis like that, is a pet hate. While they are supposed to fail safe, if it does not do that its a "Live Chassis". Also its a voltage divider, which like a shielded transformer, will put a charge onto the chassis and may cause hum if the chassis is not grounded.

With the line caps to chassis on US radios, that was more about an RF path to ground. That should be chassis to Neutral, which is where regulations and reversible plugs there cause issues.

An unrelated warning is with regulated power supplies, many of these will go berserk if directly grounded to anything. In such cases a 0.01mfd series line cap, needs to be used.

Do note with the introduction of RCD's, While the neutral is still at earth potential it is a separate entity after the RCD. An RCD will not trip if the earth leakage fault is onthe secondary.

Thanks for that Marcc. I agree with your comment about the 450 volt electros! That's why I only use 600 voters. My interest is in the optimal use of the safety caps. The ones I have are all .01 at 1KV.

Often the line caps are decorated with various international standard symbols and have a working voltage in AC eg 250~. Also that AC voltage is often marked on MOV's.

If its a DC voltage on them, that may be the punch through, albeit that's around 1600VDC to 3KVDC. RF does not usually require much more than 0.01μF bearing in mind it will be drawing current and with RF less is often better. Not withstanding some B+ decoupling caps can be around 0.25μF.

The line cap differs by way of it being designed to fail open.