Specifying Safety Capacitor Capacitance

Thread Starter

SamR

Joined Mar 19, 2019
5,019
Working on old equipment and have added fuses and 3-line cord sets for grounding. I understand the X & Y classifications and voltages but can't seem to find anything that says just how to specify their capacitance? Even the information on this site fails to mention anything about capacitance value. What determines the correct device capacitance?
 

Ian0

Joined Aug 7, 2020
9,499
The live-to-earth capacitor will be determined by the maximum current you allow to flow to earth. If you make it too big, it will trip the RCD (Residual Current Circuit Breaker).
The neutral-to-earth capacitor needs to be the same value as the live-to-earth capacitor, because you are making a common-mode filter.
The live-to-neutral capacitor can be as big as you like, but you have to make sure it discharges within a certain time, otherwise you can get a shock of the mains plug after it is unplugged, so it might need a discharge resistor. 100nF is a very popular value.
 

eetech00

Joined Jun 8, 2013
3,820
Working on old equipment and have added fuses and 3-line cord sets for grounding. I understand the X & Y classifications and voltages but can't seem to find anything that says just how to specify their capacitance? Even the information on this site fails to mention anything about capacitance value. What determines the correct device capacitance?
It depends on how the cap is used.

Here is a link that explains safety caps. Doesn't give specifics on values but may be worth watching the video.
https://www.utmel.com/blog/categories/capacitors/what-is-a-safety-capacitor#4
 

Thread Starter

SamR

Joined Mar 19, 2019
5,019
it will trip the RCD
No RCD involved here and I do have X2 0.1uF 10% 275V safety caps. But was simply a shot in the dark purchase...

Here is a link that explains safety caps
Yes, I found that link when I was trying to find my answer. However, he never really gets around to HOW to specify the capacitance needed.

"1. Selection of Safety Capacitor Parameters
Safety capacitors are generally used in power circuits for bypass, decoupling, filtering, and energy storage. It is very important to select the parameters of safety capacitors. If the selected capacitor does not meet the actual requirements, it is likely to cause serious problems."

He describes everything BUT safety capacitors! He does do a good job explaining the X & Y parameters previously though. But safety capacitors are not bypass, decoupling, or filtering devices unless sized for use at a specific frequency? I already have plenty of surge suppression on both my distribution panel and use suppressed power strips at my outlets as well where I have solid-state electronics powered. I am looking at them as input protection in addition to fusing which with my already suppressed wiring may not be really needed... Even so, it still leaves the question as to just how to specify their capacitance parameter. Is it simply based on the frequencies that you wish to filter against or...
 

Ian0

Joined Aug 7, 2020
9,499
"1. Selection of Safety Capacitor Parameters
Safety capacitors are generally used in power circuits for bypass, decoupling, filtering, and energy storage. It is very important to select the parameters of safety capacitors. If the selected capacitor does not meet the actual requirements, it is likely to cause serious problems."

But safety capacitors are not bypass, decoupling, or filtering devices unless sized for use at a specific frequency? I already have plenty of surge suppression on both my distribution panel and use suppressed power strips at my outlets as well where I have solid-state electronics powered. I am looking at them as input protection in addition to fusing which with my already suppressed wiring may not be really needed... Even so, it still leaves the question as to just how to specify their capacitance parameter. Is it simply based on the frequencies that you wish to filter against or...
What you are creating with the live-to-neutral and live/neutral-to-earth capacitors along with a common-mode choke with plenty of leakage inductance are a pair of filters to remove common-mode and differential signals in the band 150kHz to 30MHz.
Most Class-X capacitors have self-resonant frequencies lower than 30MHz, when you get to that frequency your capacitors are inductive and your inductors are capacitive, so it becomes very difficult to calculate anything with any degree of accuracy.
 

Thread Starter

SamR

Joined Mar 19, 2019
5,019
Ok...so..what is the capacitors' function?
To either open (class Y) or short (class X) on overvoltage was my understanding for "Safety" caps. But by being capacitors they are also going to be coupling/decoupling and depending on the resistance/inductance of the circuit attached to them also filtering. Finding the resistance/inductance of the attached circuit is the problem. I think I will simply put an X2 0.1uF after the fuse to short and blow the fuse. Somewhere (as shown on the schematic) there is now a capacitor to ground which should either be removed or replaced with a Y2 that does have the value stated. Or at least that's my take on it so far...
 

Thread Starter

SamR

Joined Mar 19, 2019
5,019
I went looking for Y2 caps and found a lot of 2 lead X1 Y2 caps. I've seen the 3 lead X Y devices but 2 lead? Does that mean they be used for X1 across the line or be used as Y2 line to ground? What's with the dual rating on 2 lead devices?
 

Ian0

Joined Aug 7, 2020
9,499
I went looking for Y2 caps and found a lot of 2 lead X1 Y2 caps. I've seen the 3 lead X Y devices but 2 lead? Does that mean they be used for X1 across the line or be used as Y2 line to ground? What's with the dual rating on 2 lead devices?
Presumably they can fail both short and open simultaneously on fault conditions?
 

eetech00

Joined Jun 8, 2013
3,820
To either open (class Y) or short (class X) on overvoltage was my understanding for "Safety" caps. But by being capacitors they are also going to be coupling/decoupling and depending on the resistance/inductance of the circuit attached to them also filtering. Finding the resistance/inductance of the attached circuit is the problem. I think I will simply put an X2 0.1uF after the fuse to short and blow the fuse. Somewhere (as shown on the schematic) there is now a capacitor to ground which should either be removed or replaced with a Y2 that does have the value stated. Or at least that's my take on it so far...
if the intent is to protect a device from line surges why not use TVS device?
 

Thread Starter

SamR

Joined Mar 19, 2019
5,019
Intent is to give a 60-year-old piece of tube gear some modern standards protection. Adding a 3-wire cord, X2 across the line capacitor, and fuse to the chassis has done that. But there is still the 0.05uF "capacitor of death" from line to chassis which will be removed even though the chassis ground and fuse also protects the chassis from becoming live. To remove any line noise (I have a noise problem at my location) a capacitor to ground is helpful and part of the original design. So was thinking of replacing it with a Y2 safety capacitor instead of simply replacing it with another poly film cap that is capable of shorting to the chassis. The Y2 Cornell Dublier 0.05 caps available on Mouser and DigiKey are designed for PCB installation with short legs. While looking I also found the X1 Y2 caps with long leads for point-to-point wiring which this chassis has. I have seen 3-legged X2 Y2 caps and understand them. But I have no idea just what these 2-legged X1 Y2 caps are or how they are intended to be used. Something new to me...
 
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