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Old 18th June 2005, 07:14 AM   #1
Vladco is offline Vladco  United States
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Default Power conditioner caps

Which caps should I use for power conditioner? I red about X, 2X and Y rated caps. Are they interchangeable for PC? Does any one know polypropylene caps I could use for PC? If yes digikey or mouser parts preferable but not necessary. I tryed to search but cann't find right thead.
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Old 18th June 2005, 08:00 AM   #2
AndrewT is offline AndrewT  Scotland
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X & Y caps are for connecting direct to the mains.
X2 connect live to neutral (hot to cold?)
Y connect live to earth and neutral to earth (hot or cold to ground?)
Keep the earth caps (Y) smaller to reduce earth leakage, try 10nF to 47nF.
A few 3 terminal Y caps are available to connect across all three poles of a single phase supply (live/neutral/earth).
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Old 18th June 2005, 08:54 AM   #3
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Bear in mind that to stay within earth leakage limits, 4.7nF is the max you can connect across live/neutral and to earth, or at least it used to be in the UK.
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Old 18th June 2005, 02:54 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
how do you calculate the earth leakage related to cap value connected to earth?
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Old 19th June 2005, 11:59 AM   #5
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I guess it's simply

1 / (2 * pi * F * C)

to get the reactance then good old Ohm's law

I = V / R

to get the current
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Old 19th June 2005, 11:19 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
thanks for confirming that calculation.
An RCCB (rcd) protected circuit should have an operating earth leakage current NOT exceeding 25% of the rated trip current of the breaker.
Normal domestic mains protection usually use a 30mA RCCB.
Maximum operating leakage would therefore be 7.5mA.
A 4n7F cap from live to earth will transfer 0.354mA (50Hz & 240Vac) to earth.
This allows a further factor of safety in excess of 21.
I recommend a sensible limit for the Y cap to earth of 10nF to 47nF. This cap range would allow multiple appliances on the protected circuit using upto about 100nF total capacitance to earth and stay below the recommended 25% limit.
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Old 20th June 2005, 08:02 AM   #7
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Yes Andrew good logic there. However, the 4.7nF I stated came from BS-EN60065 which used to be the standard for electrical equipment. If you had more capacitance than this your equipment would fail regardless. This was about 8 years ago or maybe more though and I believe the Standard has been superseeded.
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Old 20th June 2005, 06:33 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
"If you had more capacitance than this your equipment would fail regardless"

Why?
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Old 21st June 2005, 07:20 AM   #9
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Because the standard specifies the max capacitance allowed and it's an iron rule.
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