Hi John, Though you are fully right theoretically, i think parts of your story are a bit overdone.
Most people do not make a habit-of-disconnecting-their-heavy-equipment-while carrying-it-over-their-feet-at-the-mains-voltage-phase-point-of-90-degrees-w hile-actively-seeking- both points-of-the-power-plug-simultaneously-within-the-discharge-time-of-a-paper -capacitor of-less-then-0.1-uF-often-enough-to-really-make-the-manufacturer-very-"popul ar" as you say, ....not to mention that they also need to react involuntary by dropping the equipment...... I mean: a lot of conditions need to be satisfied a lot of times..... (but i also know that exactly this is what happens when an accident arrives) Most handheld equipment does not contain capacitors between phases and/or ground BEFORE the momentary mains switch (drills etc), and in cabinets where the mains is connected permanently or by big power plugs, service people do not have a habit of touching the bare parts that fast after disconnecting. Lets try some numbers: Paper capacitors (and they are as they need to be self healing) do not hold their charge that long. At a selfdischarge impedance of 10 M the discharge time equals 1 second. After a number of self healing discharges the capacitor actually has a bleeding resistor build in. Did you ever unfold (unroll) a filter cap after a few years duty and hold it to the light ? Literally 100's of tiny holes can be seen with the naked eye. Possibly a new cap may hold it's charge longer.... ? If a human body is to function as a discharge path , under the premise that the discharge is to be felt substantially, a current of say 3.0 mA is required. This requires at a charge voltage of 300 Volts 100 K of human impedance. Time constant 10 mS (half a period) Thats peak current. Average current during this time span is much less . If you are wet handed (not recommended) the peak current may rise substantially but the pulse with decreases propotionally, so does the effect. This effect can easily be verified with an experiment using your own pulse generator and a inversely connected mains transformer and a volunteer : that was me during a time.... I won't explain in detail why i volunteered though ......;<))) At a pulse with of 1 mS one may accept much higher voltages before anything is noticed then at 100 mS (BTW don't start with square waves, but sinus pulses) If one takes out a mains plug and voluntarily tries to touch ground and phase plug contacts at least 1 S have been elapsed. I mean, imagine your self pulling with one hand and touching the pins with the same (as the other carries the heavy equipment). The capacitor would be almost discharged. Possibly your experiences have been based upon equipment with other type of capacitors with much better dielectricum, but possibly not the well established paper type X or Y type capacitor. I DO support your point about the discharge resistor though. Any accident prevented is a step forward on the world wide scale of safety.. Gert Gremmen ce-test, qualified testing -----Original Message----- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of John Allen Sent: woensdag 18 september 2002 19:11 To: emc-p...@majordomo.ieee.org Subject: Question: Discharge capacitance 0.1 uF Hello Folks Tomonori Sato commented "However, I think discharge from 0.1uF capacitor charged to the mains peak voltage can be quite uncomfortable." I believe that to be true from personal experience and from having to investigate the results of a number of such incidents, and so would remind member of a point that I made several years ago on this forum: The primary shock almost certainly will NOT hurt a person, but the involuntary reaction TO the shock may well have much more seriousconsequences. This type of shock is often encountered by people who pick up equipment which they have just unplugged from the AC mains in order to carry it elsewhere. If they then touch the pins of the plug there are numerous reported incidences of them involuntarily dropping the unit - and that can possibly be on their own feet - and from a height of about 3ft/1m! If the unit is more than a couple of pounds (about one kilo) then the injury to t! he feet can be substantial. Worse situations could occur in industrial equipment when a service engineer opens a cabinet to perform a service operation - the reaction from the "shock" could cause him to strike touch other hazardous electrical or mechanical parts (which probably should also not be there, I do agree!) which then cause him serious actual injury. These types of incident do not make the equipment supplier very "popular" to say the least, and could result in product liability claims. The main basis for the claims would be that the supplier had not adequately assessed the hazards and taken the appropriate simple precautions which are easily and cheaply available - fit a bleeder resistor across the capacitor, or use a filter with a resistor already built in (or with transformer/inductor windings directly across the capacitor - which achieve the same result) ! Again from personal experience I can say that it is a very "embarassing" and un! comfortable experience to have to write to an injured or anno! yed person, or to his employer, to say "sorry, but that is what the safety standard allows". It is just not good "business sense". Therefore, regardless of the requirements of the various standards and this argument over capacitor value and/or charging voltage, I firmly believe that the use of bleeder resistors should be considered effectively mandatory, and have always recommended it to engineers I have advised on product safety. Regards John Allen Technical Consultant Electromagnetics, Safety and Reliability Group ERA Technology Ltd Cleeve Rd Leatherhead Surrey KT22 7SA Tel: +44 (0) 1372-367025 (Direct) +44 (0) 1372-367000 (Switchboard) Fax: +44 (0) 1372-367102 (Fax) ---------------------------------------------------------------------- Replies to this message may be posted in the following public forum: Question: Discharge capacitance 0.1 uF
