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 the 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
uncomfortable experience to have to write to an injured or annoyed 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)
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Replies to this message may be posted in the following public forum:
Question: Discharge capacitance 0.1 uF
(http://ieeepstc.mindcruiser.com/direct/topic/a/ID509830)
