Dear Jae Won Yoo,
Thank you for your recent inquiry about leakage current. Sorry for
not answering sooner, I've been out of town on business.
You have raised several important questions about leakage current.
You are correct, Dalziel's work is a key basis for leakage current limits,
but it does not clearly delineate all of the levels that are seen in
standards today. So, why these differences??? Here's a simplified
explanation.
First, Dalziel placed the threshold of perception at 0.5 ma - altho
this is probably better called the threshold of reaction in that most
people will jerk away from the shock at this level. This was traditionally
chosen as the limit of leakage current for many products. When double
insulated equipment became common, this value was cut in half (as 2
insulation resistances would now be in series between the hazardous voltage
and the user). Some equipment (such as with heaters) had trouble meeting
this requirement and were set at 0.75 mA leakage current. It was obvious
that permanently wired equipment presented less hazard in that it was
easier to provide a solid earth ground connection so a higher limit was
selected - quite often on the order of 5 or 10 mA or so (which still was
under the threshold of fibrillation for most cases).
With the advent of modern equipment - switcher power supplies, etc.
- plus the need to control emi, it became more difficult/costly to meet the
low level of leakage current traditionally used and justification was made
to go to higher levels in these equipments. This has led to the 3.5 mA
levels used in many standards. Although 3.5 mA is a value that most people
say 'hurts', it is not considered dangerous - it will not cause heart
fibrillation in a healthy person.
Secondly, the choice of leakage current limits is a complicated
process. Altho the basic information is commonly available - Dalziel's
papers, IEC 479, etc., the particular levels chosen for various classes of
equipment are selected by equipment committees - there is no centrally
dictated leakage current limit. Thus the values chosen and the rationale
for these values vary from committee to committee (since they are different
groups of folks). So, as you have seen, there are different limits for
different equipment.
Finally, in spite of the limits set in any standard, product
manufacturers need to ask themselves what level of electric shock do they
want their customers to receive when using their equipment - in normal use
and under fault conditions. Most engineers feel very uncomfortable when
experiencing the higher levels of allowable leakage current and usually
express the feeling that they can control the leakage current to some
lesser value in the equipment even considering the the usual design
limitations - technical, time and cost constraints. Designing equipment
with lower levels of leakage current instill confidence in the use of
electrical equipment by a broad group of users. Just as with any other
bad experience with a product, a user will tell 10 times more people of
their bad experience than they will of their good experience with a
product.
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Peter E Perkins
Principal Product Safety Consultant
Tigard, ORe 97281-3427
+1/503/452-1201 phone/fax
[email protected] email
visit our website:
http://ourworld.compuserve.com/homepages/peperkins
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