PSNet,
Thanx to John Woodgate for his additional remarks on sensation and
current. Yes, the current is the key parameter - designers would really
like voltage limits but the conditions need to be controlled to provide
proper protection.
From a historical point of view the traditional limit to the leakage
current that was exposed in a way it could be touched was 0.5mArms AC. This
level has come to be called perception/reaction level. Whether or not there
is a sensation varies from person to person, depending on their individual
physiology. It is low enough that there will not generally be an
involuntary reaction which might lead to a secondary injury. As time went
on this requirement was applied to appliances under normal operating
conditions; under abnormal or fault conditions a higher level of current was
accepted - less than the let-go (b curve) level of 5 mArms AC.
If you have ever touched circuits that develop these currents you
will be surprised at the intense feeling of a current of a few mA.
Personally, I can feel the 0.5mA current and don't like the 3.5mA current -
I usually get off of it before the meter stabilizes to get a reading.
The development of modern switching techniques, now directly in
mains circuits, made it more difficult to control the leakage to earth and
standards, such as IEC 60950, set a limit under fault conditions of 3.5mArms
AC (no normal operating condition was established under IEC 60950 - it was
difficult and expensive to try to get to 0.5 mArms - most supplies, from my
experience, were ooo 1.5mArms under normal conditions & can be reduced using
techniques developed to suppress EMC). Further, these Touch Current
waveforms are no longer sinusoidal - which means that peak measurements need
to be made on all modern equipment.
The new TC108 Hazard Based Standard is defining the conditions for
circuits accessible to ordinary persons which will again limit the Touch
Current to 0.5mArms AC but allow 5mArms AC for circuits accessible to
instructed persons. This is a step in the right direction.
:>) br, Pete
Peter E Perkins, PE
Principal Product Safety Engineer
PO Box 23427
Tigard, ORe 97281-3427
503/452-1201 fone/fax
p.perk...@ieee.org
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