Kim et al,
Leakage current discussion can go in a couple of directions.
Ted E & John W give a good, basic understanding as to the
fundamental issues. Permanently wired equipment is set up from installation
with a reliable earth/ground as that is what is required by the installation
rules and competent electrical installers follow these rules to provide
protection from current available on exposed conductive parts (code
inspectors check on the mechanical installation but do not normally do any
test for voltage or current from exposed conductive surfaces). Because of
this practice the requirement for residual current in the earth/ground is
quite often set at a fraction of the operating current of the equipment
(e.g. 5%)in some standards; this current would be available as an electric
shock issue if the earth/ground is not reliable. If there is a fault the
bonding and earthing/grounding conductors carry the current to earth/ground
until the fusing/breaker interrupts the power to the equipment. This system
of protection has a good record of providing the needed protection for folks
around the equipment while it is operating.
As has been mentioned, the advent of electronic switching has
confounded the currents in the earthing/grounding system for much equipment.
SMPS typically shunt switching impulses to earth/ground; where there is an
installation where the earth/ground is not reliably present (usually cord
connected equipment) there is a specification for the maximum 'touch
current' that is allowed. This limit comes from fundamental research as to
the effects of current on the human body; product committees get this
fundamental data from IEC 60479 series and adapt it to their application.
It is clear that touch current from electronic based equipment is
non-sinusoidal and that the peak value of the current must be used to
determine acceptability for protection. Some product standards have invoked
the measuring circuits and techniques from IEC 60990 to properly make these
measurements for some time. These techniques must spread to more product
standards as switching electronics are moving into more products. SMPS are
being applied in lighting (LED & CFL's); variable speed motors are being
applied in many applications from industrial motors to household appliances.
All of these products need to review their electric shock protection
requirements in the light of these fundamental changes in the application of
electronics in their equipment.
Finally, there may be yet undetermined interaction consequences to
using earth/ground as the dumping ground for these residual switching
currents. This is not yet explored in any definitive way and the
consequences are unknown. This is an opportunity for researchers; this is a
warning to equipment manufacturers who may find unexplained operational
changes in their equipment and may have to search outside their own
equipment for the root cause.
To finish up the original discussion, if equipment is installed in a
region where more relaxed installation rules could give rise to
earthing/grounding which is less reliable then the leakage current (touch
current) should be measured and controlled in that installation to minimize
the risk of electric shock from the normal operation of the machine. It is
not clear to me that any product standards are yet ready to deal with some
of these issues. In such cases the liability falls upon the equipment
manufacturer and the installation professionals to provide a safe
environment.
:>) br, Pete
Peter E Perkins, PE
Principal Product Safety & Regulatory Consultant
Tigard, ORe 97281-3427
503/452-1201 fone/fax
p.perk...@ieee.org
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