PSNet;
Looking at touch current specs
Lately there has been quite a bit of interesting dialog here on the
PSNet
concerning touch current measurements as covered in the 3rd edition of the
60950 family of standards. Most of the problems come about because of the
use of Pluggable Type A connections (usually North American NEMA-15 or EURO
10A plugs). Equipment with a reliable earth/ground connection is given more
latitude (markings, etc).
3 specific instances are covered in 5.1.2.
A1) Systems of interconnected equipment with individual power connections
tested separately
A2) Systems of interconnected equipment with a single power connection
tested together.
No questions have been raised about the A1, A2 instances, these have
been
in place for some time and the interpretation is not questioned.
The new wording is intended to clarify the measurement under other uses
that are common. (The abbreviated wording is my own – meant to help those
folks who do not use the 60950 family of standards regularly). This change
came about considering some specific usages, but is intended to be a broader
reference to the measurement conditions.
B) Equipment connected to multiple power sources but only one required at a
time, tested with only one source connected.
An instance considered here is that of equipment that would work from 2
power sources – an AC supply and a DC supply, but only one would be needed
at a time for operation. In this case, the EUT would be connected to the AC
supply (with the DC supply disconnected) and the required tests run next the
EUT would be connected to the DC supply with the AC supply disconnected and
the measurements repeated.
Based upon my own experience with telecom equipment, I would believe
that
the disconnection of the DC source would also remove the Frame Ground
connection – whether as part of a connector assembly or wires installed in a
field-wiring terminal – which would negate the argument given for a
permanent ground connection in that case.
C) Equipment running simultaneously from multiple power sources, tested with
all power sources connected.
This is intended to cover the use of redundant power supplies in
operating
the equipment. All of the power supplies would be connected into one source
(usually a power strip) and the combined touch current would be measured. I
would expect that the individual supplies would be disconnected, one then
another and the touch current again measured to insure that the touch
current under more load wasn’t any worse than before.
In all of this the intent is to continue to provide products that
present
the same level of electric shock protection to be found in the standard.
br, Pete
Peter E Perkins, PE
Principal Product Safety Consultant
Tigard, ORe 97281-3427
503/452-1201 fone/fax
[email protected]
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