Chris & PSNet,
This issue was discussed on this forum last January. Check the archives
for more details...
There is a considerable body of knowledge that shows for the usual
socket outlets (NEMA 15 in North America) or less technical, but equally
acceptable information for similar Euro outlets (for the Euro 10A plugs)
that the earthing is not reliable. This condition is only mitigated when a
separate, dedicated outlet is provided for each cord and plug. (Dedicated
outlet is a special, defined outlet according to the US/NEC wherein each
single, grounded outlet is provided with a separate wiring run from its own
breaker at the breaker panel). Further, this issue does not arise for
higher current plugs (NEMA 20, etc. as the earthing is considered reliable).
Because of this, the level of protection desired to be maintained in a
product is not evaluated when one PE connection is maintained during the
Touch Current evaluation proposed. For this reason I have continually
recommended to equipment committees and test houses that for this test all
power cords be connected into one plug strip and the Touch Current
evaluation be done at one time. All other redundant grounds are to be
lifted for this test.
This is in agreement with the principles of IEC 60990 (Measurement of
Touch Current and Protective Conductor Current) and the associated
procedures for which the intention is to provide the same level of
protection to users under all conditions of use.
So, your initial inkling is correct... You need to meet the Touch
Current requirements with all the grounds tied together...
br,
Pete
Peter E Perkins, PE
Principal Product Safety Consultant to Merix
Tigard, ORe 97281-3427
503/452-1201 fone/fax
p.perk...@ieee.org email
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