Several years ago I had the need to explain this same issue.
Similarly most of the inquiries came from the U.K. Here are
some exerpts from a "white paper" done to address this issue:
***************
This document addresses the nature and safety of two-wire
ITE products.
The international safety standard for ITE is IEC 60950. The
referenced sections of IEC 60950 below are generally the same
in unique country standards.
SHOCK PROTECTION
A major aspect of product safety for ITE is protection against
electric shock. There are three equally acceptable methods of
achieving this protection (section 1.2.4). Class I equipment
employs only "basic" insulation, but ties all accessible
conductive parts to a "ground" pin to protect the user in the
event of a failure of the basic insulation. Such equipment requires
a three-wire line cord and a reliable path to earth ground.
Class II equipment uses "double" or "reinforced" insulation
between primary voltage and accessible conductive parts to protect
against electric shock. Such equipment uses a two-wire line cord,
is not dependent on the integrity of the building's grounding system,
and typically displays the "square within a square" symbol denoting
double insulation. ITE products which contain no hazardous voltages
(e.g. less than 42.4Vpk/60Vdc) are approved as Class III devices.
NATURE OF CLASS II
There is one aspect of Class II equipment that can confuse end users.
Because no earth ground path is required or available, accessible
conductive parts will "float" to some voltage less than the applied
mains voltage. Typically this may be one-half of the mains voltage.
This voltage is not considered a hazard under the standards as the
available current cannot exceed the 250uA specified in Table 17 of
section 5.2.2. Some individuals may be able to feel a slight "tingle"
or "shock" at this low current level, although no electric shock
injury should result.
FINAL COMMENTS
If a user is aware of a voltage on an accessible part, and suspects a hazardous
condition, some simple tests can be performed to eliminate this concern.
George Alspaugh
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Michael Garretson: pstc_ad...@garretson.org
Dave Heald davehe...@mediaone.net
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
Jim Bacher: j.bac...@ieee.org
All emc-pstc postings are archived and searchable on the web at:
No longer online until our new server is brought online and the old
messages are imported into the new server.
