----- Original Message -----
Sent: Friday, August 11, 2000 12:13
When testing a system UL will do abnormals.
On an offline power supply they will OPEN and SHORT EVERY COMPONENT on the
design to test failure mechanisms.
Must not present to a user:
No dangerous smoke.
No dangerous voltages, 240VA power limits, SELV etc.
No traces on boards can vaporize.
If active failure such as lots of smoke and sparks, must not
ignite cheese cloth placed in proximity to failure area.
There are lots more.
Regarding current protection: If a non UL approved device is used for the primary protection
then a secondary means of protection is
required. UL will disable the primary means of protection and rely on the
secondary protection to work. Example: standard Mosfet used for current
limit will assume to be able to fail shorted. UL will place a wire across it
then short the output. Supposing a secondary means is a PTC then this must not
allow enough current to fail the system. UL approved/recognized active
protection devices available which might negate the need for secondary
protection. A UL approved current limit device that meets the tight tolerance
and repetitive operation of 802.3 would be beneficial.
There are lots of areas in this 802.3
implementation that need a safety aspect. I suggest that the group contact UL
and ask for a representative to become part of the group.
THIS IS INVALUABLE.
I see no way that a POWER system can be specified
properly unless you bring in the safety agencies, UL, TUV, European (EN?),