Old story, mid 80's, when PC's were being placed on every desk as office work
stations.
Two different separate events occurred.
The high volume of switching power supplies feeding off the big inhouse
transformers caused some heating issues due to harmonics, etc. Thus the
harmonic standard.
The second issue, related somewhat to USB, a secretary going to lunch caught,
pinched, the keyboard cable in the desk drawer. That mashed the +5vdc towards
the ground with a resistive path. There was enough current that the wire
heated up and caused smoke from the insulation.
This set off the smoke alarm and caused an evacuation of the building, which
was a high rise of considerable height, floors. Thus some more revisions to
the safety standards of the time concerning limiting current and smoke from
insulation.
And internal safety testing then included resistive shorting to discover heat
rise, etc.
The prior testing verified that fuse opened as expected, testing after the
incident verified that indeed the the heat generated would burn the insulation
that really did smoke.
Low smoke insulation and resettable fuses became very fashionable after that.
-------------
I am not a safety engineer, but I know a few.
On Wednesday, October 20, 2021, 03:11:28 PM EDT, John Woodgate
<[email protected]> wrote:
Hi, Pete. Isn't Charlie's option to say, with test results, that if he plugs
his product into a USBzilla source, it still remains safe under any
single-fault condition?
======================================================================================
Best wishes John Woodgate OOO-Own Opinions Only
www.woodjohn.uk
Rayleigh, Essex UK
Istae nunc praetereunt nisi non ubicumque
On 2021-10-20 19:52, Pete Perkins wrote:
Charlie, Remember that 62368-3 was developed at the direction of
ACOS to move the powered communication interfaces info from the 62368-1 base
standard to a separate document that would be available for use outside of
62368-1. The goal is to move this from the Wild West into an orderly set of
requirements that any product standard can use.
You claim that for the ‘simple circuit’ you mentioned the
limited power is safe as you see it. How does the user know if that simple
solution is implemented in any particular product and that it is safe?
The 62368-3 standard itself seems limiting so the requirements
are being rewritten into 2 new documents which will be more tailored to the two
realms under which these circuits work, <60Vdc and >60Vdc. TC108 has this work
underway.
Whether or not any of this is required is a matter for product
committees to specify and test labs to check.
Whether you want to use the requirements or not come down to
your understanding of the risk associated with the use (and misuse) of products
providing power over these communication interfaces and/or having them
specified in a mother standard.
:>) br, Pete
Peter E Perkins, PE
Principal Product Safety & Regulatory Affairs Consultant
PO Box 1067
Albany, ORe 97321-0413
503/452-1201
IEEE Life Fellow
IEEE PSES 2020 Distinguished Lecturer
www.researchgate.net search my name
[email protected]
Entropy ain’t what it used to be
From: Charlie Blackham <[email protected]>
Sent: Wednesday, October 20, 2021 10:12 AM
To: [email protected]
Subject: [PSES] IEC 62368-3:2017 evaluation of USB 2.0 ports
All
I realise there are now applications where 100W or more can be transferred, but
for devices with only USB2.0 ports, that are coming from the same chip, is
there really any need for testing and CB certification to this standard as part
of an assessment to IEC 62368-1?
The particular products I’m looking at are powered from external 5V LPS power
supplies that shutdown at around 4.5 A
I may just be being cynical, but I can’t see how the -3 standard makes such a
product any safer, it just seems to make the test lab richer 😊
Is there something in the CB scheme rules that mandates using the -3 standard
as well when seeking CB certificate?
Best regards
Charlie
Charlie Blackham
Sulis Consultants Ltd
Mead House
Longwater Road
Eversley
RG27 0NW
UK
Tel: +44 (0)7946 624317
Email: [email protected]
Web: https://sulisconsultants.com/
Registered in England and Wales, number 05466247
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