Amund, I support Rich’s approach.
It does leave a lingering question, though. (removing my rose
colored glasses and putting on my dark, pessimistic glasses)
Imagine a downstream case such as this: The unit works well and
is popular. A customer request comes to the manufacturer something to the
effect that the unit works well except does not provide the full operational
reliability in cases where there is significant EMC generated in the use area;
they ask for an output (USB , PoE, etc) so that they can cable connect the unit
for these applications. A (different) company designer believes that this is
easy to do and starts to work on this project. If you are lucky, he consults
the earlier safety lab report to understand the details to properly implement
this.
Where in the report do you clearly state that the requirements,
including isolation/insulation (creepage and clearance) were not evaluated and
the ‘secondary’ is considered mains in a clear way?
With this understanding the designer will know that the full
mains isolation/insulation will have to be done for the output circuit since it
wasn’t done for the mains/secondary interface initially. (Since, reasonably
often, the unit won’t meet the mains/secondary requirements in some way and the
manufacturer will not be willing to change it in this redesign cycle.)
If this is not clearly taken care of initially then the process
starts down the slippery slope of believing that everything was completed
earlier and not fully reviewed at the modification step. If not caught by the
designer then the test lab catch will be a major complication in the project
schedule. If not caught by the test lab (your associate down the hall) then
the product is inadequate and does not meet the requirements of the standard;
hopefully this gets caught in the review but what if it doesn’t?
My point is that simplifications need to be clearly stated in
the documentation for downstream users. Don’t leave anything to chance.
:>) 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
<http://www.researchgate.net/Peter%20Perkins> www.researchgate.net search my
name
<mailto:[email protected]> [email protected]
Entropy ain’t what it used to be
From: Richard Nute <[email protected]>
Sent: Sunday, September 12, 2021 3:05 PM
To: [email protected]
Subject: Re: [PSES] Creepage and clearance requirements
Hi Amund:
If no accessible conductive parts, then you can designate the secondary
circuits as part of the primary circuits, which means there is no need for
isolation between primary and secondary circuits. No creepage or clearance
requirements! OVC would not apply primary-to-(a primary) secondary.
The plastic enclosure would probably constitute reinforced insulation
throughout. For electric shock, you would wrap in foil and measure touch
current. Should be comfortably below the limit. And, you would need to do a
dielectric test to the same foil at twice the voltage necessary for basic
insulation. Should easily pass.
I have assumed the antenna is within the enclosure so no accessible conductive
parts. If the antenna is an accessible conductive part, then the above
scenario is not valid.
Stay safe, and best regards,
Rich
From: Amund Westin <[email protected] <mailto:[email protected]>
>
Sent: Sunday, September 12, 2021 10:16 AM
To: [email protected] <mailto:[email protected]>
Subject: [PSES] Creepage and clearance requirements
IEC60950-1:
How about the Creepage and clearance requirements for an AC driven radio HUB
device.
* One input: 230VAC (direct into wall socket)
* No physical output ports, just radio communication.
* Insulated plastic enclosure (UL94 V-0)
The Creepage and clearance requirements between primary and secondary circuits,
does it make any sense as long as the device has no cables and is encapsulated
by a plastic enclosure
I understand that there should be some Creepage and clearance to withstand OVC
II (250V transient).
Best regards Amund
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