First I am not a safety engineer, but I have met a few.
Insulation specs that I have run across in pcb layouts, creepage and clearance,
etc, such as between layers, or if the layers are like wires or other parts,
there is, or was, mentioned in an obscure passage ( I paraphrase) that air and
thus distance can be insulation. Other materials can be used as well. But !
Statistically all materials have unintended holes thru them, sometimes called
holidays, thru which the creepage and clearance requirements might not be met.
So two layers could be used which reduced the odds of two holes being aligned
up are greatly reduced.
And thus three layers worked out statistically that it would be good enough,
regardless of the thickness.
(single point failure stuff.)
Now we have triple coated wire, often used in transformers.
And three layers of insulation between copper in pcbs, as needed.
Transformers make things interesting in that the 2D considerations are now
bunched up into a 3D situation.
Now all situations are not pcbs or bundles of wires in a cable where each wire
is insulated, thus wire to wire has double the insulation, and if the cable is
shielded, double the insulation from wire to shield.
I will try a summary.
One layer of insulation if it meets the creepage and clearance specs assuming
there is an unintended hole thru the insulation.
Two layers of insulation assures the single point failure of holes is covered,
"reinforced", and the distance is maintained.
Three layers of insulation and the discussion is over, or not.
ps. The creepage and clearance specs already have a safety margin built in, so
more is not needed, but is okay to use. The mechanical engineer that know of
spacing requirements, and asks the safety engineer " is 8 mm good enough?"
Safety answers that yes that is good. Mechanical turns to dept and says 8 mm
is required, so the chassis needs to get bigger.And the three ring circus comes
to town !
On Thursday, February 24, 2022, 03:19:35 PM EST, Joe Randolph
<[email protected]> wrote:
Hi Amund:
In 60950-1 (and I believe similarly in 62368-1), there is no specific
requirement to have two layers of insulation. A single layer is okay to use if
it meets the performance requirements of so-called “double insulation.” The
term “reinforced insulation” is used in 60950-1 to refer to this distinction.
I’m short on time today, but I’ll try to elaborate further later.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com
From: Amund Westin [mailto:[email protected]]
Sent: Thursday, February 24, 2022 1:27 PM
To: [email protected]
Subject: [PSES] SV: [PSES] SV: [PSES] Numbers of insulation layers in
transformer (IEC62368)
A Class II or double insulated electrical appliance (bear the double insulation
symbol), is it required to have a transformer with 2 insulating layers? Or is
one layer in transformer ok, if the plastic enclosure is giving enough
insulation, thus in total you will have 2 layers of insulation?
Ref. 62368-1 product (AC powered WiFi HUB). No cabling/wiring on secondary side.
Cheers!
BR
Amund
Fra: Joe Randolph
Sendt: 18. februar 2022 05:09
Til: [email protected]
Emne: Re: [PSES] SV: [PSES] Numbers of insulation layers in transformer
(IEC62368)
Hi Amund:
In general, a safety lab does not spend time thinking about hypothetical
scenarios. They usually focus on simply complying with the requirements of the
applicable safety standard, which in this case is 62368-1.
I’m still relatively new at using 62368-1, but I have lots of experience using
60950-1. While there are differences in how the two standards approach the
issue of safety for 230 Vac mains, I think the underlying philosophy is the
same:
1) 230 Vac mains should not be “accessible” to ordinary users
2) 230 Vac mains should not become “accessible” without a double-fault
condition
There are lots of ways to meet these criteria. Key to both of the above
criteria is the definition of “accessible.” The method for making sure that a
230 Vac circuit is not accessible is to carefully apply the definition of
“accessible” in the standard. Safety standards typically define “accessible”
with various criteria including various specialized probes that simulate a
human finger, along with various criteria regarding anticipated use.
I’m not sure how 62368-1 defines accessibility, but in 60950-1, a 230 Vac
circuit is considered to be accessible if it can be touched by the specified
probes without the use of a “tool” (such as a screwdriver). Further, even if a
“tool” is required to be able to touch a 230 Vac circuit, that circuit is
considered to be accessible if the user is instructed to use a tool to access
the area, such as a battery compartment.
I don’t know the details of the removable part on the product in question, but
perhaps the above considerations will help you evaluate the available options.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com
From: Amund Westin [mailto:[email protected]]
Sent: Thursday, February 17, 2022 2:57 AM
To: [email protected]
Subject: [PSES] SV: [PSES] Numbers of insulation layers in transformer
(IEC62368)
Thanks, guys!
I have not received a fully explanation from the safety lab, but I assume they
are concerned about this:
- With only one insulation layer between Prim/Sec winding in the
transformer, a fault in the insulation tape will result in 230VAC in the
secondary circuits. OK, but there are no cable interfaces on the secondary side
and no conductive parts. The entire enclosure is flame category V-1 or V-0.
But one part of the enclosure is a little piece that is pressed onto the main
part of the enclosure. This little part can be demounted, and since the product
is plugged into a 230VAC socket that is located 20 cm above the floor, a child
could manage to remove this little part and access the secondary circuit (now
230VAC) by its fingers.
I guess this is the reason for requiring two layers in the transformer.
Or make it impossible to remove this part which can give access to secondary
circuits.
Best regards
Amund
Fra: Richard Nute
Sendt: 16. februar 2022 23:09
Til: 'Amund Westin' <[email protected]>; [email protected]
Emne: RE: [PSES] Numbers of insulation layers in transformer (IEC62368)
Hi Amund:
Since the unit does not have grounding/earthing, it is therefore
double-insulated with respect to the input 230 volts (and must bear the
double-insulated symbol).
This requires all accessible conductive parts and accessible surfaces to be
double-insulated from the 230 volts.
If the secondary circuit has any accessible conductive parts (e.g., metal
antenna), then the secondary must be double-insulated from the 230 volts:
transformer must have two insulation layers, basic and supplementary. (This is
the most common construction.)
If the secondary has no accessible conductive parts, then the insulation
between the secondary and accessible surfaces (whether or not conductive) is
“supplementary insulation”: transformer can have one insulation layer, basic.
(This is uncommon construction and will likely be not acceptable to many
certification houses.)
Good luck, and best regards,
Rich
From: Amund Westin <[email protected]>
Sent: Wednesday, February 16, 2022 1:02 PM
To: [email protected]
Subject: [PSES] Numbers of insulation layers in transformer (IEC62368)
Do I need one or two insulation layers (tape) between the primary and secondary
windings?
Can find an absolute answer in the standard IEC62368.
Product:
Input 230VAC
Secondary 24VDC for other Low volt circuits
No grounding /earthing
No cable outputs
Plastic enclosure
Radio HUB product
Thanks.
Best regards
Amund
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