Scott et al, You are correct in that there seems to be a split between
electrical appliances and electronic equipment and that the latter is more
Class II than the former, there is chatter about an issue that is growing among
class II equipments. All of the electronic equipment which has been the
primary user of SMPS for decades typically uses EMI filters to mitigate the
noise generated by the switching action which get fed back into the line.
These are used on Class I equipment where the noise is capacitively coupled to
earth/ground to be dissipated and keep much of this noise from feeding back
into the mains/line; likewise these filters are also used on class II equipment
for the same purpose – however the class II equipment does not have any
earth/ground sink in which to drain the unwanted signals. So, apparently, this
electrical charge is fed to the equipment chassis and the chassis voltage
builds up until it reaches some equilibrium value. The EMI folks discover this
when they find that the equipment discharges to the probe before they are ready
to induce a charge into the equipment. The voltage developed on the
equipment seems to be a hi value but limited charge (due to the limited
capacitance of the chassis to absorb it).
Altho there are not yet specific numbers, this doesn’t seem to
be a safety hazard at this point. However, it is also unknown as to how this
affects the filtering of the mains noise which was the desired result. The EMC
lab techs don’t like the equipment to zap them first rather than the other way
around.
Does anyone on this thread know of a paper on this which would
contain some specific results? Or of some researchers who are chasing this?
Or have personal experience with this. It would sure be nice to get some
feedback on this.
This is a great opportunity to expand our experience and provide
a basic understanding as to the efficacy of this process in both equipment
applications for both safety and EMC.
:>) 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
<mailto:[email protected]> [email protected]
From: Richard Nute <[email protected]>
Sent: Tuesday, January 14, 2020 12:58 PM
To: [email protected]
Subject: Re: [PSES] Class I vs Class II safety constructions
Hi Josh:
Yes, you are correct. Both of these standards specify the equipment be Class
I.
60335-2-38 applies to commercial-use griddles. 60335-2-75 applies to
commercial food or drink dispensing equipment.
Thanks, and best regards,
Rich
From: Wiseman, Joshua <[email protected]
<mailto:[email protected]> >
Sent: Tuesday, January 14, 2020 11:59 AM
To: [email protected] <mailto:[email protected]> ; [email protected]
<mailto:[email protected]>
Subject: RE: [PSES] Class I vs Class II safety constructions
It’s been a few years, but I seem to recall there was a particular IEC/EN
60335-2-xx standard that required tubular sheathed heaters to be grounded.
This would force PE to be brought in.
Additionally, IEC 60335-2-38 and IEC 60335-2-75 have requirements for
Equipotential grounding. If I remember correctly IEC 60335-2-75 does not allow
Class II products.
Josh
Joshua Wiseman
Systems Engineering
Staff Engineer, Product Safety/EMC
Ortho Clinical Diagnostics
From: Richard Nute <[email protected] <mailto:[email protected]> >
Sent: Tuesday, January 14, 2020 2:38 PM
To: [email protected] <mailto:[email protected]>
Subject: Re: [PSES] Class I vs Class II safety constructions
Hi Scott:
I have seen no safety standards or codes that specify which products must be
Class I and which products must be Class II, except in the USA washers and
dryers must be Class I. As far as I know, the decision is that of the
manufacturer. I have been associated with a manufacturer who has made the same
product both ways. In my case, one of the factors in deciding Class I or Class
II was cost (e.g., a 3-wire cord was more expensive than a 2-wire cord).
I suspect a major factor is “momentum” of the manufacturer: we made it this way
last time, and we know how to do it this way.
A product with a grounding (3-wire) power cord is a Class I product regardless
whether it has no accessible conductive parts. Unlike a Class II product, a
Class I product does not bear a marking attesting that it is Class I.
Note that a Class I construction necessarily includes Class II construction,
e.g., appliance inlet which is all-insulated. We ignore the Class II
construction portions of a Class I product.
I checked our electric kettle (which has accessible metal) and electric
coffee-maker (which has the heater plate accessible metal). Both are 2-wire.
Neither has the double-insulated symbol. Both are UL-certified.
Best regards from beautiful snowy Bend, Oregon, USA,
Rich
From: Scott Xe <[email protected] <mailto:[email protected]> >
Sent: Tuesday, January 14, 2020 6:59 AM
To: [email protected] <mailto:[email protected]>
Subject: [PSES] Class I vs Class II safety constructions
In terms of safety level, both constructions are given the equivalent
protection against electric shock. In electrical appliances, Class I is used
most whereas Class II is employed in most electronic products. Is there any
background for such design route?
In some cases such as induction cookers, the enclosure is plastic/glass - no
any internal metal part exposes to the outside surfaces. The product is not
marked with a double square symbol and comes with a 3-pin plug. Why is this
type of product not classified as Class II rather than Class I with the
plastic/glass enclosure?
Thanks and regards,
Scott
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