I suspect that the SELV circuits of an electricity meter never leave the
enclosure, so SELV is OK, but PELV would still be equally safe or
better, even if a very low probability open-circuit earth occurred.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK
On 2018-03-13 19:17, Ted Eckert wrote:
Part of the reason why two levels of protection are required is
because of what happens if one level fails. If the protective earth
bonding connection fails, and the exposed metal is no longer earthed,
the product will typically continue to operate normally. There is no
indication to the user that a failure has occurred. If a second
failure occurs that bridges the insulation gap between hazardous
voltage and that exposed metal, the chassis will become energized. The
user may still have no obvious indication of a failure until they
contact that energized chassis.
Different standards committees have different ways of addressing this
issue. TC 108 generally requires that the protective earth bonding be
designed and tested in such a way to prove that it is robust enough.
In many ITE products, the user accessible circuits are earth ground
references, so providing basic insulation between SELV and earth
ground is not possible. Other standards committees may have chosen
other methods of protection based on the products they cover. TC 13
may have decided that basic insulation between SELV and earthed parts
may be the best way to provide protection in their products. In this
case, if the earth bonding fails, the basic insulation may be need to
reduce the risk or arcing between hazardous voltage and the exposed metal.
Ted Eckert
Microsoft Corporation
The opinions expressed are my own and do not necessarily reflect those
of my employer, TC 13 or TC 108.
*From:* Scott Aldous <00000220f70c299a-dmarc-requ...@ieee.org
<mailto:00000220f70c299a-dmarc-requ...@ieee.org>>
*Sent:* Tuesday, March 13, 2018 8:57 AM
*To:* EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
*Subject:* Re: [PSES] insulation between SELV and protective earthed
accessible part- IEC 62052-31
Hi Vyas,
I'm not familiar with this standard, but the general principle is that
2 levels of protection are required to protect operators from hazards.
In the case of the "protective earthed accessible part", I imagine the
standard assumes that a single fault could compromise the protective
earthing, so Basic or Supplementary Insulation is also required
between such a circuit and SELV. In the same vein, the table requires
Double or Reinforced Insulation between an "unearthed accessible part"
and SELV.
On Tue, Mar 13, 2018 at 5:56 AM, Balmukund Vyas
<balmukund.v...@ymllabs.com <mailto:balmukund.v...@ymllabs.com>> wrote:
Dear All,
IEC 62052-31 is standard for product safety requirements for
electricity metering equipment. It has a table 20 (given below)
which details out insulation requirements between various types of
circuits.
My question is, why Basic insulation is required between SELV
circuit and protective earthed accessible parts? Isn’t a
functional insulation is sufficient for this?
Thanks
B M Vyas
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