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.
The opinions expressed are my own and do not necessarily reflect those of my
employer, TC 13 or TC 108.
From: Scott Aldous
Sent: Tuesday, March 13, 2018 8:57 AM
Subject: Re: [PSES] insulation between SELV and protective earthed accessible
part- IEC 62052-31
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
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?
B M Vyas
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