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 
Sent: Tuesday, March 13, 2018 8:57 AM
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 
<<>> 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?


B M Vyas

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