I agree:  SELV as a low-voltage earth-free supply is not actually a safe concept. It's OK for short connections in the same space, because the probability of 'power cross' is extremely remote. But if SELV is extended into several spaces, including, for example, a void, the chance of power-cross is larger and it will remain *undetected* until someone touches both the SELV and earth.

John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK

On 2018-03-13 19:13, Richard Nute wrote:

Dear Mr. Vyas:

I suggest you review the definitions for SELV and PELV in IEC 62052.  These definitions are not the same throughout IEC standards.  (I don’t have IEC 62052.)

In some standards, SELV is isolated from protective earth, and PELV is connected to protective earth.  If these are the definitions in IEC 62052, then SELV must be isolated from earth by Basic Insulation.  (I have never understood why we have both SELV and PELV as only the voltage and the isolation are the essential parameters for protection against electric shock.)

In other standards, SELV is determined by the voltage and the isolation from higher voltages; earthing (or not) of SELV does not enter into this determination.

Good luck and best regards,

Rich

*From:* Balmukund Vyas <balmukund.v...@ymllabs.com>
*Sent:* Tuesday, March 13, 2018 5:56 AM
*To:* EMC-PSTC@LISTSERV.IEEE.ORG
*Subject:* [PSES] insulation between SELV and protective earthed accessible part- IEC 62052-31

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

Table 20 – Insulation requirements between circuits and between circuits and accessible parts

        

HLV
mains-circuit^1)

        

ELV circuit

        

SELV circuit

        

PELV circuit

        

PEB circuit^2)

        

Protective earthed HLV non-mains-circuit^2), 3)

        

Unearthed HLV non-mains circuit^3)

HLV mains-circuit ^1)

        

F/B^1) 6)

Table 8Table 9

        

B

Table 8Table 9

        

D

Table 8Table 9

        

D

Table 8Table 9

        

B^5)

Table 8Table 9

        

B

Table 8Table 9

        

B

Table 8Table 9

ELV circuit

        

B

Table 8Table 9

        

F/B^6)

Table 13Table 14

        

B

Table 13Table 14

        

B

Table 13Table 14

        

F/B^5), 6)

Table 13Table 14

        

B

Table 13Table 14

        

B

Table 13Table 14

SELV circuit

        

D

Table 8Table 9

        

B

Table 13Table 14

        

F/B^6)

Table 13Table 14

        

F/B^8)

Table 13Table 14

        

B

Table 13Table 14

        

D

Table 13Table 14

        

D

Table 13Table 14

PELV circuit^2)

        

D

Table 8Table 9

        

B

Table 13Table 14

        

F/B^8)

Table 13Table 14

        

F/B^6)

Table 13Table 14

        

B

Table 13Table 14

        

D

Table 13Table 14

        

D

Table 13Table 14

PEB circuit^2)

        

B^5)

Table 8Table 9

        

F/B^5), 6)

Table 13Table 14

        

B

Table 13Table 14

        

B

Table 13Table 14

        

F/B^5)

Table 13Table 14

        

B

Table 13Table 14

        

B

Table 13Table 14

Protective earthed HLV non-mains circuit^2), 3)

        

B

Table 8Table 9

        

B

Table 8Table 9

        

D

Table 8Table 9

        

D

Table 8Table 9

        

B

Table 8Table 9

        

F/B^6)

Table 8Table 9

        

B

Table 8Table 9

Unearthed HLV non-mains circuit^3)

        

B

Table 13Table 14

        

B

Table 13Table 14

        

D

Table 13Table 14

        

D

Table 13Table 14

        

B

Table 13Table 14

        

B

Table 13Table 14

        

F/B^6)

Table 13Table 14

Protective earthed accessible part^2), 7)

        

B

Table 8Table 9

        

F/B^6)

Table 13Table 14

        

B

Table 13Table 14

        

B

Table 13Table 14

        

F/B^6)

Table 13Table 14

        

B

Table 13Table 14

        

B

Table 13Table 14

Unearthed accessible part^7)

        

D

Table 8Table 9

        

B

Table 13Table 14

        

B

Table 13Table 14

        

F/B^6)

Table 13Table 14

        

B

Table 13Table 14

        

D

Table 13Table 14

        

B/D ^4)

Table 13Table 14

Abbreviations for insulations:

B: Basic insulation or supplementary insulation

D: double insulation or reinforced insulation

F: Functional insulation

S: Supplementary insulation

1)If the functional voltage (not relative to earth/ground) is greater than the rated insulation voltage, the creepage distance for the functional insulation may be greater than that for the basic insulation. An example is a terminal block of a three-phase metering equipment with /U/_n = 230/400 V, where the functional phase-to-phase voltage is 400 V r.m.s. For an indoor meter with material group III, the creepage distance for basic insulation from Table 9 is 3,2 mm, but the creepage distance for functional insulationfrom Table 14 is 4,0 mm.

2)Connections to the protective conductor shall comply with 6.5.2.4. Otherwise, this shall be considered to be an unearthed circuit.

3)There shall be at least basic insulation between HLV non-mains circuits and HLV mains circuits.

4)Insulation between an unearthed non-mains circuit at hazardous voltage and an unearthed accessible conductive part shall satisfy the more onerous of the following:

-double/reinforced insulation, the working voltage of which is equal to the hazardous voltage; or

-supplementary insulation, the working voltage of which is equal to the voltage between the non-mains circuit athazardous voltage; and

·another non-mains circuit at hazardous voltage; or

·a mains circuit.

5)See Annex Bfor the conditional use of basic insulation for PEB.

6)Supplementary or basic insulation shall be used if one of the circuits is an independent circuit or is adjacent to a conductive part which may be earthed when the equipment is installed.

7)A functional earthed circuit shall be treated as an unearthed accessible part. The exception is where the functional earth is bonded to the protective conductor and this meets the relevant requirements, then it may be treated as an earthed accessible part.

8)For a PELV system earthed at one end of a cable run, it would be permissible to connect a SELV system at the other end.

NOTE Reference is made to the respective tables specifying clearances and creepage distances.

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