Richard,

In EN50491-3 (HBES & BACS safety) ELV, SELV, PELV are defined I think the same way as you show. But except definition there are also Notes. I have only Polish translation so may be not precise writing what was in original. Under SELV definition there is Note 1: SELV circuit is separated from PELV circuits, other SELV systems and _ground_ with simple separation, and from other circuits with protective separation.
simple separation - using basic isolation
basic isolation - giving basic protection

I can imagine a kind of capacitor/resistor divider from AC to PELV with other resistor end connected to other circuit that is grounded. If this grounding fails the voltage at PELV can be higher, so it is PELV and not SELV. If we think about Gom resistances than even if PELV would be in such situation 200V but 1nA it should be no problem I think. So if it have to be lower value resistances than if PELV is connected by this to other circuit is it still other circuit or they are boith the one circuit.

I think It would help everyone if examples of situations in mind would be just written everywhere where PELV and SELV are defined.

Regards
Piotr Galka


W dniu 2017-11-23 o 02:08, Richard Nute pisze:
For definitions of ELV, SELV, and PELV, I turn to IEC TC64, Electrical 
installations and protection against electric shock, and its publication 61140, 
Protection against electric shock – Common aspects for installation and 
equipment. TC 64 is “to lay down basic safety requirements for protection 
against electric shock for use by technical committees.” IEC 61140 is “… a 
Basic Safety Publication intended for use by technical committees in the 
preparation of standards in accordance with the principles of IEC Guide 104 and 
ISO/IEC Guide 51.”

IEC 61140 defines ELV, SELV, and PELV:

3.26
extra-low-voltage (ELV)
any voltage not exceeding the relevant voltage limit specified in IEC 61201

3.26.1
SELV system
an electrical system in which the voltage cannot exceed ELV:
– under normal conditions; and
– under single-fault conditions, including earth faults in other circuits

3.26.2
PELV system
an electrical system in which the voltage cannot exceed ELV:
– under normal conditions, and
– under single-fault conditions, except earth faults in other circuits

Most product safety standards prohibit access to ELV on the assumption that ELV 
is not adequately separated from a higher voltage.

I admit that I don't understand the statements about earth faults in other 
circuits.  I don't know how an earth fault in another circuit can increase the 
voltage of SELV or PELV.

However, in my reading of 61140, I did not find any requirement that either 
SELV or PELV could or could not be connected to earth.

Rich



-----Original Message-----
From: John Woodgate [mailto:j...@woodjohn.uk]
Sent: Wednesday, November 22, 2017 2:52 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Safe powering 12V doorlock

The definition of PELV that I have is, I think, from IEC 60364, and it says 
'not electrically separated from earth', which I interpret as meaning 
'connected to earth but not necessarily with a connection that can withstand a 
large fault current'.

You must do as you see fit, but I think that if a potentially fatal fault, 
however improbable, could exist undetected, but could be neutralized by a 
simple earth connection, it would be good practice to provide the earth 
connection.

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

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