Hi Rich,
Thank you for devoting so much time to reply. This is just what I needed.
I am familiar with IEC 60950 and I wanted to be able to compare the
definitions. Thank you.
One other question to Rich or anyone else that can help.
Is IEC 536 the base document for the definition of SELV? In addition to
how SELV is expressed in IEC 364 I am also trying to confirm the base
standard in the IEC responsible for the SELV definition. Is it IEC 536 or
some other document? In which standard or guide is the term SELV spelt out
(where it says that "SELV" means "Safety Extra-Low Voltage")? There must
be a base IEC document somewhere that defines the term.
Best regards,
Kevin
>
> Hi Kevin:
>
>
> > Can anyone please provide the SELV definition from IEC 364?
>
> I'm working from some old documents, but...
>
> SELV is not expressly defined in IEC 364-4-41. Instead,
> it specifies "Protection by safety extra-low voltage."
>
> It says:
>
> "Protection against electric shock in normal service
> and in case of a fault is deemed to be ensured when:
>
> - the nominal voltage cannot exceed the upper limit
> of Voltage Band I*,
>
> - the supply is from one of the safety sources listed
> in Sub-clause 411.1.2, and
>
> - the conditions of Sub-clause 411.1.3 are fulfilled."
>
> "*See IEC Publication 449: Voltage Bands for Electrical
> Installations of Buildings."
>
> The safety sources are:
>
> - a safety isolating transformer,
>
> - a source of current providing a degree of safety
> equivalent to that of the safety isolating transformer
> (e.g., motor generators with windings providing
> equivalent isolation),
>
> - an electrochemical source (e.g., a battery) or another
> source independent of a higher-voltage circuit (e.g., a
> diesel-driven generator,
>
> - certain electronic devices complying with appropriate
> standards where measure have been taken to ensure that,
> even in the case of aninternal fault, the voltage at the
> outgoing termainals cannot exceed the values specified in
> Sub-clause 411.1.1.
>
> The definition of SELV appears in IEC 536:
>
> "A voltage which does not exceed 50 V ac rms between
> conductors, or between any conductor and earth, in a
> circuit which is isolated from the supply mains by means
> such as a safety isolating transformer or converter with
> separate windings."
>
> IEC 950 defines SELV CIRCUIT:
>
> "A secondary circuit which is so designed and protected
> that under normal and single fault conditions, its
> voltages do not exceed a safe value."
>
>
> SELV is a special case of ELV, Extra Low Voltage. ELV is
> defined (or implied) as a maximum voltage value deemed not to
> cause an electric shock. SELV applies to an ELV where, in
> the absence of specific protective mechanisms, the ELV would
> exceed ELV under fault conditions. So, SELV implies ELV with
> some means to prevent the voltage from exceeding ELV limits
> in the event of a fault.
>
> A single-insulated transformer ELV secondary is ELV.
>
> A double-insulated transformer ELV secondary is SELV.
>
> An ELV battery is ELV. It may or may not be SELV, depending
> on your point of view.
>
> 1: Since the battery itself cannot exceed ELV under single-
> fault conditions, then it cannot be SELV. Or,
>
> 2: Since the battery itself cannot exceed ELV cannot exceed
> ELV under single-fault conditions, then it must be SELV.
>
> However, as defined, SELV implies an included protective
> mechanism to limit the voltage in the event of a fault. SELV
> also implies that the ELV is derived from a higher, non-ELV
> source.
>
> If you accept that a battery is ELV, then the safety standards
> require that the battery voltages cannot be touched!
>
>
> Best regards,
> Rich
>
>
>
>
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Best regards,
Kevin Richardson
Stanimore Pty Limited
(Technology Products Standards, Regulations and Compliance Specialists)
Ph: 02-4329-4070 Fax: 02-4328-5639
Int'l: +61-2-44329-4070 +61-2-4328-5639
Email: k...@compuserve.com (Internet)
kevin.richard...@ieee.org (Alternate Internet #1)
k...@technologist.com (Alternate Internet #2)
100356,374 (Compuserve)
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