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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