Hi John:
> Rich, you said below "Assuming you are using IEC 60950, a fire
> enclosure is required for any circuit exceeding the limits of ELV,
> independent of available current or power."
>
> Why did you mention "exceeding the limits of ELV" ? Surely specific
> components require a fire enclosure, regardless of voltage ?
ALL components require a fire enclosure.
However, IEC 60950 has one alternative and one
exception to "ALL."
1. Base requirement:
Use of flame-retardant materials within a unit, AND
Use of a fire enclosure.
2. Alternative base requirement:
Use of extensive fault testing to prove a fire
cannot escape from the unit.
No need for a fire enclosure
3. Exception to fire enclosure (for components):
Specified wire insulations;
Motors that comply with Annex B;
Components supplied by a Limited Power Source provided:
-components are mounted on V-1 or better material;
-specified wire insulations;
Components in a 15 VA TNV circuit.
No need for a fire enclosure for these components.
A fire enclosure is required for all other components.
(For example, a power supply would require a fire
enclosure, but the low-voltage secondary circuits would
not require a fire enclosure. This is a fairly common
construction for low-power products, and for products
where the power can be divided into low-power circuits.)
There are THREE sets of criteria for a Limited Power Source:
1. Open-circuit voltage.
2. Short-circuit current.
3. Maximum VA.
If a low-power inverter is supplied by a Limited Power
Source, but the inverter increases the voltage to a value
exceeding the open-circuit voltage criteria, then the
inverter output, according to the Limited Power Source
criteria, is not Limited Power and therefore requires a
fire enclosure.
By definition, any voltage exceeding the VOLTAGE LIMITS of
a Limited Power Source (i.e., exceeding the limits of ELV)
does not meet the criteria of a Limited Power Source and
therefore requires a fire enclosure.
Because this is contrary to the principle of the conservation
of energy, and because power represents the heating energy of
electric energy, some certification houses consider such
circuits and the components in them to also be supplied from
a Limited Power Source.
Best regards,
Rich
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