From: Gregg Kervill
Sent: Tuesday, January 28, 2003 3:21 PM
> Like most regulatory issues the answer is YES and NO.
>
> Therefore it is dangerous and extremely misleading (to
many
> lurkers) to apply a general answer to all conditions:
>
> Generally the equipment is expected to protect itself with
> internal over current and short-circuit protection.
I disagree. It may be possible for equipment to possess
inherent features that do not require supplementary
protection or that a piece of equipment is adequately
protected by the branch circuit protection. Much depends on
the application and design.
> The equipment will be tested with the worst case fault
> condition – 60 kAmp or more is not uncommon
This value may be true for equipment installed in Category
IV and possibly Category III environments (per IEC60664-1),
but is neither the general case nor very common for
equipment installed in Category II environments. Consider
that every finite (though minute) impedance at every
electrical connection will serve to limit the fault current
available at equipment. Impedances will also exist at each
switchable contact (molded case switches and circuit
breakers) and each wiping contact (fuse holders for
replaceable cartridge fuses, outlets/plugs, knife switches).
For North America:
In a typical household operating at 120V, it's unlikely
that a fault current available to Pluggable Equipment Type A
will be much above 5kA, even if the outlet supplying the
equipment is within 5 ft. of the service entrance. In a
typical household operating at 240V, the available fault
current will be somewhat higher (approximately doubled +).
This is also, in large measure, due to the relatively small
last distribution transformer before the household and other
loads on the transformer from other households supplied by
the same transformer.
In a typical business the fault current available will be
larger, due largely to a larger transformer connected to the
service entrance and that the service entrance voltages are
greater (480V three phase is not uncommon for larger
facilities; 120/208V is most common). I'm not certain of
the typical fault currents available at an outlet in this
case, but I'd venture a guess that its not more than 25kA at
a ANSI/NEMA 5-15 outlet.
I'm interested in hearing typical values for other regions
of the world, so if anyone has any data or reasonable
guesses, please let us all know.
> Where we rely upon the ‘breaker’ for - non-domestic
> equipment - it is ALWAYS mandatory (and common sense)
> to specify the characteristics of that breaker in
> terms of ‘tripping (operating) current’ – time
> characteristics (Type I, II or III) and Breaking
> Current 2,500 Amps is low for most domestic situations.
This must be for Europe. In the US (and I believe Canada is
the same), 5kA is a gimme interrupting rating for molded
case switches and circuit breakers. The minimum
interrupting rating for branch circuit fuses is 10kA.
> A failure to provide the necessary information
> WILL eventually result in a fire or nuisance tripping.
Or not, depending on the installation environment.
> Best regards
> Gregg
Regards,
Peter L. Tarver, PE
Product Safety Manager
Sanmina-SCI Homologation Services
San Jose, CA
peter.tar...@sanmina-sci.com
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