Hi Peter:
> For safety, it is not clear from the standards whether
> the main branch circuit breaker tripping during fault
> conditions is an acceptable result.
>
> I see no reason why this should not be acceptable. What
> is your view? Some third party labs find it acceptable
> and others do not.
Some products are provided with internal overcurrent
protection and some are not.
Clearly, those that do not have internal overcurrent
protection rely on the branch circuit protection.
If a product has an internal overcurrent protective
device, and the fault is on the load side of that
device, then the internal device should provide the
protection and not the branch circuit device.
(Otherwise, the internal device provides no
protection, and might as well be removed.)
If the fault is on the supply side of the internal
device, then clearly the internal device cannot
provide protection, and the branch circuit must
provide the protection.
The real question is whether or not the product is
safe when the fault current is just below the
operating point of the branch circuit device.
Examining this question requires an understanding
of the fault and whether its resistance can be high
enough to not trip the branch circuit yet not create
a hazardous condition (such as a fire). If the
fault resistance always is no more than 120/20 = 6
ohms, then I would say that the branch circuit
could be relied upon to provide protection against
the fault.
Note that in the USA, a 120-volt branch circuit can
be provided with either a 15-amp or a 20-amp
overcurrent device. Therefore, the product must be
safe when the fault current is 20 amps, just below
the overcurrent device operating point. That means
that the product must be capable of dissipating
2400 watts without catching fire or destroying
internal insulation that serves a safety purpose.
Best regards,
Rich
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