Rich and group,
Just one interesting point continuing the thought of fuse sizes for
branch circuit protection. Awhile back, I was conducting a series of UL tests
on a household food mixer. For one of the abnormal tests, UL required me to
use a 30 A time-delay branch circuit fuse (lamp base) in the power supply to
the unit under test. I thought this was extreme, believing that a 20 A fuse
would be the maximum anyone would ever use. When questioned, I was told that
such fuses are available and still in use, primarily in old residential areas.
I visited my local hardware store and, sure enough, they were readily
available, right along with the other more common sizes. You may want to
consider this when considering such tests in the future.
Good day,
Richard Pittenger
Agency Approval Engineer
Hobart Corporation
Rich Nute <ri...@sdd.hp.com>
Sent by: owner-emc-p...@majordomo.ieee.org
01/28/2003 07:23 PM
Please respond to Rich Nute
To: pmerguerian2...@yahoo.com
cc: emc-p...@majordomo.ieee.org
Subject: Re: Circuit Breaker Tripping Dring Fault Tests
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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