Airpax calls some models "instant," don't they?
----- Original Message -----
From: "art davis" <n4uc@hotmail.com>
To: "TenTec reflector" <tentec@contesting.com>
Sent: Tuesday, December 14, 2010 1:24 AM
Subject: Re: [TenTec] Airpax Breaker Part and Source
I'm no expert, BUT...
be very careful with the use of the term "instant trip" when referring to
a circuit breaker or fuse. All circuit protection devices take a finite
amount of time to operate, no matter what type they happen to be. Look at
the response curves (i.e. trip curves) for any breaker or fuse and you'll
see that it may be very quick indeed (less than a millisecond in many
cases) if the fault current is high enough, but not "instant". And in the
world of semiconductors very quick might not be fast enough. And look at
the curves to see the level of current that can flow through the device
without it tripping at all. You might be surprised. I may be wrong but I
doubt that there is any circuit breaker on the market that, when located
upstream at the power supply, can operate fast enough to protect
semiconductors from damage due strictly to overcurrent. If the fault
current is high enough (many times the circuit breaker rating) they may
offer some limited protection from the heat generated
by the overcurrent flowing through the device, but the semiconductor
itself must be capable of surviving the current in the first place.
The primary reason to specify different response time characteristics
(trip curves) for circuit breakers (slow blow, fast, etc...) is to make
sure that the power feeder circuit maintains proper trip coordination
among the various circuit protection devices in the circuit in case of a
fault in the wiring upstream of the load. In most power distribution
systems the fuses and circuit breakers upstream of an end user (i.e.
radio) are there only to protect the wiring between the power source and
the load. The load (in this case, the radio) must protect itself
internally. My guess is that there could be significant damage done
inside a transceiver long before a 20A breaker all the way back at the
power supply knew what was happening.
Of course, the flip side to all this is that it couldn't hurt, right? You
certainly won't have any protection if you don't use something, so put the
fastest breaker in there that you can find and keep your fingers crossed!
Just don't call it "instant"
Art, N4UC
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