Jim,
You touch on an important issue concerning a fuse - just how does it "blow"?
Years ago I discovered by accident that fuses were designed with some
remarkable properties, when we had to make our own transient generator to
verify some telcom equipment's compliance to a BABT power supply transient
spec.
The BABT spec required that you simulate some very husky power transients.
It was like a short occurs in adjacent electronics followed by the inductive
kick. The -48 voltage would clamp to around 10 volts then "pop" up to over
300 volts capable of supplying 500A for something like more than 50mS. If
you didn't design your protection properly you would have a lot of
unintentional PCB trace fuses. [ Actually heard that the spec originated
because a workman had dropped his wrench across the 1 inch diameter rods
which supply the -48 to the telco building from the battery building. After
the wrench evaporated, they found the whole room of equipment was blown,
thus the spec. Somebody verify that? ]
The simulator used 4 deep discharge current vehicle batteries supplying the
telcom equipment through 50uH of inductance (that was 0000 cable on a
spool). Parallel to that you used a starter solenoid to short out a fuse
with a dead short. Amazingly the larger fuses never produced much kick
back. They were designed to blow gently away. Tried all kinds. Most of
the 8AG didn't do much, other types, nothing, even the 100 amp cartridge
types, nothing, The absolute best was a 1A 8AG type. When that went, you'd
get a flash of light, 300 volts trying to drive 500 amps into everything,
and even the coil would "jump" up off the floor.
Talk about PCB traces acting like fuses.
Anyway, I learned a respect for people who design fuses to make them go away
so gently when there is an incredible potential for some extremely high
voltage transients.
- Robert -
Robert A. Macy, PE [email protected]
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
619 North First St, San Jose, CA 95112
-----Original Message-----
From: Jim Bacher <[email protected]>
To: 'Cortland Richmond' <[email protected]>; Chris Maxwell
<[email protected]>; ieee pstc list <[email protected]>
List-Post: [email protected]
Date: Monday, February 25, 2002 3:04 PM
Subject: RE: Use of PCB Traces as Fuse and Voltage suppressor
>
>Long time ago we found that the traces worked well as fuses when the
>batteries were fully charged. However, when the batteries were mostly
>discharged, the PC Board traces did not work well as fuses. At lower
battery
>charge levels, the traces became very hot and ignited the PC Board rather
>than opening the traces up. I therefore would recommend against using PC
>Board traces as fuses.
>
>
>Jim
>
>Jim Bacher, Senior Engineer
>Paxar Corp.
>e-mail: [email protected] or [email protected]
>voice: 1-937-865-2020
>fax: 1-937-865-2048
>
>-----Original Message-----
>From: Cortland Richmond [mailto:[email protected]]
>Sent: Thursday, February 21, 2002 12:53 AM
>To: Chris Maxwell; ieee pstc list
>Subject: RE: Use of PCB Traces as Fuse and Voltage suppressor
>
>
>
>When do you need a fuse? Level II is the only time you are allowed to lose
>functionality, and the requirement for THAT is, it can't catch fire or
>explode. I've seen "trace fuses" tried. The problem comes after the trace
>blows. You are at the mercy of your board shop, and if you use a number of
>them, results might not be all that repeatable. AS i said earlier, I've
>had a board catch fire in my hand (though not as a result of stress, but a
>solder splash). It is instructive.
>
>Cortland
>
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