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jgamtmann2(at)gmail.com
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 Posted: Wed Jul 18, 2012 7:13 am Post subject: Qualifying CB's |
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Bob, I have a question: How can a CB be qualified at currents many times (3000A) it's rated breaker current? Is this a calculated current or real measured current? Does the breaker not trip a long time before this type of overcurrent is reached? What happens to the contacts (they must arc) during this type of overcurrent, or, if I imagine (I have no access to them) the MIL specs correctly, does the CB go completely undamaged?
Do not archive.
Regards, Jurgen Amtmann
[quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jul 18, 2012 6:25 pm Post subject: Qualifying CB's |
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At 10:12 AM 7/18/2012, you wrote:
| Quote: | Bob, I have a question: How can a CB be qualified at currents many
times (3000A) it's rated breaker current? Is this a calculated
current or real measured current? Does the breaker not trip a long
time before this type of overcurrent is reached? What happens to
the contacts (they must arc) during this type of overcurrent, or, if
I imagine (I have no access to them) the MIL specs correctly, does
the CB go completely undamaged?
|
When you connect a breaker to a bus for the purpose
of protecting a downstream wire, there is also a practical
limit on how long a breaker can sustain current flow
at what ever the bus will deliver while it's deciding
to trip.
For example, a 28v aircraft battery can easily deliver
2000 to 4000 amps of current if you hard-fault the
battery bus to ground. Admittedly, the little ol'
5A breaker trips VERY fast . . . on the order of 1 to
2 milliseconds. The point of this test is to insure
that devices crafted to protect say 22AWG wires
from burning are not themselves at risk for damage
by what ever current the energy source can deliver.
This is why all breakers have two classes of current
rating (1) trip current for downstream protection
and (2) fault/rupture/interruption current ratings
where the breaker will not be damaged during the
time between onset and clearing of fault.
The third breaker featured in the ShopNotes I cited
was a good example of a 5A breaker that had perhaps
a 100A or less fault current rating. It may be
attractive to some system designers but it's certainly
not something you'd find in a TC aircraft.
Bob . . .
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