Circuit Breakers

uwochris

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Hey guys,

I understand that circuit breakers provide a form of protection against excessive current. Looking at electrical system diagrams of various a/c, I see that the circuit breakers have many different amp limits (i.e. 5 amp CB, 10 am CB, etc).

This may seem like a dumb question, but what is the significance of the amperage of the CB itself? For example, here is an exert from a Metro book:

" The pilot trim motor is supplied 28 volts DC through a 10 amp CB from the left essential bus."

I just don't understand what the significance of that "10 amps" is. Does it mean the CB will pop once it is 10 amps over the prescribed limit? Also, how do they choose what amperage of CB to use (in the above example, would a 5 amp CB do the same job?)

Thanks in advance,
Chris.
 

A Squared

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uwochris said:
Hey guys,

I understand that circuit breakers provide a form of protection against excessive current. Looking at electrical system diagrams of various a/c, I see that the circuit breakers have many different amp limits (i.e. 5 amp CB, 10 am CB, etc).

This may seem like a dumb question, but what is the significance of the amperage of the CB itself? For example, here is an exert from a Metro book:

" The pilot trim motor is supplied 28 volts DC through a 10 amp CB from the left essential bus."

I just don't understand what the significance of that "10 amps" is. Does it mean the CB will pop once it is 10 amps over the prescribed limit? Also, how do they choose what amperage of CB to use (in the above example, would a 5 amp CB do the same job?)

Thanks in advance,
Chris.

It's the amperagge at which the circuit breaker trips.

for example, if you have a piece of equipment which has a peak load of 7 amps, it might be powered through a 10 amp breaker (there's actually guidelines for how much extra capacity is required of hte breaker but I can't quote it off the top of my head) anyway, in normal; operation the there's only 7 amps flowing. Now if that equipment has a short circuit the current flow goes to 10 amps, then the breaker trips.
 

AdlerDriver

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uwochris said:
Hey guys,

I understand that circuit breakers provide a form of protection against excessive current. Looking at electrical system diagrams of various a/c, I see that the circuit breakers have many different amp limits (i.e. 5 amp CB, 10 am CB, etc).

This may seem like a dumb question, but what is the significance of the amperage of the CB itself? For example, here is an exert from a Metro book:

" The pilot trim motor is supplied 28 volts DC through a 10 amp CB from the left essential bus."

I just don't understand what the significance of that "10 amps" is. Does it mean the CB will pop once it is 10 amps over the prescribed limit? Also, how do they choose what amperage of CB to use (in the above example, would a 5 amp CB do the same job?)

Thanks in advance,
Chris.

I believe the 10 amps is the actual limit rating of the c/b itself. If you have a 10 amp c/b it will pop when the current in the circuit exceeds 10 amps - not 10 amps over some other limit.
 

Groucho

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From Glenco Aircraft Electricity and Electronics. A circuit breaker is a circuit protection device similar to that of a fuse. It differes from a fuse in that it can be reset. Circuit breakers and fuses should in all cases protect the wire in the circuit from overload and should be located as close as possible to the source bus.

Most circuit breakers are bimetalic thermal devices in that current in excess of the rated capacity will cause the bimetal leaf device to trip the breaker open. You will see button pushed out. The circuit breaker responds to heat caused by excessive current. Aviation breakers are designed to be trip free, that is the bimaetalic element must cool enough to allow the breaker to be reset.

Circuit breakers are designed to protect the wiring not necessarily the component they are connected to.

In the case of a trim motor in the SA-227 the 28V/10amp rating means that if the motor draws significantly more that 10amps for any period of time the breaker will pop out and the circuit will be open. In terms of power you can visualize the circuit breaker bimetallic strip as the heating element in a hair dryer or toaster. When it gets more than the equivalent of 280watts, that is 28Volts X 10amps of current it will become to hot and trip the breaker open.

I hpe this confuses you at a higher level.

"The Alps are a simple folk loving on a diet of old shoes. And the Lord Alps those who Alp themselves." ....... Groucho
 

mtrv

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And to add to everything else,

By using a 28 volt system, the amperage draw on each device is about half the rated amps of a 14 volt system. This allows for smaller wiring sizes which reduces total weight. However, 28 volt devices usually cost more than their 14 volt counterparts, which is why you still see 14 volt systems in many smaller aircraft.

And BTW, the battery is actually supplying 12+ or 24+ volts, but the alternator is putting out 14+ or 28+ in order to force current back into the battery for charging.
 

NoahWerka

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Flight Attendants regularly pop Circuit Breakers when the cabin temp gets too low. They are easy to spot sometimes. Other times they are not. Has nothing to do with amperage.
 
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Speedtree

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You can tell your high draw components by the amperage of their circuit breakers. If you lose electrical power and need to save your battery, look for the high amperage circuit breakers, (non essential ones) and look to pull those first to buy more time.
 

FN FAL

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uwochris said:
Hey guys,

I understand that circuit breakers provide a form of protection against excessive current. Looking at electrical system diagrams of various a/c, I see that the circuit breakers have many different amp limits (i.e. 5 amp CB, 10 am CB, etc).

This may seem like a dumb question, but what is the significance of the amperage of the CB itself? For example, here is an exert from a Metro book:

" The pilot trim motor is supplied 28 volts DC through a 10 amp CB from the left essential bus."

I just don't understand what the significance of that "10 amps" is. Does it mean the CB will pop once it is 10 amps over the prescribed limit? Also, how do they choose what amperage of CB to use (in the above example, would a 5 amp CB do the same job?)

Thanks in advance,
Chris.
When it comes to amperage the most important thing to remember is that amperage is "drawn". Draw too many amps...like in a short circuit...the circuit breaker trips.
 

SkyWestCRJPilot

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Another way to understand it is the high school physics equation V=IR. You can rearrange the equation to be V/R=I As R (resistance) drops the current (I) goes up. If you have a short circuit then your resistance drops to a very low amount. In that case your current (I) would go to a very high amount. That's a reason why you have circuit breakers, to protect against high current created from a short circuit.
 
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