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Inactive, Master Electrician
Category: Electrical
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Experience:  Electrical Contractor
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On a Cessna 172 electrical system, what is the

Customer Question

On a Cessna 172 electrical system, what is the function of the 60 amp breaker (ALT PULL OFF) between the alternator and the primary bus and the 5 am breaker (ALT FIELD) between the bus and the master switch?
Submitted: 5 years ago.
Category: Electrical
Expert:  Inactive replied 5 years ago.
HiCustomer Thank you for asking your question on Just Answer. The other Experts and I are working on your answer. By the way, it would help us to know:

-Could you explain your situation a little more?

Thank you again for trusting us with your problem. Please reply as soon as possible so that we can finish answering your question.
Customer: replied 5 years ago.
I am studying the pilots operation hand book for the Cessna 172RG and the schematic shows the electrical circuit but I am not an electrician and don't understand the two circuits. One has a 5 amp breaker between the bus and the master switch and the other has a 60 amp breaker/off switch between the bus and the alternator.

Studying for commercial pilots exam.

Jack57
Expert:  Inactive replied 5 years ago.
Ironically im studying for my private pilots exam & instrument. Do you think you can scan that page and post it here? If not - I can tell you the purpose of breakers and the reason for the different amp ratings.
Customer: replied 5 years ago.
Don't have a scanner. I understand the breakers and why some have pull/off/breaker combinations to allow you to disconnect the particular accessory like the gear pump etc. Just not sure how to trace the current through the schematic and make sense out of the two alternator circuits.
Expert:  Inactive replied 5 years ago.
Ok - I know scematics are hard to follow, especially that type. But if you look closely, you will see where the lines cross over eachother etc. They look like they connect, but they dont. So if your following a wire and it does not have a dot on top of a cross over, then the wires are not physicaly connected. If it does, it is joined. Does this make sense? Is this how your diagrams are laid out?
Inactive, Master Electrician
Category: Electrical
Satisfied Customers: 4772
Experience: Electrical Contractor
Inactive and other Electrical Specialists are ready to help you
Customer: replied 5 years ago.
That part I understand. If you have a POH for a Cessna you may be able to see the electrical schematic. For a 1981 Skyhawk 172P its on page 7-25, Hawk 172XP page 7-27, and for a 1981 Cutlass 172RG its on page 7-30.

Is the 60 amp circuit from the alternator to bus the battery charging circuit and the 5 amp circuit from the bus to the master switch the way the alternator control unit gets it power that is required to regulate the alternator?
Expert:  Inactive replied 5 years ago.
They are there to protect the wiring from burning up in the event of an overload or short. That is the sole purpose of any breaker - regardless of whether its in a house, car or airplane, and in this case the 5amp is to protect the field windings that produce the voltage. So if there is an overload or short, the breaker will kick before any major damage can be done to either the alternator or any major component of the electrical system.
Customer: replied 5 years ago.
Sorry for the need for more clarification. Just not sure what function each circuit performs. I think I understand the alternator field circuit now. This is the electrical power circuit from the battery to the alternator control unit, no? The alternator power control unit is effectively the on/off switch for the alternator and monitors alternator output in case of a charging overload.

But still not sure what the higher amperage circuit between the alternator and the bus are for. The circuits from the alternator are marked F, G and B.
One side of the B goes through a capacitor to the G side of the alternator and then to the Alternator contol unit negative sense contact.

The F circuit from the alternator goes to the field contact of the Altnernator control unit.

The B circuit connects to the 60 amp breaker and which then goes to the bus and to the positive sense contact of the alternator control unit. My guess is that this is where the power, up to 60 amps gets from the alternator to the battery and/or the airplane electrical circuit as needed.

Jack
Expert:  Inactive replied 5 years ago.
Let me put it to you this way - if you dont have the 60amp breaker engaged, you will not get any power from the alternator to the rest of the electrical system or to re-charge the batteries.
Customer: replied 5 years ago.
Got it, thanks.


Jack
Expert:  Inactive replied 5 years ago.
Your very welcome. Good luck on your exam!!!

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