Overview

The EXP Bus 2 installed in N9103D is a solid state protection device that functions very much like circuit breakers. When a circuit becomes overloaded, the EXP Bus 2 “blows,” and the offending circuit safely shuts down. Unlike a traditional circuit breaker, the EXP Bus 2 circuit resets itself, unharmed, after the load is removed for several seconds. Unlike breakers, there are no contacts to weld "on." Once tripped, the EXP Bus 2 circuit will remain "off" until the original load is completely removed for several seconds. 

Benefits

• Solid state "fuses" eliminate breakers
• Overvoltage protection
• Dual bus option for backup battery

The ’Star’s EXP Bus 2 also includes an annunciator panel which provides a warning light for each circuit in the system. When a circuit fails, the light illuminates. A dark panel indicates no problems. It also provides Over and Under Voltage annunciators, an Amp-meter indicator, and a light dimmer circuit for the instrument lights. See Owner’s Manual for details.

Description

The EXP BUS 2 DC Load Center replaces the master solenoid, all but one circuit breaker, the master switch function, and the avionics master switch function. The avionics output bus provides six overload protected, power output circuits. Switched outputs include the alternator field output, anti-collision light output, navigation light output, aux fuel pump output, and two landing light outputs. All outputs are overload protected, eliminating the need for fuses or circuit breakers.

Circuit overload system

The EXP BUS 2 uses solid state current limiting devices known as PTC current limiters, and commercially by the trade name POLYSWITCH®. These devices have unique advantages, and important differences when compared to fuses and circuit breakers. Like a fuse, the PTC device “blows” when too much current is drawn by an offending circuit.However, like a circuit breaker, the PTC can be reset, and does not need to be replaced after one use. Unlike a circuit breaker, there are no moving parts in the PTC, the device is totally solid state. Also unlike either a fuse or a circuit breaker, the PTC resets automatically when the load is completely removed.

In the past, one could obtain “self resetting” thermal circuit breakers, however these were very undesirable for aircraft use. Old style self resetting breakers would reset themselves automatically after a short delay, even if the circuit were still overloaded. In the event of a serious electrical short, such as would be caused by a frayed or loose wire contacting chassis ground, this would likely cause a periodic arcing as the breaker reset and tripped. 

PTC breakers trip once, and stay tripped, until the load is completely removed. With a PTC breaker, a small, harmless amount of current “leaks”through when the devices are tripped. This current passing through the PTC device, keeps the breaker hot, thus keeping it tripped. The small current passed poses no risk of fire or damage to the wiring or avionics, but only serves to keep the device from resetting. 

To reset the PTC, simply remove the entire load, by turning off the radio, light, or accessory being powered by the device. After about ten to fifteen seconds, the PTC will cool down, reset, and be ready for use again. The PTC can be tripped thousands of times without damaging the device. If a circuit were to trip, there is no positive indication, other than the failure of the circuit in question. Many pilots feel that if any system fails in flight, the first action to be taken is to land, then investigate the problem once safely on the ground.