APSH-3M

APSH-3M automatic bus transfer unit for aircraft power systems

The APSH-3M is a three-phase automatic bus transfer unit used in aircraft electrical power distribution systems. Its main task is to monitor the condition of the primary AC supply line and issue a switching command when the network voltage disappears or drops below the permitted level. The unit does not generate power and does not replace a contactor. It works as a control and protection device that tells the switching circuit when the load has to be transferred to a standby bus or reserve source.

Power transfer logic through APSH-3M

In normal operation, the APSH-3M monitors a three-phase AC network and keeps the connected consumers on the primary supply path. If the monitored voltage fails in one or more phases, or if the line voltage falls to an unsafe level, the unit sends a DC control signal to the load transfer contactor. This allows connected aircraft consumers, including navigation, flight control, and avionics equipment, to remain powered from a reserve source. The exact switching path depends on the aircraft power distribution diagram.

Aircraft systems controlled by the APSH-3M unit

The APSH-3M is used in aircraft where automatic transfer between power buses is required. It is associated with Soviet and Ukrainian/Russian aircraft electrical systems and appears in applications for An-124, An-140, An-148, An-225, An-70, An-72, An-74, Ka-32, Mi-26, and Mi-8 type platforms. Actual installation should still be checked against the aircraft parts catalog, because bus layout, contactor type, controlled consumers, and reserve supply logic may differ by aircraft model and modification.

Network monitoring and protection with APSH-3M

The unit reacts to loss or critical reduction of line voltage in the monitored three-phase system. In the aircraft electrical system, this prevents important consumers from remaining connected to a failed or unstable bus. The APSH-3M gives a switching command, while the actual power transfer is carried out by the connected contactor equipment. During troubleshooting, the unit should be checked together with the monitored AC line, DC control supply, switching contactor, wiring, and load bus. A transfer fault may come from any of these elements.

Identification and ordering checks for APSH-3M

Before ordering, the marking on the installed unit should be compared with the aircraft electrical documentation, wiring diagram, or parts catalog. The designation APSH-3M refers to an aviation three-phase bus transfer automatic unit and should not be confused with non-aviation equipment using similar Cyrillic abbreviations. In English-language requests it may be written as APSH-3M. In Russian documentation the correct designation is АПШ-3М. The product is also known under alternative designation АПШ-3М.

Product Type: Three-phase automatic bus transfer unit
Function: Automatic transfer command for aircraft power buses under voltage failure or abnormal voltage conditions
Application / Where Used: Aircraft electrical power distribution and protection systems
System / Installation Area: AC power bus monitoring / load transfer control circuit
Compatible Aircraft / Equipment: An-124, An-140, An-148, An-225, An-70, An-72, An-74, Ka-32, Mi-26, Mi-8, subject to aircraft documentation
Monitored Network: Three-phase AC network
Monitored Network Frequency: 400 ± 20 Hz
Monitored Line Voltage: 200 V +7 / −13 V
Monitored Phase Voltage: 36 V +1.8 / −3.6 V
DC Control Circuit Supply Voltage: 27 V +2.4 / −3 V
Current Consumption at Normal Monitored Voltage: Not more than 40 mA
Current Consumption During Voltage Failure in One, Two, or Three Phases: Not more than 100 mA
Switching Signal: +27 V DC command to load transfer contactor
Trigger Condition: Loss or critical reduction of monitored AC voltage
Weight: Not more than 0.4 kg
Dimensions: 155 × 110 × 73 mm
Technical Documentation: APSH-3M technical documentation / aircraft electrical system documentation

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