BCHF-208

BChF-208 phase sequence unit in the aircraft power system

The BChF-208 is an aircraft phase sequence control unit used in three phase onboard AC power circuits. Its task is simple but critical for the electrical system: it checks whether the connected power source has the correct phase order before the source is allowed into the aircraft network. This applies to both ground power and onboard power sources, depending on the aircraft wiring. If the phase sequence is wrong, the unit keeps the control circuit open and prevents the contactor from switching the source into the system.

Power source protection through the BChF-208

The unit monitors the phase rotation of the three phase AC supply. When the sequence is correct, the internal circuit creates the voltage condition needed for relay operation. The relay then closes its control contacts and allows the 27 V DC command circuit to operate the ground power or power source contactor. If the sequence is reversed, the relay does not pick up. In that case, the power source is not connected to the aircraft bus, which protects motors and other equipment where rotation direction or phase order matters.

BChF-208 operating principle and internal layout

The BChF-208 uses an asymmetric star circuit. One phase includes a capacitor, while the other two phases include resistors. This arrangement creates different voltage conditions depending on the phase order. With correct A-B-C rotation, the relay receives enough voltage to operate. With incorrect rotation, the relay remains inactive. The design is electromechanical and straightforward, which makes the unit easy to check during electrical system troubleshooting. In maintenance work it is usually checked together with ground power contactors, AC buses, DC control circuits and terminal connections.

Aircraft application of the BChF-208 unit

The BChF-208 is used on a wide range of Soviet and post-Soviet aircraft and helicopters. Known applications include An-14, An-22, An-28, An-72, An-74, An-124, An-140, An-225, Il-62M, Il-76, Il-78, Il-86, Tu-154, Yak-42, Ka-25, Ka-27, Ka-28, Ka-32, Mi-8, Mi-14, Mi-24, Mi-26 and Mi-28. It is also listed for several military and special platforms by internal type codes. The installation position and wiring may differ by airframe, so the aircraft electrical diagram should be checked before replacement.

Identification and replacement of BChF-208

The unit is built on a plastic mounting panel and closed with a protective cover. Connection is made through terminal screws under the cover. Before ordering, it is better to compare the marking on the unit with the aircraft parts catalog or repair documentation, especially for aircraft that have passed several overhauls. A visually similar phase control unit from another aircraft may not match the wiring or approval paperwork. The product is also known under alternative designations BChF-208 and БЧФ-208.

Product Type: Aircraft phase sequence control unit
Function: Protection against connection of a three phase power source with incorrect phase sequence
Application / Where Used: Aircraft AC power supply systems
System / Installation Area: Three phase AC power source connection and ground power control circuits
Compatible Aircraft / Equipment: An-14, An-22, An-28, An-72, An-74, An-124, An-140, An-225, Il-62M, Il-76, Il-78, Il-86, Tu-154, Yak-42, Ka-25, Ka-27, Ka-28, Ka-32, Mi-8, Mi-14, Mi-24, Mi-26, Mi-28, T-10, T-6, T-8, 9-12, 01, 4503, VP-021, VPMK
Operating Voltage: 208 ± 12 V AC, three phase
DC Control Voltage: 27 ± 3 V DC
Frequency: 400 ± 50 Hz
Current Consumption: Not more than 0.028 A
Operating Mode: Continuous
Output Signals: Relay contact closure for control circuit, typically contacts 1-2
Interface: Terminal screw connection under protective cover
Dimensions: 61 × 51 × 72 mm
Weight: Not more than 0.23 kg
Construction Type: Plastic panel with protective cover
Technical Documentation: Aircraft electrical system documentation and product technical documentation

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