BTT-40BT

BTT-40BT current transformer block in generator protection

BTT-40BT is an aircraft current transformer block used in the electrical power system of aircraft with three phase AC generators. The block is linked with differential current protection of AC generators and their feeders. Its work is not to generate power or switch the generator on its own. It gives the protection circuit a current related signal, so the system can detect an abnormal condition in the generator line.

Generator protection function of BTT-40BT

In a normal generator circuit, current on the generator side and feeder side should stay within the expected balance. BTT-40BT is used in this protection chain to monitor that condition. If a fault appears in the generator or feeder circuit, the differential protection system receives the signal needed to disconnect the faulty channel. This protects the generator, wiring and downstream electrical equipment from damage caused by internal faults or incorrect current flow.

BTT-40BT in aircraft AC power systems

The block is used with aircraft AC generators rated for single generator operation in the onboard electrical system. It works in circuits where generator output, feeder condition, protection relays and contactors are tied together. During maintenance, BTT-40BT is usually checked together with generator output wiring, current transformer circuits, protection units, generator control panels and feeder contactors. A fault in the protection channel is not always caused by the block itself, so the full circuit should be checked before replacement.

Aircraft application of the BTT-40BT block

BTT-40BT is found on a wide range of Soviet and post-Soviet aircraft and helicopters. Known applications include Il-18, Il-62M, Il-76, Il-86, Il-96-300, Tu-154, Tu-204, Yak-42, An-22, An-74, An-124, Mi-8 variants, Mi-8MTV-1, Mi-171, Mi-26 and Mi-28. The exact installation depends on the generator type, power distribution layout and protection scheme of the specific aircraft. For this reason, the part should be checked against the aircraft electrical documentation, not only by the aircraft model.

Identification and ordering of BTT-40BT

The marking on the unit and the factory label should be compared with the aircraft parts catalog, repair paperwork or product acceptance label. Older stock units may have factory packaging, acceptance stamps and storage documentation, which should be checked before supply. It is also worth noting that BT40 without the BTT prefix can refer to a machine tool holder taper standard, which is unrelated to this aircraft electrical component. The product is also known under alternative designations BTT-40BT and БТТ-40БТ.

Product Type: Aircraft current transformer block
Function: Differential current protection signal for AC generator and feeder circuits
Application / Where Used: Aircraft electrical power generation systems
System / Installation Area: AC generator protection circuit / generator feeder protection
Compatible Aircraft / Equipment: Il-18, Il-62M, Il-76, Il-86, Il-96-300, Tu-154, Tu-204, Yak-42, An-22, An-74, An-124, Mi-8 variants, Mi-8MTV-1, Mi-171, Mi-26, Mi-28
Generator Power Rating: 40 kVA
Nominal Primary Current: 111 A at 208 V, 400 Hz
Output Voltage: 3.3–4.9 V average rectified voltage
Dimensions: 168 × 72 × 95 mm
Weight: Not more than 0.85 kg
Technical Documentation: Product label, acceptance documentation and aircraft electrical system documentation

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