400 Hz network applications of 3VA molded case circuit breakers
400 Hz networks are used for:
Ground power supply systems for aircraft
On-board electrical systems of marine craft and aircraft
Other external stations of airports which are protected by uninterruptible power supplies, e.g. radar installations, data processing equipment, communication systems
It is important to supply the on-board electrical systems of aircraft with a very smooth DC voltage. This can be generated more easily by 400 Hz power supplies than by 50 Hz networks. Moreover, the power transmitters (transformers) are smaller and lighter by comparison with 50 Hz networks, two important arguments for using 400 Hz on-board electrical systems in aircraft and some marine craft.
Suitable 3VA molded case circuit breakers and trip units
3VA molded case circuit breakers equipped with 2-series thermal-magnetic trip units are suitable for use in 400 Hz networks.
Features specific to 400 Hz supplies
The thermal overload release of the 3VA1 molded case circuit breaker comprises a bimetal through which current flows, causing temperature rise. With currents at 400 Hz frequency, the temperature rise in the bimetal is higher than at 50 Hz. This is due to eddy-current losses and the reduction in the available conductor cross section as a result of the skin effect. The rated operational current must therefore be reduced by 10 % of the value used in 50/60 Hz applications.
Furthermore, a correction factor must be applied to the magnetic trip unit.
3VA1 160A, TM240 ATAM:
Reduction by 10%:
Maximum permissible rated operational current for 400 Hz applications = 160 A x 0.9 = 144 A
Correction factor 0.7:
If the switching device is to trip instantaneously in response to an overcurrent of 1200 A, the parameter Ii for instantaneous short-circuit protection (I) must be set to Ii = 1200 A x 0.7 = 840 A.
For further information on trip units and details about the setting parameters, please refer to chapter Line protection applications of 3VA molded case circuit breakers.
It is not possible to use electronic trip units for this application.