The device is used for the monitoring of the insulation of DC cable systems. It works without auxiliary power supply. The connection is made between the insulating outer shell and a reference ground.
For the reporting, there are two potential free changeover contacts available. The device is integrated in a plastic housing that is made for the mounting on top-hat railways. The CFM is a self-resetting cable monitoring device for the detection of cable errors. Between Signal and triggering a time delay of 4.8 sec is integrated.
The voltage between a cable sheath and the earth is measured by a rectifier. If the voltage limit is exceeded a signalling relay is addressed.
The circuit operates with the constant-current principle. This means that even after the voltage increases to more than 1000 V the demand for electricity is not increasing. The leakage current flowing to earth is dimensioned with only 2 mA. Only in cases of failure the current load will be increased to 6 mA. The state of switching is displayed locally by a signal lamp.
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