Application CM400
The CM 400 is a self-resetting cable monitoring device for railway power systems and other power systems. It detects cable faults in high-voltage direct-current systems, particularly in the overhead contact line and power supply infrastructure of electrified railway lines, thereby contributing to the operational safety and availability of the cable systems.
Description
The CM 400 continuously monitors the condition of cables in DC railway systems. It detects when a cable’s insulation is damaged or the cable shield is broken—and automatically reports it.
Specifically, the CM400 tests and monitors the following:
- the insulation between the conductor and the shield,
- the insulation between the shield and ground,
- as well as whether the shield itself is continuous (uninterrupted).
If the CM400 detects an error, it transmits the message via potential-free relay contacts so that it can be integrated into a higher-level control or signaling system.
The CM 400 is housed in a compact, rugged plastic enclosure that can be mounted either on a wall or on a DIN rail—making it ideal for installation in control cabinets or equipment rooms along the railway line.
Optional
An external display unit (bar graph display) can be connected. It clearly shows the current insulation value and the set threshold, so that the condition of the cable can be determined at a glance, without additional diagnostic equipment.
Function
To achieve defined and reproducible measurement conditions, resistors are connected in parallel with the insulation sections to be monitored. Depending on the device model, these resistors are either integrated into the device or—depending on the specific connection type—must be installed externally. The rating of the resistors limits the nominal shield voltage to approximately 40 V and the measurement current through the shield to approximately 1 mA, thereby complying with the electrical safety requirements for insulation monitoring in railway installations as specified in EN 50122-1 (Fixed Installations—Electrical Safety, Earthing, and Protective Conductors).
Überwachung des Kabelschirms auf Unterbrechung
Monitoring the cable shield for breaks
- Standard method – external resistance of 68 kΩ between the shield and the track ground: If the shield is interrupted, the error message “Internal short circuit (contact wire–shield)” is triggered. This type of fault requires the immediate shutdown of the affected cable, as there is a direct connection to the contact wire voltage and the requirements for protection against electric shock according to EN 50122-1 are no longer met.
- Alternative Method – 1,100 kΩ external resistor between the contact wire and the shield: If the shield is interrupted, the error message “Ground fault (shield–ground)” is triggered. Since there is no immediate hazard from the contact wire potential in this case, immediate deactivation of the cable is not absolutely necessary; the fault can be rectified as part of scheduled maintenance.
The choice of the appropriate method depends on the specific system configuration, the operator’s protection concepts (e.g., in accordance with EN 50122-1 and internal company guidelines for grounding and bonding), and the permissible response time in the event of a fault.
