Cable insulation monitoring improves preventive maintenance

What happens in your plant when cable insulation fails? The answer is anything from a difficult to find intermittent fault, to a major shutdown or even a fire. Using PLC based cable insulation monitoring improves earth leakage countermeasures without reworking major equipment.

With older plants it is not unusual for the wiring to be over fifty years old. Early detection of insulation deterioration of production equipment is essential. The leading causes of plant electrical fires are overheating at loosened connections, and electrical leakage such as earth faults. Regular thermography check will find lose connections, but not earth faults.

One problem with high impedance earth faults is that unless there is an RCD in circuit they are unlikely to trip the local circuit breaker. Even then, RCDs feeding multiple parallel loads can make finding faults locations a problem. In order to eliminate leakage current problems, users need a system that can monitor leaks individually close to the load and inform maintenance.

As part of Mitsubishi’s general purpose MELSEC-Q Series PLCs, the QE82LG module can measure leakage current for individual circuits. Moreover, it can accurately monitor insulation deterioration through measuring the resistance leakage current due to wiring or device deterioration.

PLC based cable insulation monitoring

Zero-phase Current Transformer (ZCT) mounted on the cables at the base of the panel board sends leakage current information to the QE82LG. The QE82LG allows two types of alarm threshold value to be set. For personnel safety an electric-shock level of 30mA, and a potentially fire causing leakage-level of 200mA. If any of the monitoring devices exceeds these levels, the HMI displays the leakage point and emits a warning. It also detects if there is a wiring error allowing a load current to flow to earth.

Identifies transient faults

In cases involving phenomena like moisture-caused leakage, where the cause naturally evaporates, identification is now easily done at panel board level. This means it is now possible to renovate leakage points much earlier and prevent fire-related accidents. Furthermore, being able to see leakage current by circuit numerically is a saving grace for the maintenance staff. The usual practice is alarm-point management, but being able to see the circuit numbers all the time lets them respond more quickly.

Improved preventive maintenance

Fire prevention through cable insulation monitoring is not the only benefit of introducing insulation monitoring modules. Another major advantage is its promise of future maintenance cost reduction. Using devices and wiring beyond the manufacturer’s recommended replacement times can also lead to problems. A fixed cycle of maintenance may not be enough to completely prevent accidents. The QE82LG provides regular system monitoring that detects problems earlier.

Capacitive loads

Cable insulation monitoring of long cables and inverter circuits with large capacitive leakage currents present no problems. The modules can measure resistive leakage current, and removes the capacitive leakage current. It then monitors the accurate leakage current caused by insulation deterioration.

By constantly monitoring of insulation resistive leakage current, a two-stage warning is available. This gives maintenance engineers more time to put in place a planned response.

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