PLC module reduces fire risk by monitoring cable insulation.

At Nitto Denko Corporation’s Plant 2, Mitsubishi Electric’s Q Series PLC QE82LG insulation monitoring module is used to monitor electrical devices and wiring for insulation breakdown. An electrical fault could lead to a fire, and by using PLC based modules to monitor insulation deterioration they have been able to improve leakage countermeasures without reworking major equipment.

Some plant wiring over 40 years old

According to Nitto Denko, Plant 2 has no shortage of devices and wiring that have been in use for over 40 years, since it began operation. But it’s not realistic to replace them all for new. They manage the problem by raising the everyday monitoring level.

As a general rule, the two major causes of plant electrical fires are overheating at loosened connection or contact areas of circuits, and electrical leakage such as earth faults. Nitto handles the former with yearly maintenance checks using thermography, but the problem was in the latter case.

They used to check the insulation monitoring equipment set up for each transformer, but with multiple loads connected in parallel, it was difficult to pinpoint problem locations. In order to eliminate leakage current problems, they needed a system that could monitor leaks individually, close to the load, and inform the supervisor immediately.

Accurately monitor insulation deterioration

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

Nitto placed 30 QE82LGs in the transformer room and 22 in the cubicles of its Plant 2. The Zero-phase Current Transformer (ZCT) mounted on the cables at the base of the panel board is used to send leakage current information to the QE82LG which processes it. Measurement results are displayed on the HMI (GOT- Graphic Operation Terminal) display. The QE82LG allows two types of alarm threshold value to be set; in this case Nitto set an electric-shock level of 30mA and a potentially fire causing leakage-level of 200mA. If any of the total 52 monitoring devices should exceed these levels, even once, the GOT displays the leakage point in yellow or red and emits a warning.

By changing leakage monitoring from per-transformer to per-distribution panel board, leakage causes which could not be determined in the past have now become increasingly clear. In one instance, they had a case where the company doing wiring installation got the connection wrong, so that load current flowed onto earth. At the time they couldn’t find the cause, but after introducing leakage monitoring, they were able to identify causes immediately.

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, meaning that it is now possible to renovate leakage points that much earlier and prevent fire-related accidents. In addition, 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 numbers all the time lets them grasp the status trends and respond more quickly.

Reduced maintenance costs

Nor is fire prevention through insulation monitoring the only effect of the introduction of the insulation monitoring module. Another major advantage is its promise of future maintenance cost reduction. When devices and wiring on site are used beyond the manufacturer’s recommended replacement times, things can go wrong at any time. A fixed cycle of maintenance may not be enough to prevent accidents completely. What is essential is to have regular system monitoring which detects trouble more quickly, and Nitto judged that insulation monitoring using the QE82LG would be effective in this role.

Open QE82LG leaflet here

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