When looking at the causes of unscheduled plant downtimes, it is often found that it is not errors in the instrumentation or control technology that stop the production process, but the connections between the levels. It is the information paths, converters and connection points on site that turn out to be the weak points in the analysis. This infrastructure is usually not suitable for direct exposure to the harsh ambient atmosphere in process plants. This is why these devices are housed in enclosures that protect them from moisture, temperature fluctuations and other risks such as mechanical interference.

Only temporary protection

As long as the enclosure has no defects, the devices installed in it are very well shielded against external factors. With increasing operating time or stress, however, the protection can deteriorate - caused by mechanical damage, ageing of the sealing materials or carelessness such as improper sealing. Very often, these are gradual processes that eventually lead to a failure of the devices installed in the enclosures and, in the worst case, to a complete system shutdown.

The control cabinet monitor can also be retrofitted in well-filled control cabinets. The CCM multifunction sensor (CCM stands for 'Cabinet Condition Monitoring') automatically adapts to the conditions on site.

© Turck

Modern electronic devices have their own temperature monitoring 'on board': measurements record the temperatures at critical points on the circuit boards. This data can be read out and further processed via fieldbus systems, for example. In principle, this is a good option - but some important points are not taken into account, as not every system concept is based on a fieldbus topology. In these cases, additional diagnostic information could only be obtained via additional interfaces on the devices. However, only a few companies make this effort; system components without special communication technology can therefore not be monitored.

In addition, the temperature on a circuit board cannot be used as a reliable indicator of the temperature in the control cabinet. This is particularly true for larger units. The measurement at a local hotspot can obscure the general temperature; the device temperature provides little information about the status of an enclosure, which can lead to misinterpretations. Furthermore, the temperature alone is not a parameter from which the general degree of protection of an enclosure or control cabinet can be derived. Rather, additional parameters such as humidity, incidence of light and position must be integrated into the monitoring concept in order to ensure reliable operation.

Condition monitoring for the control cabinet

To counter this problem, Turck has developed a device that can be retrofitted in almost any control cabinet or switch box and is taught to the conditions on site using a simple teach-in process. The control cabinet monitor called 'IMX12-CCM' reports the degree of protection of the control cabinet to the outside with a simple switching signal. In order to be able to use the 12 mm wide device in hazardous areas, it has an intrinsically safe 2-wire transmitter supply interface. This means that a maximum of four wires and a free space on a DIN top-hat rail are required to install and commission the control cabinet monitor. The teach-in process can be started without a computer or additional tools. The HART interface is available for further diagnostic options - for example to read out the absolute measured values.

In addition to the interface technology, the control cabinet monitor comes with several sensors that record the current status of the environment: A temperature sensor, an absolute humidity sensor and a triangulation sensor have been installed. The latter detects the distance to the lid or door. If the door is not closed properly, the device reports this status so that the operator can rectify the fault in a targeted manner.

As moisture is always a problem in closed systems, its continuous detection is an important element of condition monitoring. Reasons for moisture can include seals that have become porous and leaky due to environmental influences, or defective ventilation systems. Humidity often rises slowly but steadily, which can lead to equipment failure in the medium term. These effects can usually only be detected over a long period of time - the enclosure monitor records these long-term trends and sends a signal to the control level if limit values are exceeded in order to anticipate an instrumentation failure. The device continuously processes the data recorded by the sensors and compares it with the taught-in good status. As soon as defined limit values are exceeded, a signal is sent to the control level via a potential-free contact.

Author: Klaus Ebinger is Head of Product Management Interface Technology at Turck.