Maintenance, Test & Measurement, Calibration

Fibre optic temperature monitoring of switchgear

June 2018 Maintenance, Test & Measurement, Calibration

AP Sensing’s passive fibre optic cable provides accurate temperature measurements along the length of equipment such as a conveyor belt, or inside switchgear. The system enables cost-effective monitoring along extended or short distances, allowing overheating to be quickly detected and localised to within one-metre accuracy.

Case study

With all the possibilities that the AP Sensing FO DTS (fibre optic distributed temperature measurement) has to offer, a simple but very important question arises: Can it be used to detect hot spots within a switchgear cabinet? Can the DTS detect the change in temperature inside the cabinet caused by a hot component with a temperature between 100°C and 150°C? And, can it detect such a hot spot within 48 hours of the component reaching the specified temperatures?

Purpose of the test

The purpose of the test was to investigate if the DTS system is able to detect a typical hot contact/conductor anywhere inside a switching cabinet, even without the sensing cable touching the hot component.


• A cabinet was placed inside an air-conditioned room to keep the ambient temperature between 22°C and 24°C.

• The dimensions of the cabinet were 2620 x 500 x 2320 mm, with six ‘buckets’ varying in sizes between 200 and 600 mm.

• The top two buckets were closed off on the sides of the cabinet to simulate a real installation and to prevent convection to influence the result.

• A 100 W heat source 20 x 20 x 10 mm, with a surface temperature between 120°C and 150°C was placed at the bottom bucket inside the cabinet to simulate a hot connection point.


The results showed that the heat source caused a very small but detectable rise in the average internal temperature of the cabinet. Since the buckets were not completely isolated from each other, hot air could rise in the cabinet due to natural convection.

Detecting a hot contact/component inside the switchgear cabinet is not only a function of the source temperature, but also of the available power from the heat source. As long as the temperature sensing happens at a higher location than the source, any location inside the cabinet will suffice to detect a difference in temperature. If the cabinets are stacked end-to-end and isolated from each other, the sensing cable only needs to be inside the top most bucket. If the cabinets are not isolated from each other, the sensing cable needs to be inside the cabinet, running vertically for the height of the cabinet.

The closer the sensing cable is to the source the faster it will respond, but it is important to note that the development of a hot contact is gradual and the entire cabinet temperature will change over an extended period of time.

Questions and answers regarding the test results

Why did the system only register 35°C when the heat source was at 120°C?

• The system measures the temperature of the actual glass molecules of the optical fibre. Any change in temperature must be conducted from the outside of the cable to the glass cores inside. If the cable is not in direct contact with the heat source, the measured temperature will always be lower than the source temperature.

• The cable is sensitive to radiated heat (infrared radiation heating up the surface) as well as ambient heat (hot air surrounding the cable).

• The internal components of a typical electrical cabinet will be an optical (line-of-sight) obstruction to most of the fibre cable, thus the detection cannot rely on radiation only.

• The heat source will warm the air surrounding it and natural convection will transport the hot air to where the cable is located.

• The test proved that the 120°C source with a heating power of 100 W, sufficiently heats up the air to cause a temperature rise of more than 10°C at the top edges of the bucket where the source is located.

• 35°C was the air temperature leaving the bucket by natural convection.

How long will it take to detect a hot contact within a bucket?

• The test results show that a significant change in temperature is evident within 30 minutes, with the temperature stabilising after three hours.

• The suggestion is to have a fibre cable running vertically for the entire height of the cabinet.

• Any heat source similar or more powerful than the test source will cause hot air to escape the problem bucket and come into contact with the cable.

How should the alarm parameters be configured to detect a change in temperature?

• The best alarming method to use will be rate-of-rise. The entire length of the fibre cable inside the cabinet will be allocated to one detection zone. The system allows for up to three separate rate-of-rise parameters per zone, usually implemented as short term, medium term and long-term parameters. (A detailed copy of the test results is available on request.)

Fibre optic sensing technology offers multiple measurement solutions for measuring operating conditions in the mining and industrial sectors. It performs exceptionally well in harsh conditions and is virtually maintenance free, offering the user reliability and low cost of ownership.

For more information contact Marihette Hattingh, Sperosens, +27 (0)12 665 0317,,


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