FAQ
How does overheating in electrical equipment happen?
Overheating in electrical compartments is a common occurrence. Possible reasons can include a loose connection, wrong selection of equipment/cross-sections, and overloads. But even overload cannot lead to a fire if all connection points are fine and there are no decent contact resistances across the circuit, which brings us to the problem of loose connection again.
A loose connection can appear because of the following reasons:
1. Improper tightening during assembly
2. Vibration over time
3. Large number of connection operations (especially for withdrawable contacts)
4. Repetitive heating and cooling process over time
Since the connection is loosened, it starts to generate excessive heat because of increased resistance. Another possible threat, in that case, is oxidation, which is accelerated by overheating. Oxidation, in turn, covers the contact with an oxide film, which has significantly higher resistance than the base metal. As a result, the contact becomes degraded, and every overload now can lead to a fire.
What temperature is needed for an ignition?
Usually, the weakest point in terms of heat resistance is cable insulation. It starts to lose dielectric properties at 170-200°C; after 280°C, it can begin to melt and smoke. Other points that need to consider are plastic epoxy enclosures of current/voltage transformers, fuses, and capacitors banks.
Why is thermography inspection not the best solution for your equipment?
Nowadays, scheduled infrared thermography is the primary solution for inspection and reducing fire risks. Nevertheless, this method has serious flaws:
1. Scheduled basis.
The period between two inspections can be measured in weeks or months. Of course, it doesn’t allow maintenance personnel to be up to date regarding equipment conditions 24/7. Moreover, the scheduled inspection date and time can be unlucky, e.g., a period of low load, which doesn’t show the whole picture regarding equipment conditions. The minimum load should be at least 60% on average to see overheating.
2. Limited or no visibility during an inspection.
Inspection of most of the electrical panels is being done from one single angle. In the case of a complex internal panel layout, it’s impossible to check all concerning points and make the correct conclusion.
The situation is even worse when it comes to MV equipment since it is not advisable to open the cabinet during its operation. Thus, it’s not possible to check the condition and prevent dangerous situations at all. Solutions like infrared windows can solve this problem only, particularly keeping the problem of limited visibility.
3. Human factors and risk for personnel.
The quality of the report and understanding of equipment conditions also depend on the quality of the inspection team's service. A person, the inspector, can make wrong decisions or even skip some inspection program points. On top of that, the survey implies the immediate proximity of the inspector to live parts that pose a real threat to life and health.
What threshold temperature to choose?
From a technical perspective, the main concerns in terms of fire safety in electrical switchgear are possible deterioration of cable insulation, leading to flashover or melting and further ignition.
Typical materials for cable insulation, such PVC / XLPE / EPR, are subject to degradation, starting at 170-200°C. After 200°C, it starts changing color, and at approximately 280-300°C, it starts to smoke and melt, leading to a fire. From this point of view, the threshold temperature of ALARM should be less than 130-140 degrees to prevent the cable from negative consequences.
From another hand, there are electrical standards. The main standards describing this issue are IEC 62271-1 (for MV switchgear) and IEC 60947-1 (for LV switchgear). Unfortunately, standards do not give us absolute temperatures, which are more important in terms of fire safety. According to them, bare cooper bolted connections temperature and terminals temperature rise shouldn’t exceed 50-65 degrees compared to ambient air temperature in the enclosure, which means 90-105 degrees of absolute value at 40°C of ambient temperature. For ambient temperature as 50°C (which is the normal temperature for most switchgears in countries with warm climates) these limits are 100-115 degrees.
If we combine these two approaches and consider that absolute values are more important than relative temperature rise values, the best pick will be 100 °C for most cases. 80 and 130°C products should be used only in specific projects with a defined purpose.