How overheating in electrical equipment happens?
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 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 of 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 risks of fire. 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 date and time of inspection can be unlucky, e.g., a period of low load, which doesn’t show the full picture regarding equipment conditions. The minimum load should be at least 60% on average to be able 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 case of a complex internal layout of the panel, 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.
How rFPT works?
rFPT is made of composite material with special signal gas inside, which is encapsulated so that it releases gas only at certain temperature. After releasing, the gas spreads over the electrical cabinet, making concentration detectable by FPA. There are 4 different sizes of rFPT. Each size has a different amount of gas encapsulated in it to make proper concentration in the corresponding volume. Every rFPT has 3 or 4 thermoindication dots with varying activation temperatures, which irreversibly turn black after reaching activation temperature.
Is the gas in rFPT dangerous for the environment, humans, or equipment?
No. The gas we use is non-toxic, it’s chemically stable and doesn’t react with other chemicals inside the cabinet and coatings of equipment. This signal gas has no flash point, which means it cannot explode at any concentration. On top of that rFPT generates minimal concentration in overall air composition (0.025% - 0.1% of gas presence in air after releasing). But thanks to selective gas sensors, this amount is more than enough to detect and give ALARM signal.
How do we ensure that rFPT validity period is 10 years?
Encapsulation. It’s needed not only burst at certain temperatures and release gas inside but also to protect gas from diffusion over time. Using the scientific approach (we put rFPT inside the climatic chamber for several months and precisely measure the material's weight loss, which might indicate that gas is being released). These tests are done at different temperature levels and all the results we’ve got tell that even after 10-15 years, there will be enough gas to trigger FPA properly.
How good the adhesion of rFPT?
We use an extra adhesion glue layer from 3M company. Thanks to this and thanks to the low weight of sticker, rFPT will be reliably glued to all 10 years of validity. For sure, adhesion depends on both surfaces, so we highly recommend to clean and degrease the surface before gluing rFPT. Moreover, we recommend gluing rFPT in a ring (with overlap) to provide maximal reliability.
Is the gas lighter than air?
At the initial moment, when it is just released from rFPT, the gas is hot and it’s lighter than air hence it moves from bottom to top. As soon as it’s cooled by ambient air it gets heavier than air and might go down (if it wasn’t dispersed yet). Thus, we recommend placing FPA at the top side of the compartment to detect gas faster since it moves upwards.
Do I need to replace rFPT after its operation?
Even if the rFPT still remains some residual gas inside, we cannot be sure that it will be enough for the next triggering. Please, make a required repair for the overheated spot and replace rFPT sticker with a new one.
How can I understand that rFPT has been triggered?
By checking thermoindication dots. If all the dots are black, this indicates that this rFPT most likely operated.
What rFPT (80, 100, or 130 °C) 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, which starts 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 compare 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 normal temperature for most switchgears in countries with warm climate) these limits are 100-115 degrees.
If we combine these two approaches and consider that absolute values are more important than relative values of temperature rise, the best pick will be 100 °C for most of the cases. It’s even better than 80°C rFPT since the 80°C might be reached if the high load met hot ambient temperature, but it will not be a dangerous condition for the equipment. 80 and 130°C products should be used only in specific projects with a defined purpose.
What might the maximum distance between rFPT and FPA be?
From the point of view of detection reliability, this does not really matter. Two factors are needed to ensure only:
1. Correct rFPT size and FPA model (volume dependent)
2. Correct placement of the FPA inside the compartment. Ensure that the FPA is positioned above the rFPT and that there are no obstacles to signal gas to reach the FPA.Placing the FPA closer to the rFPT only makes sense to increase the detection rate, but 80, 100 and 130 degrees are not emergency temperatures, and this is not a big difference between 10 or 40 seconds of the detection time.