Download document () of 20
Send email
Download
You have exceeded the download limit
Delta T (ΔT) is the method of measuring the temperature difference between an object, such as a piece of equipment or an electrical connection, and the surrounding ambient environment.
In the case of electrical switchgear, this is generally the busbars and main connection points, as these are areas which will typically fail despite thermographic inspections being carried out as part of routine maintenance.
ΔT = Tcomponent − Tambient
For example, if a busbar is operating at 100°C and the ambient temperature is 35°C, the Delta T is:
ΔT = 100°C − 35°C = 65°C
Absolute measures the actual temperature of a component. For example, absolute temperature measurement is used to calculate if an object is within a maximum or minimum design threshold.
This is useful for mechanical items such as bearings or motor windings, and in certain extreme cases, these items of equipment require de-rating to operate in harsh environments.
Absolute temperature does not tell us if that same piece of equipment or connection has a fault due to either fatigue or item failure. This is because we do not understand what the surrounding ambient conditions are - it could actually be that the ambient is causing the temperature rise and not a faulty component.
Absolute only tells us that the object is operating either within or outside the design thresholds. Therefore, we require another method of temperature measurement that can provide more detailed information/understanding about the equipment being monitored.
A connection in an electrical system that is in poor condition, has become loose or compromised, will always try to dissipate the power generated across the joint as heat.
The connection or component will heat to a level above ambient sufficient to move the extra power away into the surrounding ambient.
This is obeying the theory known as ‘Newton’s Law of Cooling’ (see Fig 1.) which states that a body will raise its own temperature infinitely to dissipate the extra power due to the compromised conditions. This therefore shows why we need to use a Delta T (ΔT) method of temperature measurement.
If we only measure absolute and the joint on the busbar is operating at 100°C, then we can correctly state that it is within the IEC 61439 standards for maximum temperature thresholds and it is operating within electrical standards.
However, if we now take an ambient measurement in the vicinity of the bar (as a thermographer would do during inspection) and this ambient was only 35°C, then we actually have a Delta T (ΔT) of 65°C. This is outside the NETA (International Electrical Testing Association) guidelines.
These guidelines dictate: “Immediate action when the difference in temperature (ΔT) between similar electrical components under similar loading exceeds 15°C (27°F) or when the (ΔT) between an electrical component and the ambient air temperatures exceeds 40°C (72°F)”.
Please also note that 0°C Delta T (ΔT) is in fact 0°F Delta T (ΔT) and not 32°F!
These guidelines are all based on Delta T (ΔT) and NOT absolute.
Remember, measuring absolute temperature only informs us if a component is operating within a specific temperature band.
Absolute temperature measurement will NOT detect a compromised connection or failing piece of equipment.
Thermography provides valuable insights for a single moment in time. However, electrical systems can develop issues any day of the year. Continuous Thermal Monitoring (CTM) gives you 'always on' awareness of system health delivering precise alarm thresholds aligned with NETA standards.
Discuss your specific application requirements with our expert engineers, obtain additional technical information, or learn more about our other applications.
MyEatonFAQs
