Due to improved technology, thermal imagers are more affordable and easier to use than ever, making them a viable tool for routine electrical maintenance.
Thermal imagers work by producing pictures of heat emissions, revealing hot spots and unusual patterns. A qualified electrician just has to point the imager at the equipment they want to examine to scan it. To capture the image, it’s a matter of squeezing a trigger and then uploading the images to a computer or device for further investigation.
Imagers are simple to operate, but they should be used by professionals who have a thorough understanding of electrical measurements and equipment.
The following points are important to keep in mind:
Safety
Personal protective equipment (PPE) is essential when standing in proximity to open and live electrical panels. The protective equipment required will vary depending on the circumstances and energy level involved. Options include flame-resistant clothes, leather work boots, leather-over-rubber gloves, a hard hat, hearing protection, a face shield or a flash suit. A second person trained in low voltage rescue should also be present with a low voltage rescue kit.
Emissivity
This term describes how an object emits heat. Emissivity affects how accurately a thermal imager can measure the objects surface temperature. All objects and materials have a specific emissivity, rated from 0 to 1.0. The higher the emissivity, the more accurate the thermal imagers reading will be. Painted objects tend to have an emissivity of around 0.9 to 0.98. Materials such as electrical tape, rubber, ceramic and conduct insulation also have high emissivities. Materials with low emissivity include aluminum, copper and some types of stainless steel.
To accurately calculate an objects surface temperature, the emissivity of the object must be high. The imagers emissivity level should also be set close to the object’s emissivity. This makes it difficult to calculate the temperature of objects with low emissivity. Fortunately, a specific temperature is not needed for most electrical inspections as images are used for comparative or qualitative purposes. This means hots spots are identified not by the specific temperature, but by looking for unusual occurrences or comparing the spot with similar equipment operating under the same load conditions.
Loading
To detect any problems using a thermal imager, the equipment being examined must be under 40% of nominal load at minimum. Maximum load conditions produce the best results.
Troubleshooting tips
· Load performance or breaker issues
When investigating these issues, complete your repairs and then conduct another thermal scan. If the repair was performed correctly, the hot spot detected in the first scan should be gone. Keep in mind that not all hot spots are caused by loose connections, which is why it’s wise to have a qualified electrician perform the thermal scan.
· Three-phase imbalance
First capture thermal images of panels and high-load connection points, following the circuit when you discover higher temperatures to examine related branches and loads. Check for temperature differences by comparing the three-phases side by side.
Cooler than expected circuits could indicate a failed component. Hot conductors may be overloaded or undersized. Bear in mind that an overload, unbalanced load, bad connection or harmonics can give similar patterns, so it’s essential to investigate further with electrical quality measurements. (Remember voltage drops across switches and fuses can also manifest as unbalance at the motor and heat at the trouble spot so double-check with a thermal imager and multimeter).
· Connections and wiring
Connections with higher temperatures than similar connections under comparable loads could indicate a loose or over-tightened connection, or corrosion causing increased resistance. Usually hot spots caused by connections are warmest at the site of resistance, and then cool with distance, but this is not always the case. Sometimes a component is cold because the current is being shunted away from a high-resistance connection. Keep an eye out for defective insulation and undersized or broken wires.
· Fuses
A hot fuse on a thermal scan could be near or at its capacity, but not all problems will appear as heat. Other issues, including a blown fuse, will produce cooler than expected temperatures.
· Motor control centres
When evaluating a motor control centre (MCC) under load, compare the relative temperatures of controllers, bus bars, starters, relays, fuses, breakers, contractors, disconnects, transformers and feeders. It’s a good idea to measure the load at the beginning of each scan so you’re able to assess the measurements against typical operating conditions.
· Transformers.
With oil-filled transformers, you can use a thermal imager to examine external bushing connections, cooling tubes, fans and pumps and the surface areas of critical transformers. (The coil temperatures of dry transformers are far higher than ambient, making it difficult to identity problems with thermal imagery).
Cooling tubes should be warm when scanned. If one or more is cool in comparison, oil flow may be restricted. Don’t forget that a transformer, like an electric motor, has a minimum operating temperature. This is the maximum allowable temperature rise above ambient (standard is 40 °C). A 10 °C rise in temperature over the motor nameplate operating temperature will cut the transformers lifespan by up to 50%.
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