Thermal images are an easy way to identify apparent temperature differences in industrial three-phase electrical circuits, compared to their normal operating conditions. By inspecting the thermal gradients of all three phases side-by-side, technicians can quickly spot performance anomalies on individual legs due to unbalance or overloading.
Electrical unbalance can be caused by several difference sources: a power delivery problem, low voltage on one leg, or an insulation resistance breakdown inside the motor windings.
Even a small voltage unbalance can cause connections to deteriorate, reducing the amount of voltage supplied, which motors and other loads will draw excessive current, delivery lower torque (with associated mechanical stress), and fail sooner. A severe unbalance can blow a fuse, reducing operations down to a single phase. Meanwhile, the unbalanced current will return on the neutral, causing the utility to fine the facility for peak power usage.
In practice, it is virtually impossible to perfectly balance the voltages across three phases. The National Electrical Manufacturers Association (NEMA) defines unbalances as a percentage: % unbalance – [(100)(maximum deviation from average voltage)] / average voltage. To help equipment operators determine acceptable levels of unbalance, NEMA has drafted specifications for multiple devices. These baselines are a useful point of comparison during maintenance and troubleshooting.
Thermal imaging is key to discovering and diagnosing electrical unbalance and insulation resistance breakdown.
Commonly Inspected Components
Capture thermal images of all electrical panels and other high-load connection points such as drives, disconnects, and controls. Where you discover higher temperatures, follow that circuit and examine associated branches and loads.
Check panels and other connections with the covers off. Ideally, you should check electrical devices when they are fully warmed up and at steady state conditions with at least 40% of the typical load. This allows measurements to be properly evaluated and compared to normal operating conditions.
Abnormal heating associated with high resistance or excessive current flow is the main cause of many problems in electrical systems. Infrared thermography allows us to see these invisible thermal signatures of impending damage before the damage occurs. When current flows through an electric circuit, part of the electrical energy is converted into heat energy. This is normal. But, if there is an abnormally high resistance in the circuit or abnormally high current flow, abnormally high heat is generated which is wasteful, potentially damaging and not normal.
Ohm's law (P=I2R) describes the relationship between current, electrical resistance, and the power or heat energy generated. We use high electrical resistance for positive results like heat in a toaster or light in a light bulb. However sometimes unwanted heat is generated that result in costly damage. Under-sized conductors, loose connections or excessive current flow may cause abnormally high unwanted heating that result in dangerously hot electrical circuits. Components can literally become hot enough to melt.
Thermal cameras enable us to see the heat signatures associated with high electrical resistance long before the circuit becomes hot enough to cause an outage or explosion. Be aware of two basic thermal patterns associated with electrical failure: 1) a high resistance caused by poor surface contact and 2) an over loaded circuit or multi-phase imbalance problem.
What to Look For
Equal load should equate to equal temperatures. In an unbalanced load situation, the more heavily loaded phase(s) will appear warmer than others due to the heat generated by resistance. However, an unbalanced load, an overload, a bad connection, and a harmonic imbalance can all create a similar pattern. Measuring the electrical load is required to diagnose the problem.
It is sound procedure to create a regular inspection route that includes all key electrical connections. Using the software that comes with your Fluke thermal camera, save each image you capture on a computer and track your measurements over time. This allows you to create a baseline of images to compare with images captured at later dates. This procedure will also help you determine whether a hot or cool spot is unusual. Following corrective action, new images will help you determine if repairs were successful.
Heat is produced by current flow through a contact with high electrical resistance. This type of problem is typically associated with switch contacts and connectors. The actual point of heating may often be very small, less than a 1/16 inch when it begins. Below are several examples found with the IR SnapShot during customer demonstrations.
Thermogram A) is a motor controller for an elevator in a large hotel. One of the three phase connections was loose, causing increased resistance at the connector. The excess heating produced a temperature rise of 50 degrees C (90F). Thermogram B) is a 3-phase fuse installation where one end of one fuse has poor electrical contact with the circuit. The increased contact resistance caused a 45C (81F) hotter temperature at that connection than at the other fuse connections. Thermogram C) is a fuse clip where one contact is 55C (99F) hotter than the others. And thermogram D) is a two-phase wall plug-in where the wire connections were loose causing the terminals to heat 55C (100F) hotter than the ambient.
All four of these examples were serious and needed immediate attention. Thermogram B) shows an interesting principal used in interpreting thermal patterns of electrical circuit. The fuse is hot at one end only. If the fuse were hot at both ends, the problem would be interpreted differently. An overloaded circuit, phase imbalance, or an undersized fuse would cause both ends of the fuse to overheat. Being hot at one end only suggests that the problem is high contact resistance at the heated end.
The wall plug in Thermogram D) was seriously damaged as seen in the visual picture below, however, it continued to operate until it was replaced.
What represents a “red alert”?
Repairs should be prioritized by safety first—i.e., equipment conditions that pose a safety risk—followed by criticality of the equipment and the extent of the temperature rise. NETA (InterNational Electrical Testing Association) guidelines dictate immediate action when the difference in temperature between similar electrical components under similar loads exceeds 15 °C (27 °F) or when the difference in temperature between an electrical component and the ambient air temperatures exceeds 40 °C (72 °F).
NEMA standards warn against operating any motor at a voltage unbalance exceeding one percent. In fact, NEMA recommends that motors be de-rated if operating at a higher unbalance. Safe unbalance percentage vary for other equipment.
The following thermograms show overloaded circuits. Thermogram E) shows a circuit panel in which the main breaker at the top is over heated 75C (135F) above ambient. This total panel is overloaded and in need of immediate attention. Thermograms E) and F) show all the standard circuit breakers over heated. Their temperatures were 60C (108F) above ambient. Although in the thermogram the wires are blue in color they are also hot, 45 to 50C (81 to 90F). This entire electrical system needs to be redone.
Thermogram G) shows one line of a controller that is about 20C (36F) above the others. This needs further investigation to determine why one wire is that much hotter than the others are and to determine the repair needed. Thermogram H) shows a current transformer that is 14C (25F) warmer than the other two transformers in a 3-phase service installation. This indicates a serious imbalance of the service or a faulty current transformer that could seriously impact the customer's utility bill.
When making an inspection it is important that the system is under load. Wait with the inspection for "worst case" or peak loads, or when the load is at least 40% (according to NFPA 70B). Heat generated by a loose connection rises as the square of the load; the higher the load, the easier it is to find problems.
Don't forget to consider the cooling effect of wind or other air movement.
Surface Temperatures Only
Infrared cameras cannot see through electrical cabinets or solid metal bus trays. Whenever possible open enclosures so the camera can directly see the electrical circuits and components. If you find an abnormally high temperature on the outside surface of an enclosure, rest assured that the temperature is even higher, and usually much higher, inside the enclosure. Below are some thermograms taken of a bus enclosure, which identify a serious problem with the electrical buses inside the enclosure. The hot spots were on the order of 10C hotter than the ambient and 6C hotter than other parts of the bus enclosure.
Literally hundreds of different pieces of equipment may be found in an electrical system. They start with the utility electricity production, high voltage distribution, switchyards and substations, and end with service transformers, switchgear, breakers, meters, local distribution, and appliance panels. Many utilities have purchased the FlexCam® or SnapShot® to help with their maintenance. And nearly every type of industry has bought Infrared Solutions cameras to help with maintenance on their end of the electrical distribution system.
Thermogram M) is a service transformer that had leaked some cooling oil, resulting in dangerously over heated coils near the top. One connection was 160C (288F) above ambient. This transformer needed immediate replacement but the company wanted to delay the repair one month so it could be done during a scheduled total plant shutdown. They used the IR SnapShot camera to monitor the state of the transformer and successfully delayed the repair. Thermogram N) is for a pole mounted service transformer that has a connection 30C (54F) hotter than ambient. Such a condition required maintenance at the next convenient opportunity. Thermogram O) shows a hot main connection on an interrupter at a substation in
Mexico. The connection was found to be 14C (25F) hotter than the others. This was believed to be a problem that needed attention. Thermogram P) shows an overhead connection in a Peru substation. It was less than 10C or (18F) above ambient and not of immediate concern.
What’s the potential cost of failure?
Motor failure is a common result of voltage unbalance. Total cost combines the cost of a motor, the labor required to change out a motor, the cost of product discarded due to uneven production, line operation, and the revenue lost during the time a line is down.
Assume the cost to replace a 50 hp motor each year is $5000, including labor. Assume 4 hours of downtime per year with income loss of $6000 per hour. Total cost: $5000 + (4 x $6000) = $29,000 annually.
When a thermal image shows an entire conductor is warmer than other components throughout part of a circuit, the conductor could be undersized or overloaded. Check the conductor rating and the actual load to determine which is the case.
Use a multimeter with a clamp, a clamp meter, or a power quality analyzer to check current balance and loading on each phase.
On the voltage side, check the protection and switchgear for voltage drops. In general, line voltage should be within 10% of the nameplate rating. Neutral-to-ground voltage tells you how heavily your system is loaded and helps you track harmonic current. Neutral-to-ground voltage higher than 3% should trigger further investigation.
Loads do change, and a phase can suddenly be 5% lower on one leg, if a significantly large single-phase load comes online. Voltage drops across the fuses and switches can also show up as unbalance at the motor and excess heat at the root trouble spot. Before you assume the cause has been found, double check with both the thermal imager and multimeter or clamp meter current measurements.
Neither feeder nor branch circuits should be loaded to the maximum allowable limit. Circuit load equations should also allow for harmonics. The most common solution to overloading is to redistribute loads among the circuits, or to manage when loads come on during the process.
Using the associated software, each suspected problem uncovered with a thermal imager can be documented in a report that includes a thermal image and a digital image of the equipment. This is the best way to communicate problems and suggest repairs.