Tuesday, February 09, 2021
Using Thermal Imaging Cameras for Electrical Inspections [Copied from Fluke Article October 2020]
Thermal imaging
Commonly Inspected Components
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 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 results in costly damage. Under-sized conductors, loose connections or excessive current flow may cause abnormally high unwanted heat that result in dangerously hot electrical circuits. Components can literally become hot enough to melt.
Thermal imagers 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 overloaded circuit or multi-phase imbalance problem.
Contact Problems
Heat is produced by current flow through 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
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. 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.
Thermogram A
Thermogram B
Thermogram C
Thermogram D
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.
Overloaded Circuit Problems
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.
Thermogram E
Thermogram F
Thermogram G
Thermogram H
Load Requirements
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.
Comments