Using Thermal Images for Electrical Inspections
Heat: The Symptom of a Faulty Electric System
An infrared electrical inspection of your facility can provide you with valuable information about the condition of your electrical system and equipment that can save thousands of repair dollars and hundreds of hours of lost productivity. Even worse, unchecked electrical system failures can result in fires or dangerous explosions. Infrared inspections are designed to identify potential equipment failures before they happen so that appropriate preventative action can be taken. The decreased liability of a thermally inspected electrical system even encourages some insurance providers to offer discounted rates.
When an electrical system begins to have problems, it gives off heat as a byproduct of electrical resistance. Infrared cameras record and visually display this emitted heat to show the exact location of the electrical defect. Thermal imaging simplifies diagnosis and repairs while protecting adjacent electrical components from hot spots. Routine thermal electrical inspections will reduce repair costs and decrease lost production hours that result during unexpected down time.
A thermal scan of your electrical system increases efficiency, greatly reduces the risk of fires, and allows preventative and predictive maintenance to be scheduled during planned down-times. In business, this alone can mean mean big cost savings.
More About Predictive Maintenance
Thermal Imaging has proven to be an ideal inspection method for all types of predictive electrical maintenance. Infrared technology gives thermographers the ability to “see”, measure, and record temperatures on defective components and the normal wear, chemical contamination, corrosion, fatigue and faulty assembly that occurs in many electrical systems. The molecular friction produced by problem electrical components is visible within the infrared spectrum and recordable by infrared cameras. Overheating can occur in any electrical component including generators, transformers, pole top connections, insulators, disconnects, jumpers, shoe connections, fuse connections, switchgear, starters, contactors.
Ohm’s law states that power dissipated in the form of heat exponentially increases as the current increases. This means that the amount of heat generated in a conductor is proportional to its resistance and to the square of the current it carries, while the temperature rise depends on the rate at which the heat is dissipated through convection, radiation, and conduction.
By allowing a small amount of current to pass through electrical devices, localized "hot spots" can be detected using a thermal camera making thermal imaging a non-destructive method of detecting problems on electrical devices.
How Electrical Inspectors Use Thermal Imaging Technology:
- Connectors, Relays, and Switches: Find poorly secured, corroded, or current overloaded hardware
- Semiconductors: Find poorly bonded, die attached, open, shorted, or leaky active devices
- Circuit Boards: Find overstressed components, plated through holes, poor heat sinks, and bad solder joints
- Discrete Components: Find overstressed transformers, capacitors, and resistors
Standards for Thermal Electrical Inspections
7.1.1. Current-carrying capacity of a bus - The current-carrying capacity of a bus is limited by the temperature rise produced by the current and other factors. Buses for generating stations and substations are generally rated on the basis of the temperature rise which can be permitted without danger of overheating equipment terminals, bus connections, and joints. ANSI C37.20C-1974 (IEEE standard 27-1974) permits a hottest spot temperature rise for plain copper buses of 30 ºC (54 ºF) above an ambient temperature of 40 ºC (104 ºF), with a hottest spot total temperature limit of 70 ºC (158 ºF). The standard allows a hot spot temperature rise of 65 ºC (117 ºF) for metal-enclosed applications where silver contact surfaces are used on connections and a hot-spot temperature rise of 45 ºC (81 ºF) for silver-surfaces terminals of outgoing circuits. Aluminum used for bus work ordinarily has a conductivity of 63 percent, as compared to copper at 99 percent. For a given current rating and for equal limiting temperatures, the area of an aluminum bus will be about 133 percent of the area of a copper bus.
7.1.2. Allowable current density in a bus - Allowable current density in a bus is the amount of current that the bus can carry per square inch or cross-sectional area without exceeding the permissible temperature rise. For both ac and dc buses, densities may vary from values of 9.3. x 105 and 1.09 x 106 A/in2 (600 and 700 A/in2) in heavy current-carrying copper buses to 1.86 x 106 and 2.17 x 106 A/in2 (1,200 and 1,400 A/in2) in light buses under favorable conditions. For aluminum, densities of 75 percent of the above values are usually permitted.
7.1.3. Current-carrying capacity of conductors - No method has been generally accepted by the industry for the calculation of the current-carrying capacity of conductors for overhead power transmission lines. However, Reclamation designs its ACSR transmission line capacity in accordance with tables, charts, and procedures in the "Aluminum Electrical Conductor Handbook," 1971 edition or specific instructions from the conductor manufacturer.
7.2. Temperature of Connections - The principal function of an electrical connection is to satisfactorily carry the electrical load over its entire service life. The electrical load can be expected to have daily fluctuations from no load to full load and frequently to very heavy overloads, thus causing wide fluctuations of operating temperature. In addition, the ambient temperature can be expected to fluctuate between daily extremes and between seasonal extremes. The effect of this heat cycling on a poorly designed or improperly installed connection is frequently progressive deterioration and ultimate failure of the connection or associated equipment. Therefore, temperature rise provides an important and quite convenient method of monitoring the condition of electrical connections.
7.4. Thermographic (IR) Surveys - Sophisticated infrared equipment in vans or helicopters is now widely used throughout the electric power industry for rapid scanning of substation and switchyard bus and equipment as well as transmission lines. Available equipment can survey an average-sized switchyard in about 30 minutes. Experience in the industry has proven the technical worth and economic justification of infrared testing as a preventive maintenance tool.
7.4.1. Thermal Imaging devices - Thermal imaging devices are useful in permitting a rapid scan of switchyard buses and equipment terminals to detect temperature differences. Infrared signals from photovoltaic detectors are electronically amplified and transmitted to a small television viewing screen producing an image corresponding to the thermal patterns within the viewed scene. Temperature differences are shown in varying shades of gray or in different colors. An accessory instant camera may be used to provide a permanent record of temperature differences.
7.4.3. Scheduling of infrared surveys - Scheduling of infrared surveys to detect hot spots in electrical connections and equipment is an important facet of an effective infrared testing program. Such surveys should be conducted during periods of high system loads, which circuits in a given station are sufficiently loaded to reveal existing or potential hot spots. Subsequent surveys should be scheduled to test transfer of auxiliary buses under loaded conditions. Each hot spot detected by an infrared survey should be physically inspected and evaluated. Judging the severity of a faulty connection by temperature readings alone can be misleading. A reading of 50 ºC (90 ºF) above ambient in one case can be as serious as one 180 ºC (325 ºF) above ambient, especially if the load is low in the first case. A follow up inspection of suspect and/or repaired connections should be performed with a hand-held infrared thermometer, temperature-sensitive tape, or other suitable means.