What is Infrared Scanning? How is it done?
Infrared scanning is a method that is utilized to locate high-resistance connections ("hot spots") by using a camera that turns infrared radiation into a visible image.
This test is performed with the equipment in service carrying normal load current, which is a major advantage because it does not interrupt normal production. Exposure to energized equipment, of course, carries the possibility of exposure to electrical hazards.
The operator shall recognize and deal with such potential hazards accordingly.
The most common use of infrared scanning is to locate loose or corroded connections in switchboards, panel boards, bus ways, and motor starters.
It is a comparative type test in which the person who performs the scan is looking for an area that appears brighter (hotter) than a similar area, such as a lug connection on phase "A" as it compares to similar connections on phases "B" and "C"
The person should be aware of how unbalanced loading may affect heating, thereby giving an indication similar to looseness.
One limitation of infrared scanning is that the equipment has to be carrying enough load for the hot spots to be visible.
At lower loads, there may not be enough heat generated to locate a problem, even when the connections are significantly looser than they should be.
MOTOR SURGE COMPARISON TESTING BASIC INFORMATION AND TUTORIALS
What is Motor surge comparison testing? How is it done?
Motor surge comparison testing addresses the problem of insufficient test voltage to find the weak insulation between turns by utilizing a high-voltage pulse. Two identical high-voltage pulses are introduced into two windings of a motor.
The propagation of the pulse through one winding is compared to the propagation of the identical pulse through the winding next to it.
An oscilloscope (usually built into the surge tester) is used to look at the traces and to compare them. The patterns should be identical (or very nearly so) and can appear as one trace (two superimposed traces) if both windings are good.
A turn-to-turn failure (or a failure to ground) is indicated by two distinctly different traces appearing on the oscilloscope.
Motor surge comparison testing has been used by motor winding shops for many years. There are now portable models that are available for field testing.
Motor surge comparison testing has proven to be a valuable tool in detecting the early stages of a winding failure, both from the standpoint of preventing an unexpected failure during operation and preventing a catastrophic failure of the motor so it can be repaired instead of needing to be replaced.
Motor surge comparison testing addresses the problem of insufficient test voltage to find the weak insulation between turns by utilizing a high-voltage pulse. Two identical high-voltage pulses are introduced into two windings of a motor.
The propagation of the pulse through one winding is compared to the propagation of the identical pulse through the winding next to it.
An oscilloscope (usually built into the surge tester) is used to look at the traces and to compare them. The patterns should be identical (or very nearly so) and can appear as one trace (two superimposed traces) if both windings are good.
A turn-to-turn failure (or a failure to ground) is indicated by two distinctly different traces appearing on the oscilloscope.
Motor surge comparison testing has been used by motor winding shops for many years. There are now portable models that are available for field testing.
Motor surge comparison testing has proven to be a valuable tool in detecting the early stages of a winding failure, both from the standpoint of preventing an unexpected failure during operation and preventing a catastrophic failure of the motor so it can be repaired instead of needing to be replaced.
TRANSFORMER TURNS RATIO TESTING BASIC INFORMATION AND TUTORIALS
What is Transformer turns ratio (TTR) testing? How is it done?
The voltage across the primary of a transformer is directly proportional to the voltage across the secondary, multiplied by the ratio of primary winding turns to secondary winding turns.
In order to ensure that the transformer was wound properly when it was new, and to help locate subsequent turn-to-turn faults in the winding, it is common practice to perform a TTR test.
The simplest method would be to energize one primary winding with a known voltage (that is less than or equal to the windingÕs rating) and measure the voltage on the other winding.
Since source test voltages can fluctuate, it is often more accurate to use a test set, designed for this purpose, that creates the test voltage internally, thus giving a direct read-out of the ratio measured.
The voltage across the primary of a transformer is directly proportional to the voltage across the secondary, multiplied by the ratio of primary winding turns to secondary winding turns.
In order to ensure that the transformer was wound properly when it was new, and to help locate subsequent turn-to-turn faults in the winding, it is common practice to perform a TTR test.
The simplest method would be to energize one primary winding with a known voltage (that is less than or equal to the windingÕs rating) and measure the voltage on the other winding.
Since source test voltages can fluctuate, it is often more accurate to use a test set, designed for this purpose, that creates the test voltage internally, thus giving a direct read-out of the ratio measured.
WINDING AND CONTACT RESISTANCE TEST BASIC INFORMATION AND TUTORIALS
What are winding and contact resistance testing?
Winding and contact resistance are similar in that both are looking for a very low ohmic value, since they are measuring the "resistance" of a component that is supposed to conduct electricity.
A Kelvin Bridge has long been a standard method of measuring low values of resistance, and is still in use today.
With the advent of electronics, there are digital meters available that also are capable of measuring very low values (milliohms or microohms) of resistance.
The typical low-resistance ohmmeter uses four terminals (to eliminate lead resistance) in which a dc current is injected into the conductor to be measured and the voltage drop across the conductor is measured.
Contact resistance test sets can be used to measure the resistance of bus joints and cable joints, as well as the closed contacts of a circuit breaker or motor starter.
In many cases, it is a comparative type test in which the resistance of one set of contacts is compared to the readings obtained from the other two phases of the same, or a similar, piece of equipment.
Winding resistance differs from contact resistance in that the inductance of large windings can interfere with the operation of the test set.
There are test sets, available commercially, that are designed specifically for large transformer and motor windings, for cases in which a standard low-resistance ohmmeter is not adequate.
Winding and contact resistance are similar in that both are looking for a very low ohmic value, since they are measuring the "resistance" of a component that is supposed to conduct electricity.
A Kelvin Bridge has long been a standard method of measuring low values of resistance, and is still in use today.
With the advent of electronics, there are digital meters available that also are capable of measuring very low values (milliohms or microohms) of resistance.
The typical low-resistance ohmmeter uses four terminals (to eliminate lead resistance) in which a dc current is injected into the conductor to be measured and the voltage drop across the conductor is measured.
Contact resistance test sets can be used to measure the resistance of bus joints and cable joints, as well as the closed contacts of a circuit breaker or motor starter.
In many cases, it is a comparative type test in which the resistance of one set of contacts is compared to the readings obtained from the other two phases of the same, or a similar, piece of equipment.
Winding resistance differs from contact resistance in that the inductance of large windings can interfere with the operation of the test set.
There are test sets, available commercially, that are designed specifically for large transformer and motor windings, for cases in which a standard low-resistance ohmmeter is not adequate.
PREVENTIVE MAINTENANCE DESIGN CONSIDERATIONS BASIC INFORMATION
The best preventive maintenance programs start during the design of the facility. A key design consideration in order to support preventive maintenance is to accommodate planned power outages so that maintenance activities can proceed.
For example, if delivery of power is not a 24 hour necessity, then extended outages after normal work hours can be allowed for maintenance activities. Otherwise, consider design features that can speed up the maintenance process or reduce the duration of the outage to loads.
These might include redundant circuits, alternate power sources, or protective devices such as drawout circuit breakers (rather than fixed-mount circuit breakers).
Additional consideration should be given to the accessibility of the electrical equipment for maintenance. Circuit breaker location can be critical to the maintenance process.
An example would be circuit breakers that are installed in a basement that has only stairway access through which equipment can be brought down to the circuit breaker location. In addition, access to the back of switchboards or switchgear, as opposed to their being mounted against the wall, may be necessary in order to perform thorough maintenance.
The environment in which the equipment is installed can play an important part in maintenance. Where equipment is mounted (inside or outside) and whether it is properly enclosed and protected from dust, moisture, and chemical contamination are all factors that influence the frequency with which maintenance tasks should be performed.
The design phase is also the period in which the establishment of baseline data for the equipment should be considered. This can be done by including in the design specifications the acceptance or start-up testing of the equipment when it is Þrst installed. The InterNational Electrical Testing Association (NETA) provides detailed specifications for electrical power equipment in NETA ATS-1995 [B1].
Design drawings are very important to an effective maintenance program. As-built drawings should be kept up-to-date. An accurate single-line diagram is crucial to the effective and safe operation of the equipment.
This helps the operator to understand the consequences of switching a circuit that can interrupt power in an undesirable or unplanned mode. More significantly, it can help avoid the accidental energization of equipment.
As part of the procurement of the electrical equipment, consideration should be given to the tools and instruments that are required to perform effective maintenance, such as hoists or manual-lift trucks that are used to remove and install circuit breakers. These tools and instruments will help to ensure safety and productivity. Finally, the installation, operation, and maintenance manuals should be obtained and filed.
For example, if delivery of power is not a 24 hour necessity, then extended outages after normal work hours can be allowed for maintenance activities. Otherwise, consider design features that can speed up the maintenance process or reduce the duration of the outage to loads.
These might include redundant circuits, alternate power sources, or protective devices such as drawout circuit breakers (rather than fixed-mount circuit breakers).
Additional consideration should be given to the accessibility of the electrical equipment for maintenance. Circuit breaker location can be critical to the maintenance process.
An example would be circuit breakers that are installed in a basement that has only stairway access through which equipment can be brought down to the circuit breaker location. In addition, access to the back of switchboards or switchgear, as opposed to their being mounted against the wall, may be necessary in order to perform thorough maintenance.
The environment in which the equipment is installed can play an important part in maintenance. Where equipment is mounted (inside or outside) and whether it is properly enclosed and protected from dust, moisture, and chemical contamination are all factors that influence the frequency with which maintenance tasks should be performed.
The design phase is also the period in which the establishment of baseline data for the equipment should be considered. This can be done by including in the design specifications the acceptance or start-up testing of the equipment when it is Þrst installed. The InterNational Electrical Testing Association (NETA) provides detailed specifications for electrical power equipment in NETA ATS-1995 [B1].
Design drawings are very important to an effective maintenance program. As-built drawings should be kept up-to-date. An accurate single-line diagram is crucial to the effective and safe operation of the equipment.
This helps the operator to understand the consequences of switching a circuit that can interrupt power in an undesirable or unplanned mode. More significantly, it can help avoid the accidental energization of equipment.
As part of the procurement of the electrical equipment, consideration should be given to the tools and instruments that are required to perform effective maintenance, such as hoists or manual-lift trucks that are used to remove and install circuit breakers. These tools and instruments will help to ensure safety and productivity. Finally, the installation, operation, and maintenance manuals should be obtained and filed.
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