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DEVELOPING AND IMPLEMENTING AN EFFECTIVE ELECTRICAL MAINTENANCE PROGRAM Part I: Selection Of A Testing Firm Part II: Developing Specifications Part III: Anatomy Of a Shutdown; Implementing The Program
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Electrical Testing
Commonly referred to as a Megger test, which is a trademark of Biddle/AVO. Test sets originally were the hand crank type and now the battery operated type are becoming popular. Shown here is a 5000 volt Megger manufactured by This test can be performed at 500, 1000, 2500, 5000 volts or occasionally up to 15,000 volts. A DC voltage is applied to the equipment being tested to determine the integrity of the insulation. Results of the test are typically measured in megohms and sometimes gigohms. Battery operated 5kV Insulation Resistance Tester Insulation resistance tests can be accomplished by various methods however the three most common tests or measurements are: Short Time Reading (Spot Check): The most common test method, typically test voltage is applied for one minute or less and recorded. Test results are compared over time to determine if the insulation is deteriorating. Dielectric Absorption Ratio: The ratio of the 60 second reading to the 30 second reading. In good insulation the test results should increase over time and this ration should be above 1. Polarization Index: The ratio of the 10 minute reading to the 1 minute reading. A result of less than 1 indicates a breakdown of the insulation or requirement for maintenance. This test is typically used for transformers, motors and cables. High Potential Testing Commonly referred to as a "Hipot". This test applies a DC voltage well above the rated operating voltage for a short duration, usually around 1 minute. AC High Potential tests can also be completed however, the equipment is much larger and the test voltage is less. The most common test is the DC High Potential Test. This test, like the insulation resistance test, ensures the insulation integrity of the electrical equipment. This test should be performed in strict accordance with the manufacturer's recommendations. For maintenance purposes, this test is completed on equipment rated above 1000 volts. Two types of test are: Step-Voltage: Voltage is applied in several increments, usually more than 5 and less than 10 of the desired test voltage, and held for a short period of time, around 1 minute. After each time period, leakage current is recorded on a data form or plotted on a special graph. A skilled technician can recognized insulation breakdown or impending failure by viewing this data during the test. Proof Test: Test voltage is applied for 1 minute and leakage current recorded. Typically, this test is a Go/No Go type test, i.e., if the test set does not trip offline during the test, the equipment passes.
Transformer Turns Ratio
Determines the number of turns in one winding of a transformer in relation to the other winding. The test set applies a low voltage to one of the windings and compares it, by measurement, to the other winding and generates a ratio. This ratio should be within .5% of the calculated value. Maintenance testing is usually done at the "As Found" tap setting but can be completed for all tap settings. A deviation from the .5% can indicated shorted turns. Three phase and single phase turns ratio tester
Contact Resistance
Utilized to determine the quality of the contacts and contact connections in switches and circuit breakers. The test equipment utilized for this test applies 1, 10 or 100 amperes across the contacts and measures the resistance. The most common portable test sets are the 10 ampere. Typical readings will be in the microhm range. Alternate tests can be done to measure voltage drop across contacts and variants of this test are referred to as watts loss. High resistance across contacts can cause heating which will shorten the life of the contacts and possibly the surrounding insulation. Low Resistance Ohmmeter
Primary Current Injection An AC current well above the circuit breaker's rating is applied across each pole of the circuit breaker. After a period of time the circuit breaker will trip and the resulting time delay compared with published manufacturers trip curves to determine if the trip function is operating within specification. This test also checks proper operation of the circuit breaker current transformers or sensors as well as the trip unit. This test is usually performed on a 3-5 year interval. For practical purposes this test is usually performed on circuit breakers above 400 amperes. The equipment for this test is quite large, difficult to move and has special input power requirements. Secondary Current Injection
A specialized test set, usually supplied by the circuit breaker manufacturer, tests the trip unit of the circuit breaker. Functions tested include, timing of the various trip functions and the trip solenoid. This tests effectively tests most items in the trip circuit except for the current transformers or sensors. Square D Company Secondary Current Injection Test Set (Universal Test Set) Insulating Liquid Testing Liquid filled transformers, tap changers and oil circuit breakers require periodic testing of the insulating medium, oil, silicone, PCB, Rtemp. A small sample of the liquid is drawn from the tank, preferably through a sampling valve. This sample is sent to a laboratory and analyzed for PCB content, dissolved gas analysis, dielectric breakdown and a long list of other tests. This comprehensive testing can identify potential problems with the insulating fluid including water, dielectric breakdown. Validity of the test results depends a great deal on the quality of the sample drawn. Special procedures and containers are utilized and should be followed closely. Samples can be analyzed in accordance with the following standards.
The NEC states that ground fault systems should be performance tested when first installed. Maintenance of these systems is left to the manufacturer;s recommendation. Most ground fault relays installed in low voltage equipment have a monitor panel that allows for periodic testing. This testing can include a Trip and No Trip test. The manufacturer's procedures should be closely reviewed and followed to prevent inadvertent operation of the equipment. It is possible to test the ground fault system by applying a high current in the equipment and measuring the time delay for the ground fault relay to operate, however this type of test is time consuming. Monitor Panel for Ground Fault Relay High Pressure Contact switch with integral ground fault relay and monitor panel |
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