MAXIMUM CURRENT RATING
Maximum current is the most current an arrestor can conduct without damage. In order to be meaningful, it is also necessary to state the time for which the current is conducted. Usually maximum current is tested using an eight by twenty test wave. This means the test surge rises from zero to maximum amplitude in eight microseconds, then declines back to zero in twelve microseconds for a total of twenty microseconds. In order to achieve a very high maximum current, an arrestor must have a very high conductivity. Multi-pole arrestors are generally rated at maximum current per pole. For example, a three phase 20,000 amp arrestor would not be rated 60,000 amps.
MAXIMUM NUMBER OF SURGES
The maximum number of surges which an arrestor can conduct should be stated with a description of the surges. Some arrestors might have a high current rating for one time but a lower rating for maximum surge life. Cooling time between surges is another factor.
Response time is a measurement of how quickly an arrestor reacts to a surge. Response times of five nanoseconds or less without a current stated are usually measurements at a one milliamp bench test. A more meaningful test is to measure time to clamp a specific amount of current with leads attached.
Maximum joules rating is a measure of energy an arrestor can absorb without damage. Joule rating takes into consideration surge charge and voltage. The energy absorbed by an arrestor is converted into heat and mechanical stress. When the joule rating is exceeded the arrestor is damaged. Arrestors with a high conductivity have a high joule rating which permits them to handle more current more rapidly. Joules may be calculated by multiplying voltage by charge in coulombs. A coulomb is a charge of sixty two, followed by seventeen zeros, electrons. To have meaning, the joule rating must be taken at the arrestors rated current.
Spark-over voltage is the voltage at which the arrestor will begin to conduct if a test voltage typically limited to one milliamp is slowly increased. This test is useful to insure that the line voltage will not trigger the arrestor. A sixty cycle power frequency of 125-250 volts will have a top-of-the-wave voltage of 175 to 350 volts. It is essential that spark-over be well above those levels to assure that the arrestor does not conduct the line voltage as immediate damage would result. Spark-over voltage will also affect response time and discharge (clamping) voltage for arrestors having a low conductivity. A change in spark-over of 500 volts might change response time by one nanosecond. Arrestors will exhibit different spark-over voltage in response to different means of testing. If one wishes to insure that the arrestor will resist alternating power voltage, then alternating voltage should be used for the test. If one wishes to determine the voltage at which the arrestor reacts to a surge, then a surge impulse should be used for the test.
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