SELECTING LIGHTNING ARRESTORS for SHORE POWER

[GordMay]

HOW LIGHTNING ARRESTORS AFFECT SURGES

Surges are comprised of two elements: voltage and quantity of charge. A very high voltage surge can damage equipment by breaking down the insulating medium between elements in a circuit or between those elements and a ground. The amount of damage will be determined by the current from the charge and/or current from the power source. In order to protect a circuit from damage, a surge arrestor must conduct sufficient charge from the surge to lower the surge voltage to a safe level quickly enough to prevent circuit insulation from breaking down.

All circuits can withstand a high voltage for a short time. The shorter the time becomes, the higher the tolerable voltage becomes. Consider a fifty thousand volt surge impressed on a two-hundred-forty volt apparatus having a surge arrestor connected parallel. The surge arrestor will begin to conduct the charge, bleeding it out of the circuit. As the charge is removed from the circuit, the surge voltage will fall. As the charge approaches zero the surge voltage will approach zero. If this happens quickly enough the apparatus will be protected.

How quickly an arrestor can eliminate a surge from a circuit depends on four factors: the magnitude of the voltage, the quantity of the charge, the speed at which the arrestor starts conducting, and the conductivity of the arrestor. Given two arrestors, one having double the conductivity of the other, one will handle the surge twice as rapidly as the other. Given two arrestors of the same conductivity but one which begins to conduct more quickly, the quicker one will eliminate the surge from a circuit more quickly.

CLAMPING VOLTAGE VS. DISCHARGE VOLTAGE

There is no one clamping voltage for any arrestor. The clamping voltage will vary according to the amount of current being conducted, the internal resistance of the arrestor, the response speed of the arrestor, and the point in time at which the clamping voltage is measured. When a clamping voltage is specified, the current being clamped should be stated. For example, 500 volts at 1000 amps. Anytime there is a clamping voltage specified with no current there is no real meaning. If one uses a negligible current, such as one milliamp, any clamping voltage can be achieved. However, there is no protection afforded.

Consider a surge which rises from zero to fifty thousand volts in five nanoseconds, connected to an arrestor which starts to conduct at five nanoseconds, and clamps the surge to 500 volts in 100 nanoseconds. At any point in time during the one-hundred five nanoseconds, the clamping (discharge) voltage would be different. Even though the clamping voltage can be said to be 500 volts, if measured at twenty-five nanoseconds the clamping voltage would be above twenty-five thousand volts. An arrestor with a low ultimate clamping voltage might have a low conductivity which would cause the high voltage to exist in the circuit for a longer period of time. Arrestors with a high conductivity (low internal resistance) can conduct surges from the circuit more rapidly. Arrestors having a high current rating will have a high conductivity and will conduct a surge from the circuit more rapidly. The quicker a surge is eliminated, the more likely the equipment will be protected. Any reference to clamping voltage should always include the amount of current being clamped, and the clamping time.

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

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.

JOULES

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 (62 x 10 to the 17Th). To have meaning, the joule rating must be taken at the arrestor’s rated current.

SPARK-OVER VOLTAGE

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.