Lightning Arrestor Ratings

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The rated voltage of a lightning arrester is the maximum voltage at which the arrester is designed to perform its duty cycle.

The MCOV is the maximum continuous operating voltage at which the arrester is designed to begin clamping the surge voltage across its terminals.

The range between the MCOV and the rated voltage is the operating range of the arrester. Below the MCOV, the arrester will not perform. Above the rated voltage, there is danger of thermal runaway or permanent damage to the arrester.

The pressure relief rating is the maximum current the arrester can dissipate without damaging itself. This rating is useful in installations usually hit by lightning .

In IEC, I think relief classes are in A, B, and C with A being the highest rated. In ANSI/IEEE ratings are 100 kA, 85 kA, 10 kA, etc.

But what is LAs duty cycle?

Then MCOV should be written as minimum continuous operating voltage as per your comment?

1. Duty cycle of an arrester is its operating voltage for which its designed to operate during which the arrester must be able to withstand lightning surge with a wave shape of 8/20 at 1-minute intervals. For station class arresters, the magnitude of this surge is 10kA (10,000 Amperes) or larger. For intermediate and distribution class arresters, this surge is 5 kA (5000 Amperes). The surge wave shape is an 8/20, which means the current wave reaches a crest in 8 milliseconds and diminishes to half the crest value in 20 milliseconds.

2. Not really. Note the word 'continuous'. At MCOV, the arrester can operate almost infinitely without overheating.

Let me correct myself regarding number one. The one I'm referring here is the lightning protection level.

To go back to your question, the rated voltage is the temporary overvoltage capability of the arrester. It can be only applied temporarily, usually 10 seconds, although some manufacturers specify 100 seconds. Under this condition, a resistive leakage current of about 1 mA is dissipated to the earth. If the arrester is exposed to this phenomenon for more than 10 or 100 seconds as the case may be, it will render the arrester of recooling and will become thermally unstable until it will experience thermal runaway.

Dear,

The "Arrester Rated Voltage"-is the maximum power frequency voltage that is applied in the operating duty test for 10 s. It is furthermore, the reference parameter to establish the power frequency voltage versus time characteristic of the arrester, and for defining the test requirements for the line discharge test.

"MCOV" = Maximum Continuous Operating Voltage-The continuous operating voltage is the maximum permissible value of a sinusoidal power frequency voltage, which may be continuously applied betwen the arrester terminals.

"Pressure relief class" -A number related to the capability of an arrestor to withstand internal fault current without violent shattering of the housing.(See Clause 5.11 of IEC 60099-4)

Reference: 60099-5/IEC1996+A1:1999. Pages:31/33.

Regards:

JSPD

Angola/West Africa