To ensure proper protection, you must know which type of lightning arrester to use and what its rating means.
Lightning arresters (LAs) are among the most misunderstood and misapplied protective devices in our industry. Yet, with the increasing use of sensitive electronic equipment, they're almost a "must," because they divert the effects of extremely short-term overvoltages on an electrical system to ground. These overvoltages usually stem from lightning strikes.
How do lightning arresters divert the energy associated with lightning strikes? Lightning arresters are made up of varistors whose resistance reduces as the implied voltage increases. This reduction in resistance continues until the lightning arrester acts just like a direct short to ground. Upon reaching this condition, the lightning energy diverts to ground away from the protected equipment, thus reducing the effect of the overvoltage. That said, how do you know which type of LA to use? And what do their ratings mean?
What's in a class? ANSI/IEEE C62.1 (IEEE Standard for Gapped Silicon-Carbon Surge Arresters for AC Power Circuits) and C62.11 (IEEE Standard for Metal-Oxide Surge Arresters for Alternating Current Power Circuits) separate LAs into four classes: station, intermediate, distribution, and secondary.
Each type provides different levels of protection and energy diversion. The station class offers the best protective level and is capable of diverting the most energy. The intermediate class has the next best level; but a lower energy diversion capability than station-type LAs. The distribution class provides the worst protective levels and lowest energy diversion. Because the secondary-type LA doesn't overlap in voltage range with any of the other classes, it's difficult to make a direct comparison. Keep in mind: As you progress from station to distribution class, the cost significantly declines, but so does the protective level. By comparing cost to benefit, you can get the most efficient arrester for the application.
What's in a rating? A metal oxide varistor (MOV) arrester has two voltage ratings: duty cycle and maximum continuous operating voltage, unlike the silicon carbide that just has the duty cycle rating.
Duty cycle rating. The silicon carbide and MOV arrester have a duty cycle rating (in kV), which duty cycle testing established. This testing subjects an arrester to an AC rms voltage equal to its rating for 24 min, during which the arrester must withstand lightning surges at 1-min intervals. The magnitude of the surges is 10kA (10,000A) for station class arresters and 5kA for intermediate and distribution class arresters. The surge waveshape is an 8/20, which means the current wave reaches a crest in 8 ms (8 microseconds or 0.000008 sec) and diminishes to half the crest value in 20 ms.
Maximum continuous operating voltage rating (MCOV). The MCOV rating is usually 80% to 90% of the duty cycle rating. Table 2 lists the MCOV ratings of various MOV arresters.
The MCOV rating of an MOV arrester is important because it's the recommended magnitude limit of continuously applied voltage. If you operate the arrester at a voltage level greater than its MCOV, the metal oxide elements will operate at a higher-than-recommended temperature. This may lead to premature failure or shortened life.
A closer look. The conductive elements of most LAs are made of either silicon carbide or metal oxide. An insulated housing (either porcelain or polymer-based rubber) surrounds the conductive elements.
Silicon carbide LAs. This design uses nonlinear resistors made of a bonded silicon carbide placed in series with gaps. The function of the gaps is to isolate the resistors from the normal steady-state system voltage. One major drawback is the gaps require elaborate designs to ensure a consistent spark-over level and positive clearing (resealing) after a surge passes. This design has lost popularity due to the emergence of the MOV arrester.
MOV LAs. The MOV design usually does not require series gaps to isolate the elements from the steady-state voltages because the material (zinc oxide) is more nonlinear than silicon carbide. This trait results in negligible current through the elements when you apply normal voltage. This leads to a much simpler arrester design.
An insulated housing surrounds series disks of zinc oxide in an MOV arrester. The disks have a conducting layer (generally aluminum) applied to their flat faces to ensure a proper contact and uniform current distribution within the disk. This design results in no "gaps;" thus, the reference to the MOV arrester as the "gapless" arrester. The MOV arrester design has become the most preferred because of its simplicity and resulting reduced purchase cost.