advanced search

Arrest Your Lightning Protection Problems

Jun 1, 1999 12:00 PM, By Jerry R. Borland, P.E., Technical Editor

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.

Want to use this article? Click here for options!

© 2012 Penton Business Media, Inc.

---

Acceptable Use Policy blog comments powered by Disqus

Browse Back Issues Select an Issue EC&M Magazine | April 2012 March 2012 February 2012 January 2012 December 2011 November 2011 October 2011 September 2011 August 2011 July 2011 June 2011 May 2011 April 2011 March 2011 February 2011 January 2011 December 2010 November 2010 October 2010 September 2010 August 2010 July 2010 June 2010 May 2010 April 2010 March 2010 February 2010 January 2010 December 2009 November 2009 October 2009 September 2009 August 2009 July 2009 June 2009 May 2009 April 2009 March 2009 February 2009 January 1, 2009 December 2008 November 2008 October 1, 2008 September 2008 August 2008 July 2008 June 1, 2008 May 2008 April 2008 March 2008 February 2008 January 2008 December 2007 November 2007 October 2007 September 2007 August 2007 July 2007 June 2007 May 2007 April 2007 March 2007 February 2007 January 2007 December 2006 November 2006 October 2006 September 2006 August 2006 July 2006 June 1, 2006 May 1, 2006 April 2006 March 1, 2006 February 2006 January 2006 December 2005 November 2005 October 2005 September 2005 August 2005 July 2005 June 2005 May 2005 April 2005 March 2005 February 2005 January 2005 December 2004 November 2004 October 2004 September 2004 August 2004 July 2004 June 2004 May 2004 April 2004 March 2004 February 1, 2004 January 1, 2004 December 1, 2003 November 1, 2003 October 1, 2003 September 1, 2003 August 1, 2003 July 1, 2003 June 1, 2003 May 1, 2003 April 1, 2003 March 1, 2003 February 1, 2003 January 1, 2003 December 1, 2002 November 1, 2002 October 1, 2002 September 1, 2002 August 1, 2002 July 1, 2002 June 1, 2002 May 1, 2002 April 1, 2002 March 1, 2002 February 1, 2002 January 1, 2002 December 1, 2001 November 1, 2001 October 1, 2001 September 1, 2001 August 1, 2001 July 1, 2001 June 1, 2001 May 1, 2001 April 1, 2001 March 1, 2001 February 1, 2001 February 1, 1995 January 1, 2001 December 1, 2000 November 1, 2000 October 1, 2000 September 1, 2000 August 1, 2000 July 1, 2000 June 1, 2000 May 1, 2000 April 1, 2000 March 1, 2000 February 1, 2000 January 1, 2000 December 1, 1999 November 1, 1999 October 1, 1999 September 1, 1999 August 1, 1999 July 1, 1999 June 1, 1999 May 1, 1999 April 1, 1999 March 1, 1999 February 1, 1999 January 1, 1999 December 1, 1998 November 1, 1998 October 1, 1998 September 1, 1998 August 1, 1998 July 1, 1998 June 1, 1998 May 1, 1998 April 1, 1998 March 1, 1998 February 1, 1998 January 1, 1998 December 1, 1997 November 1, 1997 December 1, 1996 November 1, 1996 October 1, 1996 September 1, 1996 August 1, 1996 July 1, 1996 June 1, 1996 May 1, 1996 April 1, 1996 March 1, 1996 February 1, 1996 January 1, 1996 December 1, 1995 November 30, 1995 November 1, 1995 October 1, 1995 September 1, 1995 August 1, 1995 July 1, 1995 June 1, 1995 May 1, 1995 April 1, 1995 March 1, 1995

browse e-newsletters