Lightning Protection and Earth Electrode Resistance 

By on

Definition of stroke parametersMost installations involve some form of lightning protection system which is connected to an earth electrode.  The function of the earth electrode is to dissipate the lightning strike safely in to the ground.  Often the performance of the earth electrode is specified and verified by stating a maximum resistance and ensuring that the installed system meets this.  Resistance is easily understood and an easy property to measure.  However, it is only indicative of the performance of the earth electrode under lightning conditions.  Other parameters have a more signification effect.

The detailed analysis of a system is relatively complicated.  However, it is fairly easy to examine some of the concepts and see how resistance fits into the overall picture.  The attached image shows a standard short stroke  (duration less than 2 ms) as defined by IEC 62305 [1].  Although lightning strokes will be different, the important thing is the fast di/dt on the rise.  Actual parameters vary stroke by stroke,  but for a typical first stroke the mean peak current I is 33.3 kA with a dt/dt30/90% of 24.3 kA/μs [2][3].

We can use a a typical earth electrode specification of maximum resistance 10 Ω, achieved by using an earth rod to illustrate some concepts.   Typically an earth rod will have an inductance of 5x10-6 H/m [4].   If we assume the rod is 5 m long then the induced voltages due to resistance Vr and inductance Vl during the initial rise are:

myElectrical Equation 
 
and
 
 myElectrical Equation 

As can be seen the voltage due to the inductance is twice that due to the resistance.  A lightning peak of 65 kA or earth rod of 10 m the difference in voltage drops would be further exaggerated.  This clearly illustrates the importance of inductance in the design of any lighting protection earth electrode.  The use of resistance itself is not an accurate measure of performance.

This is further complicated by soil ionizations.  Due to the nature of lightning surges, soil ionization is likely to take place, reducing the apparent resistance [5].  The effect of this is to reduce resistive voltage drop and further increase the importance of the inductance of the system. 

The lighting stroke can also be looked at as a traveling wave.  In this instance the current will have to contend with the surge impedance myElectrical Equation.  In this analysis the resistance has no effect and it can be noted that the most efficient earth electrode system is one with a low  inductance and high capacitance.  An earth rod performs poorly in this regard and a better earth electrode system is a mesh type arrangement.

While the above examples are conceptual, they do illustrate the influence of inductance, capacitance are important on the performance of a lighting protection system and that of resistance is less so.  Good lightning earth electrode design should go beyond merely specifying a performance parameter based on resistance.  

References

  • [1] - IEC 62305 'Protection against lightning - Part 1 General Principals', Annex A, 2006
  • [2] - Berger K., Anderson R.B., Kroninger H., "Parameters of lightning flashes', CIGRE Electra No.41, 1975
  • [3] - Anderson R.B., Eriksson A.J., "Lighting parameters for engineering application', CIGRE, Electra No.69, 1980
  • [4] - William D. Stevenson, "Elements of power system analysis", p44-47, 1982, McGraw Hill
  • [5] - Z. Stojkovic , 'The soil ionization influence on the lightning performance of transmission lines', University of Belgrade, 1999

 



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus

  1. ram31's avatar ram31 says:
    7/2/2013 11:36 AM

    Is it allowed as per regulations to connect lightning protection earthing,telecom & ELV earthing and electrical (HV switchgear, MV switchgear, transformer, DG set) in one ring inside the ground?

    • Steven's avatar Steven says:
      7/2/2013 1:50 PM

      I think it is allowed. I would bond everything together and thats what the standards say. There are places whose local regulations disagree and if you work in one of these localities, you have choice but to keep separate.

    • 7/2/2013 5:53 PM

      Yes. As per IEC & IS all metallic non current carrying equipments including lightning protection schemes are to be bonded together. However where insulated(isolated) type lightning protection is followed, the lightning scheme components like air termination, down conductor, earth electrodes etc are to be kept isolated from other networks.


Comments are closed for this post:
  • have a question or need help, please use our Questions Section
  • spotted an error or have additional info that you think should be in this post, feel free to Contact Us



3 Phase Loads

Three phase systems are derived from three separate windings, either connected in delta or star (wye). Each winding can be treated separately, leading...

How to refer fault levels across a transformer

Over the past year or so I've been involved in on going discussions related to referring fault levels from the secondary of a transformer to the primary...

Tip – Latitude and Longitude on Large Scale Plans

If you are working on a large plan, get the real coordinates [latitude, longitude] for two or more points and add them to the drawing. That way you can...

Low Voltage Circuit Breakers

Circuit breakers are switching devices whose primary function is to isolate parts of an electrical distribution system in the even of abnormal conditions...

Voltage Levels to IEC 60038

The standard aims to consolidate AC and traction voltages within the industry and defines the following bands: band 1 - A.C. systems 100 V to 1...

Questions - Reputation and Privilege

Our question and answer system while letting you do exactly what it says, is much more.  It is a dynamic user driven system, where our users not only ask...

Electromechanical Relays

Electromechanical relays have been the traditional backbone of electrical protection systems.  While over recent years these have been replaced by microprocessor...

The dc resistance of conductors

This is the first of two posts on the resistance of conductors. In the next post I will look at the ac resistance, including skin effect and we deal with...

Skin Tapping Input

Tapping your forearm or hand with a finger could soon be the way you interact with gadgets. A new technology created by Microsoft and Carnegie Mellon ...

Large Hadron Collider

The 27 km, Euro 6 billion  Collider lies on the border between France and Switzerland, took nearly 30 years to complete.  Some of the lofty goals for the...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note