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



Voltage Drop in Installations - Concepts

Problems on achieving maximum voltage drop within an installation come up often. Depending where you live, local regulations will have different limits...

How D.C. to A.C. Inverters Work

Traditionally generation of electricity has involved rotating machines to produce alternating sinusoidal voltage and current (a.c. systems). With the development...

Introduction to Current Transformers

Current transformers (CTs) are used to convert high level currents to a smaller more reasonable level for use as inputs to protection relays and metering...

GE's Shingijutsu Factory

GE's latest thinking on product manufacturing is he Shingijutsu philosophy or Lean production system. They have started applying this at the Louisville...

Bows and Arrows

It starts with me reading one of the Horrible History books with my son (Groovy Greeks). Arrows were mentioned which lead to the discussion of the bodkin...

RLC Circuit, Resistor Power Loss - some Modelica experiments

Modelica is an open source (free) software language for modelling complex systems. Having never used it before, I thought I would download a development...

Gas Insulated or Air Insulated Switchgear

Various arguments exist around SF6 Gas Insulated (GIS) and Air Insulated (AIS) medium voltage switchgear. Recently we had to change a GIS design to AI...

Software Usage Guidelines

Using software in our  work is essential for most of us and we are becoming even more dependant on it's use.  While software is a great asset, many times...

Fault Calculation - Per Unit System

Per unit fault calculations is a method whereby system impedances and quantities are normalised across different voltage levels to a common base.  By removing...

Maximum Demand for Buildings

Estimating maximum demand is a topic frequently discussed. Working out how much power to allow for a building can be very subjective . Allowing too much...

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