Frequently Asked Questions

KNR experts in the field of Engineering and Consulting


Process of bringing insulation strengths of electrical equipment into proper relationship with expected Overvoltages and characteristics of Surge Protective Devices so as to reduce the risk of failure to an economically & operationally acceptable level.

Coordination between the insulation levels of protective devices and the device to be protected

  • A3- IEC
  • IEC 60071-1: Definitions, Principles & Rules
  • IEC 60071-2: Application Guide
  • IEC 60071-4: Computational guide to Insulation Coordination & Modelling of electrical networks
  • IEEE C 62.82.1: Insulation coordination- Definitions, principles & rules
  • IEEE C 62.82.2: Guide for the application of Insulation Coordination

General Questions

Why Insulation Coordination Study?

The overall aim of insulation coordination is to reduce to an economically and operationally acceptable level the cost and disturbance caused by insulation failure.

What are various overvoltage studies conducted while doing insulation Coordination Study?

Insulation Coordination study is carried out in accordance with the methodology and guidelines outlined in IEEE standard 1313.2-1999, IEC 71-2, IEC 71-4, CIGRE WG 33.01, CIGRE WG D1.03 recommendations and IEEE technical papers presented in various forums.

The following overvoltage studies are done:

a) Lightning Overvoltage (Fast front overvoltage)

b) Switching Overvoltage (Slow front overvoltage)

c) Temporary Overvoltage (Power frequency overvoltage)

d) Very Fast Transients Overvoltage or VFTO (Very fast front overvoltage)

What is Surge Arrester?

As per - IEC - 60099 -1, Surge Arrester is defined as, the device designed to protect the electrical apparatus from high transient voltages and to limit the duration and the amplitude of follow current.

What are the ideal characteristics of a Surge Arrester?

Ideal characteristics of a Surge Arrester are:

  • It should not draw any current at normal power frequency voltage.
  • It should draw current during transient overvoltage and should provide a low impedance path to ground.
  • It must be capable of interrupting the power frequency follow current after the surge is discharged to ground

Which overvoltage performance studies are required to ascertain Insulation Coordination of the Substation?

The following overvoltage performance studies are carried out to ascertain Insulation Coordination of the Substation:

  • Temporary Overvoltage
  • decides the rating of arrester
  • Lightning Overvoltage
  • decides number of arresters and their locations
  • Switching Overvoltage – for system voltage > 245kV
  • decides location of arrester and PIR requirement
  • Very Fast Transient Overvoltage – for GIS substations of system voltage > 245kV
No Data