Surge Arresters are classified based on their rating, the amount of energy to be dissipated and their application. 

Surge Arresters are classified into three types.

Station Class Arresters

Station Type arrestors are used in substations where the equipments are connected to the Transmission lines.  These arrestors are designed to discharge high amounts of energy and have elaborate systems of pressure relief.  They are used for equipments whose rating is above 20 MVA

Intermediate Class Arresters

These are used for Transformers and rotating machines of 1 - 20MVA.  Intermediate Class Arresters are a compromise between reliability and economy.  Typical examples of applications would be dry type transformers, switching and sectionalizing equipment. 

Distribution Class Arresters

Distribution Class arresters are used in transformers and rotating equipment with a powe rating below 1000 kVA.  These arresters are used on exposed lines connected to the equipment. 

It is important that the surge arrestor is selected properly to ensure correct operation and adequate protection to the equipment. 

There are three important steps in surge arrestor selection. 

Deciding the Surge Arrestor Voltage.  
This is decided by the nature of the earthing i.e. whether we use grounded or ungrounded systems.

Selecting the class of arrestor.
Surge Arrestors are classified into three types Station Class, Intermediate Class and Distribution Class.  The class determines the degree of protection, capacity and, ultimately, the price.  The Station Class Surge Arrestor is the most expensive. 

Deciding on the location of the surge arrestor. 
The location of the arrestor is crucial.  For an equipment to be properly protected, the arrestor should be placed as close to the equipment as possible. 

Spark Gap Arresters are the oldest form of Surge arresters.  The spark Gap lightning arrestors have two horned terminals with a distance between them.  One of the terminals is connected to the line while the other is connected to the earth.  
During normal current, the gap prevents leakage of current.  In the event of an surge, the voltage across the gap is sufficient to cause breakdown and start conducting.  This reduces the steepness of the surge. 

The arc which is formed during operation is extinguished as it rises across the airgap.

The downside of the spark gap arresters is that it is difficult to precisely set the trigger voltage.  Another difficulty with Spark Gap lightning arresters is to switch off the follow-on current after the voltage has returned to normal.

Surges in overhead lines affect substation equipments, can cause backflashovers and can affect power quality.

Surges in overhead lines can be categorized into two types
  1. Internal Surges and
  2. External Surges
Internal Surges are surges caused due to switching of capacitors, earth faults, sudden load swings, etc.  External Surges are caused due to lightning strikes on power lines.

There are two methods of protecting Transmission lines from surges. 

  1. Shielding Methods and
  2. Discharge Methods

Shielding methods use a shielding line run along the top of the tower to shield the lines from lightning.  The shielding line offers protection up to an angle of 30 degrees from the vertical on either side.  Sometimes two shielding lines are run in parallel to increase the area under protection. 

The other method of protection is the discharge method of protection.  The discharge method of protection uses surge arrestors which discharge the surge through themselves.  These arrestors are mostly non-linear resistors or spark gap based arrestors

Insulated Tools are an essential part of the maintenance equipment in all Electrical Installations.  Insulated are tools such as spanners, pliers, screw drivers which are insulated with special insulating material. 

This is necessary when working with live equipment.  In many cases, there are chances of tools getting accidentally dropped on to live bus bars or terminals. A screw driver making contact to a live circuit can get in contact with an earthed panel.    These can result in a flash over and an explosion with serious consequences. 

All insulated tools are labelled with a voltage rating.  Insulated Tools should never be used as the primary method of protection.  For instance, all insulated tools should be used with an Electrical Glove of appropriate rating