The ratings of a lightning arrester depend on three main factors such as rated voltage, rated frequency, and rated nominal discharge current.
All lightning arresters should withstand the maximum continuous operating voltage (MCOV). This is the voltage at which the leakage current flowing through the arrester is limited up to its specified rated value. The voltage rating of a lightning arrester depends on the type of neutral provided in the system. There are mainly three types of neutral used in power system networks.
Effectively Earthed System
In this system, the star point is solidly earthed. The calculation of the rated voltage of a 132KV system is given below.
- The line to line nominal voltage is 132 KV.
- The line to line highest system voltage is 132 × 1.1 = 145 KV.
- The line to earth highest system voltage is 145/√3 KV.
- The line to earth pick system voltage is 145 ×√2/ √3 KV ≈ 120 KV.
In the case of solid earth or effective earth system, this will be directly the voltage rating of the lightning arrester.
Non-Effectively Earth System
When the system is not directly earthed that means it is earthed through any resistance or reactance is known as a non-effectively earthed system. For this type of system, the voltage rating of lightning arresters will be calculated as follows.
- The line to line nominal voltage is 33KV.
- The line to line high system voltage is 33 × 1.1 = 36 KV.
- The line to earth highest system voltage is 36 / √ 3 KV.
- The line to earth pick system voltage is 36 × √2/ √3 = 30 KV.
- Factor 1.4 is multiplied with that value for calculating the rated voltage of lightning arresters in non-effectively earthed systems.
- Therefore the rated voltage of lightning arresters, in that case, will be 30 × 1.4 = 42 KV.
The standard rated frequency of a lightning arrester is nothing but the power frequency of the system and that is 50 Hz.
Rated Nominal Discharge Current
It is the allowed pick discharge current flowing through the arrester during discharge of surge energy. There are mainly four standard current ratings available and these are 1.5 KA, 2.5 KA, 5 KA, and 10 KA.
- In the system voltage above 66KV, normally lightning arresters have 10 KA rated nominal discharge current.
- In the system having a nominal voltage below 66kv but more than 22 KV, we use 5 KA rated lighting arrester.
- In the system having a nominal voltage of less than 22 kV, 1.5 or 2.5 kV LAs are used.