In this article, we shall explore the lightning arrester ratings. The article includes rated voltage, MCOV, discharge current, residual voltage, energy absorption, and many more.
Lightning Arrester Ratings based on Voltage
How to Select the Lightning Arrester Ratings based on Voltage?
For that, we have to consider two conditions. The first one is the effectively earthed condition. The other is the non-effectively earthed condition.
Effectively and Non-Effectively Earthed Condition
An effectively earthed condition means a star-connected earthed system. Here, the three-phase winding forms a star in the transformer. Then, we connect the star point solidly to the earth. This is like in our 132 KV system, 220 KV system, or 400 KV system. However, many power utilities use delta-connected transformer in 33 KV systems. A delta-connected winding does not have any direct provision for earthing. Because, a delta-connected three-phase winding does not have a readily available star point or common point. This is why we refer to it as a non-effectively earthed system.
Now, we consider a star-connected system and imagine there is a fault in one phase. Because of a solidly earthed star point, the other phase voltage can’t increase beyond 80% of phase-to-phase voltage or the line voltage of the system.
Whereas in a delta connection, if any fault occurs in any of the phases, the voltage of the other two phases may reach up to √3 times their phase voltage or a little bit higher. In an effectively earthed system, during a fault in any of the phases, the voltage of the other two phases will not exceed the healthy phase voltage.
However, in a non-effectively earthed system, the voltage of the healthy phases goes up to 100% or even little bit more of the line voltage. The rated voltage of a lightning arrester for an effectively earthed system is generally 0.8 times the phase to phase voltage or rated line voltage of the system. For a non-effectively earthed system, we select lightning arrester ratings for 100% or even more of the system line voltage.
Example – 132 kV System
Say, for a 132 KV system, the rated voltage of the selected lightning arrester will be 0.8 times of 145 KV, because the highest system voltage of 132 KV is 145 KV, or 1.1 times 132 KV. Considering 132 KV, we have to write 0.8 × 1.1 × 132 KV, and this is the same as 0.8 × 145 KV, which is around 120 KV.
Example – 220 kV System
Let us again take an example of a 220 KV system. The 220 kV system is also solidly earthed. The rated voltage of the lightning arrester equals 0.8 × 245 kV, which is nearly 198 kV. The highest system voltage for a 220 kV system is 245 kV. Therefore, 198 KV is the rated voltage of the selected lightning arrester for a 220 KV system.
Example – 33 kV System
Now, come to the 33 KV system. The 33 KV system, as I mentioned, is normally non-effectively earthed. So, in the 33 KV system, the selected lightning arrester will have a rated voltage of 33 KV × 1.1, or 36 KV only. But here we have to multiply by some safety factor. Most of the utilities choose 42 KV instead of 36 KV. This is simply for providing some percentage of margin. However, you can also go for 36 KV lightning arresters for the purpose.

What is the Maximum Continuous Operating Voltage (MCOV) of a lightning arrester?
We normally connect a lightning arrester between the phase and earth. There are two voltage conditions between phase and earth: one occurs during an earth fault in other phases, and the other under normal (healthy) conditions.
Voltage Condition During an Earth Fault
When an earth fault occurs in one phase, the voltage of the other phase will rise. At that time, the lightning arrester should not conduct any current. Because it should not operate during earth fault conditions. It should only operate during lightning or switching impulses.
Lightning Arrester Ratings based on Temporary Voltage Withstand Capability
The lightning arrester must withstand this high voltage temporarily. Since this voltage is not continuous, the protection system quickly clears it. According to Indian standards, these lightning arrester ratings refer to the power frequency voltage that the arrester can withstand for a limited duration (typically 10 seconds) without conducting any current to earth, except for its nominal leakage current.
Under normal system conditions, the phase voltage or line-to-earth voltage of the system appears across a lightning arrester. Even in this state, it should not conduct any current to earth except its nominal leakage current.
Maximum Continuous Operating Voltage (MCOV)
For this reason, we assign another voltage rating to a lightning arrester: the Maximum Continuous Operating Voltage (MCOV). The earlier discussed voltage (withstand rating for 10 seconds) is temporary, whereas MCOV refers to the continuous voltage that appears across the arrester when we connect the arrester between phase and earth. This is the phase voltage, i.e., line voltage divided by √3.
This voltage is the continuous voltage, and the arrester must not conduct any current other than allowable leakage. To ensure a safety margin, the MCOV of a lightning arrester must be slightly higher than the system’s phase voltage.
For example, in a 132 kV system, the phase voltage is 132 / √3 ≈ 76.2 kV. After adding the necessary safety margin, the MCOV becomes 102 kV. However, the rated voltage of the lightning arrester for such a system is typically 120 kV.
So, there are two main lightning arrester ratings based on voltage:
- Rated Voltage – For example, 120 kV, which it can withstand for 10 seconds during fault conditions.
- Maximum Continuous Operating Voltage (MCOV) – For example, 102 kV, which it must withstand continuously under normal operating conditions.
Lightning Arrester Ratings based on Nominal Discharge Current
Depending on the lightning arrester ratings based on current, lightning arresters are classified into different categories: distribution class and station class lightning arresters.
The distribution class lightning arrester means, the arrester takes a rated nominal discharge current 5 KA or below. For station-class lightning arresters, we take the rated nominal discharge current as 10 kA for 132 kV systems and 10 to 20 kA for 220 kV and higher systems. The nominal discharge current plays a significant role in choosing a lightning arrester. It is the current that discharges through the arrester during a surge. The waveform of this current is an 8/20 microseconds impulse.

Residual Voltage of a Lightning Arrester
When the discharge current passes through the arrester, due to the inherent impedance of the arrester, a voltage develops across it. We call this voltage the residual voltage. It must be less than the Basic Insulation Level (BIL) of the equipment connected to the system. The lightning arrester protects this equipment. The lightning impulse withstand capability is rated by the peak impulse voltage. This voltage has a 1.2/50 microseconds waveform.
For example, suppose the transformer bushing connected to a 132 kV system has a lightning impulse withstand capability of 550KV. Let us take a station class lightning arrester with a discharge current of 10 KA. For this current, the residual voltage developed across the arrester should be less than 550 kV peak. Otherwise, the arrester will fail to protect the equipment during actual lightning events.
Protective Ratio of a Lightning Arrester
Suppose for a 10 KA nominal discharge current, the residual voltage appearing across the arrester is 340 kilovolts. We define the protective ratio as:
\[ \text{Protective Ratio} = \frac{\text{Lightning Impulse Withstand Voltage of Equipment}}{\text{Residual Voltage of Arrester}} \]
This is an important parameter, we consider in choosing the lightning arrester. Hence for the above mention transformer bushing the Protective Ration will be, \[ \text{Protective Ratio} = \frac{550}{340}=1.61 \]
Maximum Discharge Current
This is also one of the important lightning arrester ratings based on current. This is the maximum discharge current rating, also called the high current impulse withstand capability. We measure it in kiloamperes peak, and it follows a 4/10 microsecond waveform.
For a station class lightning arrester, the standard value of this current is 100 KA, and for a distribution class arrester, it is 65 KA as per Indian standards.
Very high discharge currents may occur, especially when a lightning stroke hits in close proximity to the arrester. However, the energy discharged during such high current events is not very significant due to the very short duration of the impulse — just 4/10 microseconds.
Energy Absorption Capability of a Lightning Arrester
The rated discharge current develops a residual voltage while flowing through the body of the lightning arrester. If we multiply this discharge current I by the residual voltage E, we obtain the power (IE) dissipated across the lightning arrester during the discharge of the surge current (I).
If we then multiply that power by the duration for which the discharge current flows through the arrester, we get the energy. This energy is ultimately converted into heat, and the arrester’s body or structure must be capable of absorbing this energy without any permanent distortion or alteration of its characteristics.
This defines the energy absorption capability rating of a lightning arrester. These lightning arrester ratings are expressed in kilojoules per kilovolt (kJ/kV).