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Basics of Line Surge Arresters

The application of Line Surge Arresters (LSA’s) is well known for its cost-effective performance improvement in the electricity supply industry. 

Across the globe, most of the utilities and transmission system operators (TSOs) are considering LSA’s when conventional methods do not provide satisfying results.

Some users also apply this technology for different purposes as listed below:

  • switching surge control to optimize structures and reducing clearances

  • line uprating and compaction

  • safety concerns such as preventing population and injury damage

  • live line working to reduce the minimum approach distance LSA’s

EGLA Externally Gapped Line Arresters
Basics of Line Arresters: Resources and Tips

Different purpose than Station Class Arresters

Surge arresters in substations have the duty to prevent significant financial damages of valuable assets as power transformers. Indeed, an insulation breakdown inside a power transformers might lead to undesirable consequences.
Line Surge Arresters are used to prevent flashovers on the insulators strings of the overhead line. The purpose is not the protect the insulator itself but to reduce or eliminate lightning induced outages. These can have penalizing contractual consequences but also severely impact the stability of the network.

Basics of Line Arresters: Text
Line Surge Arresters for lightning performance improvement, prevent lightning flashovers

Prevent lightning flashovers

LSA are mainly used to improve lightning performance by reducing lightning induced outages. If the BIL of the insulator string is exceeded, a flashover might cause of breakers operation to clear the fault. Lightning induced outages are very significant in some regions.

Although transmission lines are shielded with OPGW and grounding resistance was achieved during tower erection, back-flashovers and shielding failure might still be an issue that needs to be addressed.

Basics of Line Arresters: Text


Non-Gapped Line Arresters

NGLA is a basic adaptation of the substation’s arresters used to protect valuable equipment like power transformers. As of today in 2022, the IEC/IEEE for NGLA are the same as for gapless tpyes (IEC 60099-4). The active part is directly connected between the phase conductor and the grounded structure. The residual voltage of the MOV column will limit the overvoltages across the insulators and prevent flashover when BIL is exceeded.

NGLA requires a clamping system to the conductor, a Ground Lead Disconnector (GLD) and in most cases a grading ring with a corona ring.

To galvanically isolate the line surge arrester from the line voltage in the unlikely event of a fault or thermal overload, a disconnector is installed in series. It automatically and immediately disconnects the line surge arrester from the line voltage. This allows the affected overhead line to be reenergized and operated until convenient replacement can be scheduled.

Relevant standards: IEC 60099-4 / IEEE C62.11. Application Guide IEC 60099-5 / IEEE C62.22, IEEE 1243

Basics of Line Arresters: Text


Externally Gapped Line Arresters

Externally Gapped Line Arresters (EGLA) have an external spark gap placed in series that galvanically isolates the active part (SVU – Series Varistor Unit) of the line surge arrester from the line voltage under normal conditions.


In case of lightning, the spark gap is ignited, and the overvoltage is safely discharged through the resulting arc. The active component (SVU) limits the subsequent current to ensure that the arc is extinguished within the first half-cycle of the operating power-frequency voltage. After this, the line surge arrester immediately returns to standby condition. No breaker operation required. The active component can have either one or two SVUs (on each side) depending on the system voltage level and user’s requirements.


Relevant standards: IEC 60099-8

Basics of Line Arresters: Text
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