The most costly mistakes when selecting surge arresters – A list of the 12 most common

When designing medium- and high-voltage networks, we often fall into pitfalls arising either from budgetary constraints or structural limitations. However, when it comes to selecting surge arresters, mistakes in this area can have very costly – and dangerous – consequences. At the same time, these mistakes are relatively easy to avoid – indeed, as early as the design stage.

If you are currently facing the challenge of selecting surge arresters, we strongly encourage you to read the article below. In it, we will advise you on what to look out for and discuss the most costly mistakes made when selecting surge arresters.

The most common (and most costly) mistakes made when selecting surge arresters, and their consequences

Without further ado, let’s take a look at the 12 most common mistakes made when selecting surge protection devices, which can result in costly repairs.

1. Start from Un instead of Us (Um) and phase-to-earth voltages (Us/√3)

The nominal voltage (Un) should not be used as the basis for selecting a surge arrester for a given circuit. This could lead to accelerated ageing of the varistors, an increased risk of thermal instability during transient overvoltages (TOV), and a rise in the number of failures following earth faults or switching operations.

Instead, attention should be paid to the system voltage (Us, particularly important in cases where temporarily elevated values occur) and the maximum operating voltage (Uc, phase-to-earth). Therefore, if you need to determine these parameters, do not focus solely on the nominal value.

2. No model for the neutral point earthing and the duration of the earth fault

The neutral point earthing model (also known as the k-factor) is a very important parameter influencing the selection of surge arresters. Transient overvoltages in healthy phases of networks that are not effectively earthed can reach higher values and last longer, which is used to determine Uc and the rated voltage (Ur).

An undefined k-factor and duration of the earth fault will result in a reduced service life of the surge arresters. Please also note that if you select them without determining these parameters, even with other values that appear correct ‘on paper’, a claim against the manufacturer may be contested.

3. Underestimating Ur without verifying TOV

Some designers assume that choosing surge arresters with a lower rated voltage will provide better protection. Nothing could be further from the truth – unfortunately, this assumption is one of the most costly mistakes on our list.

The logic seems simple at first glance: a lower Ur means a lower protection level, and therefore better protection for the devices being safeguarded. In practice, however, such a calculation very often results in damage to the surge arrester – not due to the surge itself, but due to thermal overload resulting from a prolonged event (lasting seconds or minutes).

Underestimating Ur without first calculating the TOV scenarios causes the arrester to operate in the ‘steeper’ section of the U–I characteristic: a higher leakage current flows through it, which generates additional heat and accelerates the degradation of the varistors. The result is paradoxical: an apparent improvement in protection leads to faster wear and tear of the circuit that was supposed to provide that protection.

If you wish to verify a lower Ur, always precede this decision with an analysis of transient surges. Both without a pre-charge and with the ‘pre-charge’ variant, which better reflects the actual operating conditions of the station.

4. Selection based solely on In and ignoring energy/charge (Qrs/Wth)

The rated discharge current (In) is one of the first parameters that catches designers’ eyes when reviewing data sheets. The problem is that In does not describe the energy capacity of the arrester – it merely indicates the current at which the manufacturer has specified the level of protection. For line applications or those requiring switching operations, however, the most important factors are:

• Qrs
• Wth – rated thermal energy
• Line discharge class

The result of omitting these parameters is that a limiter which would work well under laboratory conditions fails in practice. This is all due to a lack of power reserve.

5. Lack of control over the buffer zone (distance effect)

The voltage at the terminals of the protected equipment, which is located several or several dozen metres away from the surge arrester, may be higher due to the travelling wave and reflection effects. In the case of MV/HV systems, this phenomenon – known as the distance effect – can compromise the effectiveness of the protection, even if the surge arrester’s electrical parameters have been correctly selected.

This error is most often caused by the location of the arrester, where structural considerations are prioritised over protection criteria.

6. Failure to take into account the inductance of connections and earthing

A surge arrester with a correctly selected protection level may prove insufficient in the presence of loops or long earthing conductors. This usually results in an additional voltage drop across the conductors or the earth (L·di/dt), and consequently an increase in voltage across the protected equipment.

In practice, this results in significant variations in performance between seemingly identical substations. Identical surge arresters, but installed differently and with cables routed differently, can provide completely different levels of protection. Side effects also include an increased risk of insulation damage and interference in control and measurement circuits (secondary surges).

7. No insulation coordination margins

Simply installing a surge arrester does not guarantee effective protection. It is also necessary to calculate the insulation coordination margins, i.e. to ensure that the surge arrester’s protection level is sufficiently higher than the surge withstand capacity of the protected equipment.

In this case, there are two key indicators:

• LIWV – Lightning Impulse Withstand Voltage
• SIWV – Switching Impulse Withstand Voltage

The margin must take into account both the effect of the distance from the arrester and the inductance of the connections. If the coordination margins are too small, the insulation of the transformer or GIS/AIS equipment may be breached even in the presence of an arrester. The cascade risk is particularly severe here: a surge damages the insulation, leading to a short circuit, and the short circuit causes further failures in the substation. The calculation of margins should therefore be a standard part of the design documentation, rather than merely an optional extra.

8. Incorrect selection of the line surge arrester class for the application

MV/HV surge arresters are available in various duty classes, which correspond to different levels of electrical stress. Substation, line, distribution and transmission applications differ significantly in terms of the surge and switching events typical to them. Selecting a class solely on the basis of the ‘catalogue specification’, without analysing the nature of the application, often leads to underestimation.

As a result, a surge arrester with an insufficiently high class suffers accelerated degradation of its varistors, particularly at sensitive points in the network, e.g. line ends, cable entries, and the vicinity of distributed transformers. Remember that replacing surge arresters with the correct class after a failure is significantly more expensive than taking this into account at the selection stage.

9. Neglecting environmental conditions and external insulation

A surge arrester may have correctly selected Uc and Ur values, yet still be damaged by a flashover across the housing surface – especially in conditions of heavy pollution, high humidity or salt spray. The leakage path of the housing and its geometry are electrical parameters just as important as the internal characteristics of the varistors.

10. Overlooking mechanical criteria

The mechanical parameters of a surge arrester are not merely an add-on to the specifications. Underestimating loads from connections, wind forces or seismic vibrations can lead to cracks, deformation or loosening of connections. Any of these faults can, in turn, alter the insulation geometry, creating a risk of discharges and failure.

Mechanical damage – including subtle damage that is not apparent during a routine inspection – shortens service life and increases maintenance costs. This is precisely why SLL (Specified Long-term Load) and SSL (Specified Short-term Load) must always be verified in relation to the specific conditions of the installation site, rather than simply being compared with the ‘default’ catalogue value.

11. Underestimation of short-circuit capacity

In the event that a surge arrester suffers internal damage (e.g. following thermal overload), a short-circuit current may flow through its varistor column. If the arrester’s rated short-circuit current is not suited to the short-circuit conditions at the substation, the damage may be catastrophic.

The consequences will then extend beyond the arrester itself: there is a real risk of damage to adjacent equipment and a threat to the safety of operating personnel. In tender and acceptance documentation, it is always worth checking whether the manufacturer’s stated rated short-circuit current has been verified for the specific installation site.

12. Installation and assembly errors

In medium and high-voltage networks, installation ‘details’ are critical. Failure to observe the minimum distances between the arrester and the supporting structure and between phases, an incorrect installation configuration (operating position) or equipment assembly that does not comply with the documentation – any of these errors can alter the uniform voltage distribution across the varistor string. This leads to localised overheating or accelerated ageing.

The practical consequence is a lack of consistency in installation across the station fleet, and non-conformities that are only detected during inspections or maintenance checks.

How to choose the right MV/HV surge arresters and avoid costly mistakes? (CHECKLIST)

Selecting a surge arrester is a process consisting of several steps that must be carried out in the correct order. The methodology below will enable you to check that all key parameters have been taken into account before placing an order. You can also easily implement it as a standard procedure within any design or operational organisation.

Step 1: Input data

Before proceeding with the selection, gather all key network data: maximum system voltage (Us), neutral point earthing method and outage times, transient overvoltage (TOV) scenarios (amplitude and duration), surge withstand capability of protected equipment (LIWV/SIWV), substation layout (distances, connection lengths), environmental conditions and short-circuit currents.

Step 2: Minimum continuous operating voltage (Uc)

Take Uc ≥ 1.05 · Us/√3. Also take into account the specific characteristics of the network: harmonics, voltage tolerances and any prolonged increases in Us (e.g. 126.5 kV for 60 minutes in 110 kV networks, as found in operators’ guidelines and connection conditions).

Step 3: Selection under TOV (Ur)

Determine the required Ur for each TOV scenario and select the highest value. Where possible, also analyse the ‘pre-energised’ variant – this better reflects the actual operating conditions of the substation, where the arrester may have already survived surge events.

Step 4: Selection of class (Qrs/Wth)

Select the surge arrester’s duty class so that Qrs and Wth correspond to the nature of your application. Line applications, substations with capacitor banks, renewable energy facilities and frequent switching operations require a higher energy class than typical distribution substations.
Step 5: Location and connections

Design the installation as close as possible to the protected object. Keep earthing cables short and straight, eliminating loops and unnecessary bends. The value of L·di/dt depends directly on the length and geometry of the current path.

Step 6: Short-circuit and mechanical considerations

Verify the surge arrester’s rated short-circuit current (and its duration) against the conditions at the specific installation site. Also check the SLL/SSL parameters for loads from connections, wind and vibrations.

Step 7: Installation

Ensure that minimum installation clearances, operating positions and the completeness of fittings are in accordance with the manufacturer’s documentation – both at the design stage and during acceptance.

Glossary – Symbols and abbreviations

• Un – nominal mains voltage.
• Us – system peak voltage, i.e. the highest phase-to-phase voltage occurring under normal operating conditions.
• Um – maximum voltage for equipment, relating to the insulation requirements of the apparatus.
• Uc / MCOV – continuous operating voltage of the arrester; in practice, the minimum level at which the apparatus can operate without time limitation.
• Ur – rated voltage of the arrester, selected, amongst other things, for TOV.
• Upl / Ups – the surge protection level of the arrester for lightning and switching surges, respectively.
• LIWV / SIWV – the withstand voltage of the device’s insulation for lightning and switching surges.
• BIL / BSL – IEEE terminology corresponding to LIWV and SIWV, respectively.
• TOV – transient overvoltages of a specified amplitude and duration.
• k – earthing factor resulting from the method of neutral point earthing.
• Qrs / Wth – parameters describing the surge current carrying capacity and the energy or thermal withstand capability of the arrester, depending on the manufacturer’s documentation.
• SLL / SSL – Specified Long-term Load and Specified Short-term Load – basic mechanical parameters of the arrester.
• distance effect – an increase in voltage across the protected equipment due to the arrester being installed at a distance and wave phenomena.
• L·di/dt – an additional voltage component arising across the inductance of the cables due to a rapidly rising surge current.

Please note: the terminology and interpretation of parameters may vary depending on the manufacturer and the data sheet – always check the definitions in the documentation.

Protektel surge protectors – Expertise that begins before you buy

Each of the errors described above can be eliminated or their impact significantly reduced – provided that the manufacturer’s support begins not with issuing an invoice, but with verifying the selection of protective devices. In practice, this means access to full TOV and energy data in the data sheets, the ability to test variants against a specific network scenario, and consistent, comprehensive installation documentation.

The PROXAR (Protektel) range of surge arresters has been designed with precisely this approach in mind. The data sheets contain full selection data, allowing you to document and compare selection options without having to contact the manufacturer at every step.

If your project requires TOV verification, energy class assessment, mechanical compatibility checks or interpretation of requirements for non-standard voltage levels – the Protektel team is at your disposal at every stage of the selection process. Professional advice begins before purchase and is precisely when it matters most for the reliability of the installation.