How a Surge Arrester Works, and What Insulation Coordination Means

In this guide we follow one lightning surge from the sky to the ground, with animated diagrams, and see how a small device keeps a large transformer alive.

The problem: a spike taller than the insulation

Every piece of power equipment has insulation. Insulation is the material that keeps live parts apart from each other and from the ground. It can only hold back a certain voltage before it breaks down.

For short impulses, this limit is called the BIL, or Basic Insulation Level. The BIL is the peak impulse voltage the insulation is tested to survive. A transformer on a 132 kV network, for example, often has a BIL of 650 kV.

Normal grid voltage is far below the BIL. But some events create short, very tall voltage spikes. We call them overvoltages. So what happens when a spike is taller than the insulation limit?

The insulation fails. A spark punches through it. This is called a flashover if it happens through air, or a breakdown if it happens inside solid or oil insulation. Inside a transformer, this damage is often permanent.

There are two main sources of overvoltage. Lightning makes a very fast, very tall spike. Switching, which means a breaker opening or closing, makes a slower spike that is usually lower. The diagram below compares one spike with and without an arrester.

What is inside a surge arrester

A modern arrester looks like a short insulator. Inside, it is surprisingly simple. Its heart is a column of round discs made of zinc oxide (ZnO). Each disc is a metal-oxide varistor, or MOV. A varistor is a material whose resistance changes strongly with voltage.

The discs are stacked and sealed inside a housing of porcelain or polymer. The housing has sheds, the ribbed rings on the outside. They make the surface path longer, so rain and dirt cannot easily form a leak path. The top terminal connects to the line. The bottom terminal connects to the ground.

Older arresters also had spark gaps in series with the discs. Most modern arresters are gapless. The zinc-oxide discs do the whole job alone.

The core idea: millions of tiny gates

Why does zinc oxide behave like a valve? The answer is in the zoom above. The disc is made of tiny zinc-oxide grains. Between the grains are very thin grain boundaries.

Each boundary acts like a small electronic gate. Below about 3 volts, the gate blocks current. Above about 3 volts, the gate lets current through easily. A disc holds millions of these gates in series and in parallel. So the whole column has a sharp switching voltage. The designer sets it by choosing the number and height of the discs.

This makes the arrester a non-linear resistor. Its resistance is not fixed. It is huge at normal voltage and very small during a surge. The graph below shows this as a voltage–current curve. The current axis jumps by a factor of 1,000 at each tick, so we can see tiny and huge currents on one graph.

Two points on this curve matter most. The first is the normal operating voltage. Here the arrester carries only a tiny leakage current, so it does not heat up. Datasheets call the highest safe value the MCOV, or Maximum Continuous Operating Voltage. The second is the residual voltage. This is the voltage across the arrester while a standard surge current, often 10 kA, flows through it. The residual voltage is the arrester’s protective level.

Step by step: one surge, four moments

Let us follow one lightning surge from start to finish. Each card shows one moment.

Insulation coordination: making the levels fit

An arrester only helps if its protective level sits well below the equipment’s BIL. Choosing the arrester and the equipment so these levels fit together is called insulation coordination.

That space is the protective margin. Engineers often check it as a ratio: BIL divided by the arrester’s protective level. IEEE guidance (C62.22) asks for a ratio of at least about 1.2 for lightning surges, which is a 20% margin. For slower switching surges it asks for at least about 1.15. The chart below uses typical values for a 132 kV transformer.

Why the arrester must sit close to the equipment

There is one more effect that surprises many engineers. A surge is a travelling wave. When it hits the end of a line, such as a transformer winding, it reflects, which means it bounces back. The arriving wave and the reflected wave add up. So the voltage at the transformer can rise above the arrester’s protective level.

The further apart they are, the bigger this extra voltage. A simple estimate is: voltage at the transformer ≈ arrester voltage + 2 × steepness × travel time between them. Use the slider to see the effect. This simple formula gives a worst-case value, so it is good for building intuition.

With these values, the arrester protects well up to about 60 metres. Beyond that, the margin becomes too small. This is why arresters are mounted right at the transformer terminals. It is also why short, straight earth leads matter. Every extra metre of lead adds voltage during the surge.

Where surge arresters are used

Surge arresters appear across the whole power system. They guard power transformers, which are costly and slow to replace. They sit at substation entrances, where overhead lines enter the yard. They protect cable terminations, where an overhead line meets an underground cable. They also protect motors, generators and capacitor banks.

On distribution poles, we can see small arresters next to pole-mounted transformers. In buildings, much smaller surge protective devices (SPDs) follow the same idea at low voltage. The rule is always the same. The arrester is a cheap guard. It keeps the costly equipment behind it below its BIL.

Key takeaways

• A surge arrester connects a live line to ground. It works like a pressure relief valve for voltage.
• Its active part is a column of zinc-oxide (MOV) discs. Millions of tiny grain-boundary "gates" make it strongly non-linear.
• At normal voltage it carries only microamps. During a surge it carries kiloamps while the voltage stays nearly flat.
• The residual voltage at a standard current, often 10 kA, is the arrester's protective level.
• Insulation coordination keeps the BIL well above the protective level: a ratio of about 1.2 or more for lightning.
• Mount the arrester close to the equipment. Distance and long leads add voltage through wave reflection.