Insulation vs Surge Arrester

In electrical power systems, all of these devices are used together to help prevent the most costly portions of the system (transformers, generators, capacitor banks) from being damaged by abnormal voltages or currents due to lightning, switching, or short circuits.

In the case of abnormal voltage transients, this technique is called "insulation coordination". The concept is simple: every component in the system has an over-voltage rating, or Basic Impulse Level (BIL). By periodically connecting surge suppression devices (arresters) that break down at a significantly lower voltages than the BIL of the more expensive parts of the system, dangerous surges can be safely shunted to ground before they reach the higher breakdown voltages of more costly components in the system. Modern Metal Oxide Varistor (MOV) arresters will usually self-recover after the surge has passed. However, older surge protectors that employ spark gaps or horn gaps may require operating an upstream circuit breaker or cutout in order to extinguish the undesired power arc that follows flashover of the protective device.

And remember, surge arresters; particularly MOV, are not designed to withstand power frequency overvoltages for more than their temporary overvoltage rating or capability. They are designed to protect against impulse overvoltages such as lightning and switching.

Also, all insulation (including air) has a volt-time strength characteristic. The shorter the time, the higher the dielectric strength with the highest being steep wave impulse and the lowest (for all practical purposes on AC systems) power frequency.

Typical insulation coordination curves will plot the insulation withstand voltage from steep wave through power frequency versus the surge arrester discharge voltages over the same times and the difference is the protective margin or level.

Surge arresters are actually used to protect the insulation, by breaking down during temporary over voltages and conducting energy to ground.

Insulation is needed permanently to prevent any short circuits.