Surge Arresters

Surge Arresters are tireless protectors of expensive equipment against transient voltages. They have in the past received bad press regarding failures and poor reliability but a degree of this has been through mal application and poor understanding. Moisture has been the curse of the metal oxide varistor either from poor sealing inside porcelain or in fact permeation through some of the polymeric housings used. However, a large number of failures attributed to poor weather resistance have almost certainly been the arrester doing the job it was designed to do, then giving up its life due to excessive energy transfer. The modern day surge arrester is a reliable and cost effective device. It is only in the recent past that the distribution industry in the UK has started to use these devices more extensively to protect pole mounted transformers and switchgear.

The reasons being that there is a better understanding of the ineffectiveness of spark gaps, a need to eliminate switching outages (necessary to clear spark gaps), a need to better protect expensive switchgear and a significant product cost reduction with the silicone housed arrester. Cooper Power Systems are one of the worlds largest manufacturers of metal oxide varistor (MOV) disks making many thousands per day. Disks are available for 5KA and 10KA distribution arresters to  IEC 99-4 but also high energy disks for Station Class Arresters. Porcelain is still favoured by many as an insulator but the Cooper patented UltraSil silicone rubber housing is rapidly becoming the favoured material for distribution arresters. Metal Oxide Varistor disks with their non-linear voltage characteristic have become the most extensively used arrester type in the last 20 years but they have one shortcoming in that they cannot tolerate continual overvoltage such as might be experienced in situations where ferroresonance exists. In an attempt to guard against this shortcoming it has been popular practice to increase the arrester voltage by 1 disk typically 3KV. This of course is at the expense of the degree of protection. Cooper researched the overvoltage problem and came up with a solution called VariGap. This ingenious product combines the advantages of the metal oxide disk with the resistance graded gap. By careful selection it was found that the combination could pick up all the advantages of both systems yet still eliminated the disadvantages. VariGap arresters show up to 50% increase in TOV withstand and up to 30% improvement in surge protection.