Should a current transformer (CT) secondary winding be left in or inadvertently become open circuited while the primary is energised, then damage may occur to the CT – leaving it inoperable or seriously effecting its measurement accuracy. If the CTs are being utilised in particularly difficult places to access, such as in a gas insulated substation or in a power transformer bushing, this can then lead to an expensive repair to a relatively inexpensive component. To prevent damage occurring to open-circuited CT's, a range of products are available. These devices are based either on a silicon carbide varistor-based technology or on a gas discharge tube-based technology. The purpose of this study is to compare the performance of these devices.