A thermal-mechanical fatigue assessment system comprising: an electrical conductor having a resistance that is subject to change in different operating conditions when connected to an energized electrical power system; a temperature sensor positioned in the same environment as the electrical conductor, or in an environment having the same temperature as the environment in which the electrical conductor is positioned, and configured to measure the ambient temperature of the electrical conductor; one or more current sensor(s) to provide a measurement of the current flowing through the conductor; and a controller (304, 306) operable to estimate an operating temperature of the electrical conductor based on an ambient temperature input from the temperature sensor, and current flowing through the conductor as provided by the current sensor(s), wherein the electrical conductor is a fuse element, and characterized in that the controller (304, 306) is further configured to: estimate the temperature of the fuse element based on an addition of an estimated first temperature differential, an estimated second temperature differential, and the ambient temperature; wherein the estimated first temperature differential is an estimated temperature differential between a temperature of the fuse element and a temperature of an arc quenching medium surrounding the fuse element, the estimated first temperature differential estimated based on the ambient temperature and the current flowing through the fuse element; and wherein the estimated second temperature differential is an estimated temperature differential between the temperature of the arc quenching medium and the ambient temperature, the estimated second temperature differential estimated based on the ambient temperature and the current flowing through the fuse element. an electrical conductor having a resistance that is subject to change in different operating conditions when connected to an energized electrical power system; a temperature sensor positioned in the same environment as the electrical conductor, or in an environment having the same temperature as the environment in which the electrical conductor is positioned, and configured to measure the ambient temperature of the electrical conductor; one or more current sensor(s) to provide a measurement of the current flowing through the conductor; and a controller (304, 306) operable to estimate an operating temperature of the electrical conductor based on an ambient temperature input from the temperature sensor, and current flowing through the conductor as provided by the current sensor(s), wherein the electrical conductor is a fuse element, and characterized in that the controller (304, 306) is further configured to: estimate the temperature of the fuse element based on an addition of an estimated first temperature differential, an estimated second temperature differential, and the ambient temperature; wherein the estimated first temperature differential is an estimated temperature differential between a temperature of the fuse element and a temperature of an arc quenching medium surrounding the fuse element, the estimated first temperature differential estimated based on the ambient temperature and the current flowing through the fuse element; and wherein the estimated second temperature differential is an estimated temperature differential between the temperature of the arc quenching medium and the ambient temperature, the estimated second temperature differential estimated based on the ambient temperature and the current flowing through the fuse element. estimate the temperature of the fuse element based on an addition of an estimated first temperature differential, an estimated second temperature differential, and the ambient temperature; wherein the estimated first temperature differential is an estimated temperature differential between a temperature of the fuse element and a temperature of an arc quenching medium surrounding the fuse element, the estimated first temperature differential estimated based on the ambient temperature and the current flowing through the fuse element; and wherein the estimated second temperature differential is an estimated temperature differential between the temperature of the arc quenching medium and the ambient temperature, the estimated second temperature differential estimated based on the ambient temperature and the current flowing through the fuse element.