On the Protection of Thermal Processes

Standards specify an equation for the time-current characteristic of a thermal relay. When the equation is published without reference to its derivation, it may be mistaken for the characteristic of an inverse-time overcurrent relay which if implemented would provide only marginal protection. This paper explains the ambiguity introduced in the standard and addresses thermal protection from the perspective of a thermal model that provides optimum protection. The paper employs MATLAB simulations to compare the dynamics of the thermal model with that of an overcurrent implementation during cyclic overloads.