A relay is a switch; it is not usually meant to open under fault conditions (high current).
A fuse is a fault protection device. If a short circuit develops, large currents will flow, and the fuse link will melt, causing the circuit to open
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An overload relay is normally connected in series with a SCPD (Short circuit protection device). SCPDs include circuit breakers and fuses.
A relay can be reset and put but back into service after an overload.
A fuse needs to be replaced after every fault.
A circuit breaker can be reset and put back into service after clearing a fault.
When there is an overload, the relay operates, protecting the downstream load.
Under short circuit condition, it is the SCPD that operates, protecting the circuit upstream.
Both SCPD and overload relay are characterised by time-current curves plotted on a graph sheet. Both these characteristics are superimposed to determine the crossover point, above which the SCPD would operate And below which the overload relay would operate.
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