The general term “overcurrent” applies to a relatively small increase over the full load current (FLC) rating (e.g. due to mechanical overloading of a motor) rather than the massive current increase caused by a short-circuit fault.
Generally, an overcurrent, supplied from a CT, is detected by a relay with an appropriate time-delay to match the protected circuit.
Short-circuit faults in LV distribution circuits are mainly detected and cleared almost instantaneously bf fuses, MCCBs or MCBs.
Main supply feeders are usually protected against short-circuits by circuit breakers with instantaneous magnetic trip action.
Overcurrent relay types on ship:
Magnetic
Thermal
Electronic
All relay types have an inverse current- time characteristic called OCIT (over- current inverse time), i.e. the bigger the current the faster it will operate.
A magnetic relay, directly converts the current into an electromagnetic force to operate a trip switch.
An electronic overcurrent relay usually converts the measured current into a proportional voltage. This is then compared with a set voltage level within the unit which may be digital or analogue.
In analogue unit the time delay is obtained by the time taken to charge up a capacitor.
This type of relay has separate adjustments for overcurrent and time settings together with an instantaneous trip.
Both the magnetic and electronic relays can be designed to give an almost instantaneous trip (typically less than 0.05 seconds or 50 ms) to clear a short- circuit fault.
Thermal relays are commonly fitted in moulded case circuit breakers (MCCBs) and in miniature circuit-breakers (MCBs) to give a “long time” thermal overcurrent trip in addition to a magnetic action for an instantaneous trip with a short-circuit fault.
Overcurrent protection relays in large power circuits are generally driven by current transformers(CTs).
All overcurrent relays can be tested by injecting calibrated test currents into them to check their current trip levels and time delay settings.
