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EK-General : 2021/OCT/09 – 2017/DEC/09 – 2014/JUL/**
Question
Q9. With reference to overcurrent protection for electrical circuits:
(a) Explain THREE methods of protection, stating where EACH may be used; (6)
(b) explain, with the aid of a diagram, the meaning of the term inverse current time characteristic. (4)
Asked in Year/Month/Ques No.
Synopsis
Types of Overcurrent
The three major categories or types of overcurrent are;
1. Overload overcurrent
2. Short-circuit overcurrent
3. Ground-fault overcurrent
1. Overload overcurrent
>> Any current in excess of rated-load current is, an overload.
>> Overload conditions can occur at the service, feeder, or branch-circuit level of a building's electrical-power distribution system.
Trip used: OCIT
>> The Over Current Inverse Time relay function monitors general balanced overloading and has current/time settings determined by the overall protective discrimination scheme.
>> Typical setting ranges for current (I) and time (t) are:
>> current (I) = 0.7-2. In, (In = nominal or rated generator current)
>> Time t: 1-10seconds.
2. Short-circuit overcurrent
>> Short-circuit current is high-magnitude fault overcurrent
>> Short-circuit overcurrent normally involves an accidental cross-connection of at least two circuit conductors (supply and return).
Trip used: OC(INST.)
>> ''Instantaneous'' trip to protect against extremely high overcurrent caused by a short-circuit fault. >> Typical setting ranges are:
Current I> : 2-10% of nominal or rated generator current, and time t: 0.1 seconds
3. Ground-fault overcurrent
>> Ground-fault overcurrent is also a short-circuit condition that normally affects only one of the circuit conductors.
>> Usually local, therefore OC INST type of protection is provided to respective group control panel.