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Electrical Engineering – Circuit Protection

Question 1 (a, b and c)

Describe, with the aid of sketches, the mechanisms built into the design of an HRC fuse link to enable it to respond efficiently to a sustained overload as well as to a severe short-circuit condition.


Explain the significance of ‘let-through energy’ when assessing discrimination between major and minor fuses in a system of feeder protection. Describe how manufacturers’ data can be used in this assessment.


When fuses or circuit breakers cannot provide adequate earth leakage protection, an RCD may be employed. Explain how an RCD operates and state what condition it must fulfil to ensure that the circuit will be disconnected within the required time of <25 – 40 ms.                         Question 2 (a, b and c)     1 below shows a time-current characteristic typical of miniature circuit breakers. Explain how such a characteristic can be achieved in the construction of an MCB.  In what respect would the actual characteristic of Type D MCB differ from that of Type B of the same rating?  What significance does this difference have on the application of each type?           Time s       0.1s——————————————–   Prospective current A Fig 1: Typical time-current characteristic of MCB   In general, HRC fuses provide back-up protection for the overload protection relay incorporated in an induction motor starter. Explain the difference between these two forms of protection and show how the two protective devices should be co-ordinated.  Define the two types or levels of back-up protection that could be provided.       A major consideration in the design of high-voltage circuit breakers is the problem of arc extinction. Describe briefly two mechanisms incorporated in modern circuit breakers for extinguishing arcs rapidly.     Question 3 (a, b and c)   A 60kVA 415V 3-ph 50-Hz distribution circuit is supplied by a 25mm2 3-core PVC-insulated, steel wire armoured cable, which has a tabulated current-carrying capacity of 102A.   (i) Select a suitable BS88 fuse rating to protect against overload currents.   (ii)  Given that the k-value for PVC/copper cables is 115, check using the adiabatic equation and the time-current characteristics, that this fuse also provides the cable with adequate short-circuit protection if the prospective short-circuit current is 7080A.           Finally it is proposed to run a separate 16mm2 copper/PVC protective conductor such that the circuit has a total earth-fault loop impedance ZS of 0.43W. Check that this cable would be able to withstand the thermal constraints of a zero-impedance earth fault.                   A 25-kW 415-V 3-phase 50-Hz induction motor operates at a power factor of 0.85 and an efficiency of 0.89 p.u. on full load. What would be a suitable current setting for the overload trip in the starter?   When starting a high-inertia load direct-on-line, a current of six time’s normal full-load current is sustained for 12 seconds.  Use an appropriate manufacturer’s data sheet to select a suitable fuse link to provide short-circuit and back-up protection for the starter.             Recommended links   http://www.blc.lsbu.ac.uk/eservices/Module3/Module%203d.html   http://www.voltimum.co.uk/articles/hrc-fuses-when-its-good-be-weakest-link   http://www.studyelectrical.com/2014/05/principles-and-methods-of-arc.html             Mid Kent College of H & FE School of Advanced Computing & Engineering     Marking Scheme Check List                   Student’s Name:………………………   ITEM Grading Max Marks Available Marks Achieved Comments Description of the mechanisms of an HRC fuse link 5     Explanation of the “let-through energy” with respect to discrimination between major and minor fuses, with reference to manufacturers’ data.   5     Detailed description of an RCD operation 5     Explained the characteristics of the construction of an MCB 5     Justified the difference in different types of MCB characteristics 5     Explanation of the HRC back-up and overload relay protection 5     Definition of two types of back-up protection 5     Definition of two mechanisms in the modern circuit breakers to extinguish arcs with in high voltage designs. 5     Selection of suitable BS88 fuse rating from the data sheets 5     Perform adiabatic equation calculation for prospective short-circuit current. 10     Calculates the suitability of cable during zero-impedance earth fault. 10     Selection of the suitable current setting for the overload trip in the starter of the stated induction motor. 5     Ability to use suitable manufacturers’ data to select suitable fuse link to provide short-circuit and back-up protection for the starter. 5     Makes general comments on the assignment and self evaluates on the stated learning outcomes. 5     TOTAL 80     Final Grade     General Comments:-                  

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