Question

Selection of Fuse Practical

Answer 1

Practical Significance

Fuse prevents the electrical circuit in the event of over current and fault, hence to avoid the damage to an electrical circuit selection of correct rating of fuse is important.

Competency and Practical Skills

This practical is expected to develop the following skills for the industry identified competency: ‘Plan wiring/cabling activities using relevant materials following safe practices’

i. Select proper fuse element.

ii. Connect fuse in electrical circuit.

iii. Replace fuse element.

Practical Outcome

1. Select and place relevant fuses in different lighting circuits.

Relevant Affective domain related Outcome(s)

i. Practice good housekeeping with safety measures.

ii. Demonstrate working as a leader/a team member.

iii. Maintain tools and equipment.

iv. Follow ethical practices.

Minimum Theoretical Background

A fuse is a safety device used for purpose of protecting circuit against excess current.

In the event of excessive current, the fuse element melts and opens up the circuit thereby protecting it from damage.

In electrical installations, the fuses are always connected into live wires (L1, L2 and L3 as shown in Fig. 1) and never into the neutral (N) or the protective earth line (PE).

Following are the types of fuses used in domestic wiring.

1. Re-wirable type (up to 200 A)

2. Cartridge type ( up to 1250 A)

Some important terms related to fuse are as follows:

1. Current Rating: Safe maximum current that can be passed continuously without overheating.

2. Fusing Current: The current at which the fuse element melts.

3. Fusing Factor: It is a ratio of minimum fusing current and current rating.

The fusing factor for re-wirable fuse varies between 1.4 and 1.7 and may go up to 2.0, but for HRC fuse it is 1.1. However, a fuse selected for overcurrent protection should not have a fusing factor more than 1.25.

Practical set-up / Circuit diagram / Work Situation

Resources Required

Precautions to be Followed

1 Follow electrical safety rules.

2 Use insulated tools while working on installation.

3 Insure that the supply in “OFF” while replacing the fuse.

Procedure

1. Identify any electrical circuit in the department.

2. Identify the number of sub-circuits.

3. Calculate the connected load in the sub- circuit.

4. Calculate total load on a circuit.

5. Calculate individual current rating of circuit.

6. Select proper current rating of fuses for the sub-circuits and main switch.

7. Switch “OFF” the main switch.

8. Test the supply at incoming and outgoing terminals of the main switch with the help of test lamp.

9. Test the supply at DB.

10. Replace the old fuse element in DB and main switch using new fuse element of proper rating.