Testing fuses

Class practical

An opportunity to test the manufacturer's fuse rating.

Apparatus and materials

For each student group

Cells, 1.5 V, with holders, 3

Crocodile clips, 4

Lamps with holders, 6

Ammeter (0 - 1 amp), DC

Cartridge fuses, 0.25 A, (3 cm long), preferably glass type with visible wire, 2

Variable resistor or rheostat

Health & Safety and Technical notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a 'short circuit'. This discharges the cell rapidly and can make it explode. The risk is reduced by using 'low power', zinc chloride cells not 'high power', alkaline manganese ones.

Procedure

a Connect up the circuit shown, but with only one pair of lamps in circuit diagram. Note the current.

b Add a second pair of lamps and note the current.

c Add the third pair of lamps and again note the current. Does the current ever exceed the rating of the fuse?

d Now, set up a new circuit to determine the current needed to 'blow' (melt) the fuse. Connect the three cells in series with the fuse, the ammeter and the variable resistance (rheostat). Gradually decrease the resistance in the circuit, watching the current reading on the ammeter until the fuse blows.

Teaching notes

1 Students could try to determine whether the manufacturer's rating on the fuse is 'right'. The 'rating' is the current which the fuse can carry for some time (say several hours) but not for ever. If the current rises to twice the rating, the fuse should blow in less than one second.

If the current could rise to ten times the rating (e.g. in a mains circuit), the fuse could explode. Fuses for this application have ceramic tubes, not glass, and the space inside is filled with dry sand.

2 It is often surprising how difficult it is to blow a fuse - the internal resistance of the cells limits the current.

This experiment was safety-checked in December 2004