Tuesday, December 2, 2014

Electrical circuit protection

Electrical circuit protection

Every electrical installation must be protected from current overload. This is achieved by locating a protective device at the commencement of each circuit in the form of a fuse or circuit breaker. Their function is to protect the circuit conductors (cables) and not the appliance or user.

Fuses

Three types of fuse are used (Figure 100); the rewireable or semi-enclosed fuse; the cartridge fuse or fuse link; and the high rupturing capacity (HRC) fuse.

Rewireable fuse

This comprises a fuse holder, an element and a fuse carrier. The holder and carrier can be made of porcelain or bakelite. The fuse holder is colour coded as follows:

45 A green

30 A red

20 A yellow

15 A blue

5 A white

This type of fuse is popular for domestic appliances and small commercial units because of cheapness and ease of replacement. It is not recommended for commercial refrigeration duty because of these disadvantages:

1 The fuse carrier can be loaded with the wrong size fuse wire.

2 The fuse element tends to weaken after long usage owing to oxidation of the wire by heating in air. This causes it to fail under normal conditions,

i.e. normal starting current surges are sensed by the fuse as an overload.

image3 The fuse holder and carrier can be damaged as a result of arcing in the event of an overload.

Cartridge fuse

This consists of a porcelain tube with metal end caps to which the fuse element is attached. The tube is filled with silica.

These fuses may be used in plug tops with 13A socket outlets and in distribution boards. They are recommended for refrigeration duty. They have the advantages over the rewireable types of not deteriorating, of being more accurate in breaking at the rated value, and of not being subject to arcing.

HRC fuses

This is a sophisticated version of the cartridge fuse. It is normally used for the protection of motor circuits in commercial and industrial installations. It

consists of a porcelain body filled with silica and with a silver element; the body terminates in lug-type end caps.

These fuses are fast acting, and can discriminate between a starting surge and an overload. An indicating element shows when the fuse is ruptured.

Rating

The selection of fuse ratings depends on the full load current, the locked rotor current and the cable size. The current ratings for tinned copper wire are shown in Table 6.

Fusing factor

Different types of fuse provide different levels of protection. Rewireable fuses are slower to operate and are less accurate than cartridge types.

To classify the protection devices it is important to know the fusing performance. This is achieved by the use of a fusing factor:

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Here the fusing current is the minimum current causing the fuse to rupture, and the current rating is the maximum current which the fuse can sustain without rupturing. For example:

1 A 5 A fuse ruptures only when 9 A flows; it has a fusing factor of 9/5 or 1.8.

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2 The current rating of a cartridge fuse is 30 A and the fusing factor is 1.75; the fuse will rupture at 30 ð 1.75 D 52.5 A.

3 The current rating of an HRC fuse is 20 A and the fusing factor is 1.25; the fuse will rupture at 20 ð 1.25 D 25 A.

It must therefore be realized that a fuse is rated at the amount of current it can carry, and not the amount at which it will rupture. Rewireable fuses have fusing factors of approximately 1.8; cartridge fuses of between 1.25 and 1.75; HRC fuses of up to 1.25 maximum; and motor cartridge fuses of 1.75.

Circuit breakers

A fuse element provides protection by destroying itself and must be replaced. It cannot be tested without destruction; therefore the result of a test will not apply to the replacement.

The circuit breaker (CB) is an automatic switch that will open in the event of excess current and can be closed again when a fault is rectified. The switch contacts are closed against spring pressure, and held closed by a form of latch arrangement. A slight movement of the latch will release the contacts quickly under the spring pressure to open the circuit; only excessive currents will operate it.

Two types exist: thermal and magnetic.

Thermal

The load current is passed through a small heater, the temperature of which depends upon the current it carries. The heater will warm up a bimetal strip. When excessive current flows the bimetal strip will warp to trip the latch mechanism.

Some delay occurs owing to the transfer of heat produced by the load current to the bimetal strip. Thermal trips are suitable only for small overloads of long duration. Excessive heat caused by heavy overload can buckle and distort the bimetal strip.

Magnetic

The principle used in this type is the magnetic force of attraction set up by the magnetic field of a coil carrying the load current. At normal currents the

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magnetic field is not strong enough to attract the latch. Overload currents will increase the force of attraction and operate the latch to trip the main contacts.

A typical magnetic circuit breaker is shown in Figure 101