PROTECTIVE DEVICES Compiled and Presented by D. Nedrick

Why Do we need Protective Devices in Circuits?Short Circuit A condition which exists in an electric circuit when conductors of opposite polarity come into contact with each other. A circuit that contains almost no resistance to limit the flow of current. This condition results in excessive current flow.Dangers of short circuit: fire

Why Do we need Protective Devices in Circuits?OverloadAn electrical condition in an excessive amount of current flows. This condition arises when too much utilization equipment is connected into a circuit or system or when a motor is starting. When a motor is driving a load which requires more horsepower than its rating, it is overloaded and excessive current flows.

Circuit Breaker is a electro-mechanical device for making and breaking a circuit both under normal and abnormal condition such as those of a short circuit, the circuit been broken automatically.

The circuit breaker is generally opened by hand (manually) but is automatically opened under fault conditions by an over-current release.

Over-current ReleaseThe over-current release is opened by the magnetic effect of the line current flowing in the circuit. This current flows through a current coil consisting of a few turns of heavy-gauge copper wire or copper tape. When a continual overload is placed on the protected circuit the electromagnetic field, due to the current flowing in the coil, draws up a plunger which operates a mechanical trip, thus isolating the circuit from the supply.

Fig 6.9 shows the simplified layout of an electromagnetically operated over-current release. When an overload current flows through the coil, the soft-iron plunger is lifted high enough to open the control circuit.

Fig 6.10 shows an over-current release using a piston fitted in an oil dashpot to retard the movement of the soft-iron plunger, thus providing a time-lag on the operation of the over-current release. The operating or tripping, current can be varied by screwing the dashpot up or down; it is generally set to operate at 11/2 times (150 per cent) full load current.

Over-current releases are commonly used in motor starter circuits where very high starting currents (about six times full loaded current) are encountered. The time-lag action of the dashpot allows sudden surges of current in circuit without tripping, although a continuous overload current would operate the trip.

Fig 6.11 shows the circuit of a manually-operated 3-phase circuit breaker which is fitted with electromagnetic overload releases. The circuit-breaker is closed by hand and the operation of any type of the over-current releases trips the switch mechanism an open all the phases

The selection of the breakers should be based on the ability to: Provide the proper overload protection Ensure a suitable voltage rating Provide a sufficient interrupt current rating Provide short circuit protection Co-ordinate the breaker(s) with other protective devices The choice of the overload protection is based on the rating of the panel board

Fuse A fuse is a weak link in a circuit which will break when too much current flows, thus protecting the circuit conductors from damage. It must be remembered that the priority of the fuse is to protect the circuit conductors, not the appliance or the user. Calculation of cable size therefore automatically involves the correct selection of protective devices.

Types of FusesA fuse simply carries a metal element, usually tinned copper, which will melt and break the circuit when excessive current flows. There are three types of fuses:The Rewireable or semi-enclosed fuse The cartridge fuse and fuse link; and The high-rupturing-capacity (h.r.c.) fuse.

Rewireable FuseThis consist of a plastic bridge and base.The bridge has two sets of copper contacts which fit into contacts in the base.

Rewireable Fuse (contd)The Fuse element, for example, tinned copper wire, is connected between the terminals of the bridge. An asbestos tube, or pad, is generally fitted in the fuse to minimize the effect of arcing when the fuse element melts.This type of fuse is termed a semi-enclosed fuse to distinguish it from the older type of fuse which consisted simply of a piece of wire connected between two terminals.

RegulationsThe approximate sizes of tinned copper wire to be used for elements in semi-enclosed fuses includes:0.2 mm 5A current rating0.35 mm - 10A Current rating0.50 mm - 15A Current rating

The current rating is the current which the fuse element will continuously without deterioration.The fusing current is the current at which the fuse element will melt. For the rewireable fuse, this is approximately twice the current rating of the fuse element (fusing factor = 2). Fusing factor = fusing current/ current rating.

Advantages and DisadvantagesAdvantages1. Cheap2. Easy to replace fuse element

Disadvantages1. Fuse element deteriorate in use2. Any size of fuse wire can be fitted, thus defeating the purpose of the fuse.

NOTE: The fuse/ CB must be capable of protecting the smallest conductor in the circuit. 3. Lacking in discrimination. It is possible that a 15A fuse element may melt before a 10A fuse element, depending largely on the condition of the wire. Further the rewireable fuse is not capable of discriminating between a momentary high starting current and a continuous fault current.

4. Easily damaged, particularly with short circuit currents.

Cartridge FuseThis type has come into common use with the fused 13A plug used on the domestic ring circuit.

The fuse element is contained in a porcelain tube fitted with two connecting caps and has a fusing factor of 1.5.The colour code for these fuses is as follows:5A White 13A Brown 15A Blue 30A Red 60A Purple

Fig. 6.16 shows the construction of a high breaking capacity fuse. This consist of the following:Porcelain tube.Silver element.Indicating element which ignites powder under the label to show when the fuse element has opened.End caps.Silica (fine sand) filling used to quench the arc.

Note. The fuse must always be placed in the phase or non-earthed conductor of the installation, never in the neutral (earthed) conductor.

Thermal TripThe thermal trip (6.17) is operated by using the heating effect of an electric current. Two flat pieces of dissimilar metals are join together and placed inside a nichrome spiral which carries the line current of the protected equipment.

When the line current reaches a certain pre-determined level, the heat from the spiral distorts the bi-metallic strip causing it to operate a push bar mechanism which opens the switch.This type of protection is generally on low current circuits (for example: fractional horsepower starters) because the bimetallic strip tends to distort permanently if heavily overloaded.

All fuses and single pole control devices (e.g. circuit-breakers, thermostats) must be fitted in the line conductor in a 2 wire installation where one pole is connected to earth