Basic_Electrical_Engineering_4th_edition

Flexible conduit wiring. This method ofwiring is used to provide mechanical protection to cables between rigid conduits (surface conduit) and electric machine or any other electrical device. However, this is not a general method ofwiring. It is used for connecting rigid conduit with machine terminal box in case ofmotor wiring and energy meter with main switch in case of industrial and domestic wiring.

The following are the advantages and disadvantages ofconduit wiring :

Advantages :

1.It provides protection against mechanical injury to insulated cables and against short­ circuits.

2.Since conduits canbe made water tight, the conduit wiring provides protection against moisture.

3.Gives better appearance.

4.Relatively longer life.

Disadvantages :

1.It is a relatively costlier system ofwiring.

2.Highly skilled workmanship is required and it is more time consuming.

The method ofwiringis normallyrecommended where cost is not the consideration e.g. in big hotels, workshops and factories and places ofstoring inflammable materials.

1 1 .3 SPECIFICATIONS OF WIRES

Usually cables or wires have a no. ofstrands twisted together. Standard stranding consists of6 wires around 1, then 12 around 6, followed by 18, 24 and so on. A stranded conductor is expressed as 7/0. 1 where the first number stands for the no. ofstrands used and the second number corre­ sponds to the diameter ofeach strand in mm. Some times the second number given corresponds to the gauge ofthe strand used e.g., 3/20 which means a cable with three strands each of20SWG.

All electric cables consist ofthree essential points :

(a)the conductor for transmitting electrical power,

(b)the insulation, an electrical insulatingmedium, needed to insulate the conductor from direct contact with earth or other objects, and

(c) external protection against mechanical damage, chemical or electro-chemical attack, fire or any other dangerous effects external to the cable.

Copper conductor has extensively been used for cables but oflate aluminium is being used to a considerable extent. To obtain flexibility a number ofwires are made up into a strand which makes it easier to handle, less liable to kink and break and to a large extent eliminates risk to the conductor breaking through the dielectric. The wires in a stranded conductor are twisted together to form lays. The successive layers usually are stranded in opposite direction i.e., ifthe wires of one layer have a right-handed lay, the next layer has a left-handed lay.

In regard to specification ofcables, it is important to mention all possible details regarding the particular type ofcable in terms ofsize (in mm or mm2) types of conductor whether copper or aluminium, type ofinsulation PVC or VIR and voltage grading.

1 1 .3.1 Size of Conductor

Following points must be kept in mind while determining the size of the conductor for internal wiring for a given circuit.

(i)Minimum size ofthe conductor for mechanical reason

(ii)Current carrying capacity

(iii)Voltage drop

Minimum size of Conductor

(i) Wire. The minimum size of copper conductor in use is 1/1.2 mm based on permis­ sible wattage of 800 watts in the subcircuit. The aluminium conductor of size 1/1.4 mm is used for a subcircuit in domestic wiring and minimum size of conductor for power wiring is 4 mm2 and 1/2.24 mm.

(ii) Underground cables. The area of cross section of conductor for two core cables should not be less than 6 mm2 and for three and four core cables it should not be less than 25 mm2. The minimum area of cross section of a three and halfcore cable should be 50 sq. cm.

Current-carrying capacity. Whencurrent is passed through a wire a certain amount of power is wasted in the wire which appears in the form of heat which results in increase in temperature ofthe conductor which may again decrease the life of insulation and, therefore, the rating should be such that when that current is allowed to flow continuously it does not affect the insulation ofthe cable. However, it should be able to carry a relatively larger current. (Varying between 1.5 to 2 times the continuous rated current.) for a short duration without affecting the insulation.

Voltage drop.Before decidinga proper size ofcable to be used in acircuit, due consideration must be given to voltage drop. This voltage drop in the cable from distribution board to the point ofdevice (lamp, fan, motor etc.) is due to the flow ofcurrent through the cable. The permissible+ voltage drop from supply terminal to any point on the wiring system should not exceed 2% 1 volt for light loads and 7.5% for declared supply voltage of power loads.

We take a few examples to illustrate the selection ofspecification forwires:

Example 11.1. Determine the size of the conductor for power and lighting circuit from meter to main distribution board having three light/fan circuits of800 watt each and two 15A circuits (power circuits) of1500 watt each. Assume negligible voltage drop.

1 1 .5 TYPES OF CABLES

A briefdescription ofthe various cables used in domestic wiring is given here. The main requirements ofthe insulting materials used for cable are :

1.High insulation resistance.

2.High dielectric strength.

3.Good mechanical properties i.e. tenacity and elasticity.

4.It should not be affected by chemicals around it.

5.It should be non-hygroscopic because the dielectric strength ofany material goes very much down with moisture connect.

VulcanizedRubber

Rubber in its natural form is highly insulating but it absorbs moisture readily and gets oxidized into a resinous material; thereby it loses insulating properties. When it is mixed with sulphur alongwith other carefully chosen ingredients and is subjected to a particular tempera­ ture it changes into vulcanized rubberwhich does not absorb moisture and has better insulating properties than even the pure rubber. It is elastic and resilient.

The electrical properties expected ofrubber insulation are high break-down strength and high insulation resistance. In fact the insulation strength ofthe vulcanized rubber is so good that for lower voltages the radial thickness is limited due to mechanical consideration.

The physicalproperties expected ofrubber insulation are that the cable should withstand normal hazards of installation and it should give trouble-free service.

Vulcanized rubber insulated cables are used forwiringofhouses, building and factories for low power work.

PVC (Polyvinyl chloride) cables. These are thermoplastic insulating materials and not used for high temperature as it gets softened and flows down to heat. These are therefore, not used for heating appliances. PVC is harder than rubber, hence a thin layer ofPVC insulation is good enough. In fact, its thickness is decided by mechanical reasons rather than electrical. The PVC insulated cables are lesser in diameter as compared to TRS and more number ofwires can be placed in conduit as compared to TRS wires. These are used upto 1.1 kV voltages especially in concealed wiring system.

Tough Rubber Sheathed (TRS) or Cab Type Sheathed (CTS) Cables. This cable is usedwhere humidity is high and it does not deteriorate even during long duration ofexposure to moisture. As the name suggests these cables are provided withouter protective covering oftough rubber over normal insulation. These are available as single, twin and three cores with an earth continuity conductor. These are used on 250/500 volt circuits.

1 1 .6 LIGHTING CONTROL CIRCUITS

Various lighting control circuits have been developed for maximum convenience and economical use ofelectric lighting.

Fig. 11.4 (a) shows a circuit diagram for switching two incandescent lamps with a single pole lighting switch. The no. oflamps in the circuit may be one, two or more. Fig. shows

those that do not cause direct physiological but may cause involuntary muscular reactions are called secondary shock currents. It is to be noted that the threshold value ofcurrent which gives a tingling sensation on the hand or finger when touching an electrically live device differs from person to person. For a normal healthy person a current of1 mA is the threshold value to produce tingling sensation. Currents of about 10 to 30 mA can cause lack ofmuscular control.

In most of the cases a current of 100 mA will cause ventricular fibrillation. Currents of higher magnitude may even completely stop the functioning ofheart or may cause severe electri­ cal burns. Ventricular fibrillation is that condition when the heart beats in an abnormal and ineffective manner with fatal consequences. Therefore, the threshold value ofcurrent is the main criterion for propergroundingofdevices. Under threshold condition, there is rapid uncoordinated contractions ofthe ventricles ofthe heart thereby the synchronisation between the heart beat and pulse beat is lost. When individual muscle fibres contract independently, they are said to be fibrillating. Fibrillation can occur in the atrial or ventricular muscles. During ventricular fibril­ lation the ECG (Electrocardiogram) is totally disorganised.

A human heart canbe considered as a muscle operating rhythmically due to a nerve pulse that provides the heart beat. Therefore, when an external signal due to electric current is sent into the heart which will have different frequency, different from that of the normal heart, it disturbs the rhythmic flow ofoperation ofheart. This condition ofoperation is known as ventricu­ lar fibrillation or arrhythmic operation ofheart. Once this arrhythmic condition is set up, it is difficult to stop. It usually requires injection ofanother electric current to stop the fibrillation and reestablish the normal rhythm. Table 11.2 gives typical effects of Electric shocks current on humans.

Table 11.2. Typical effects of Electric shock currents on humans

It is to be noted that the effect ofelectric shock currents not only depends upon the physi­ ological features ofa person but the psychologic factors also play an important part. The effects of electric current on vital human organs not only depend upon the magnitude of currents but it depends upon the duration and frequency ofthe current. Humans are more vulnerable to electric shock current at 50-60 Hz. The human body can withstand slightly higher current at 25 Hz and almost five times larger for d.c. current. Similarly at frequencies of1 KHz or 10 KHz even larger currents can be tolerated. In case oflightning (where the frequency is very high and duration is in µ secs) the human body can withstand very high currents in terms of several hundreds of amperes.

Whenthe humanbodybecomes a part ofthe electric circuit the current that passes through it, is limited by the body resistance and the resistance of the earthing system of the device to which the person comes in contact. The human body resistance differs from person to person and also it is different when two feet or between one hand and one feet or between two hands. Gener­ ally two types ofpotentials are important from electric shock point ofview. These are

(i)Touch potential and

(ii)Step potential

When a person touches the electrically live part ofa device, the potential between the hand and the foot is known as touch potential. However the potential between the two feet ofthe person when walking on an electrically conducting and live surface is known as step potential. Usually, the step potentials are smaller in magnitude as compared to the touch potentials.

For grounding a device, a galvanised steel wire is connected to it's body and the other end ofthe wire is connected to a steel rod which is put into the ground upto a certain depth. The pit contains moisturised mixture ofsalt and charcoal to provide better conductivity. The steel rod or any other electrode is placed in this mixture. Sometimes more than one rod are used to improve the grounding i.e., reduce the earthing resistance. The earthing resistance also depends upon the resistivity ofthe soil. In transmission lines, tower footing resistance is reduced by using radial or parallel galvanised steel wires in the ground. These are known as counterpoise wires. The accept­ able value ofgrounding resistance varies upon its application e.g., for domestic appliances, the acceptable value is about 1 ohm and for substationequipment a 5 ohm resistance is acceptable. It is seen that the thickness ofthe rod does not play a major role in reducing the ground resistance as does the length ofthe rod. Therefore, it is better to use thin but long rods or many small rods.

Some ofthe safety measures to avoid electric shock are given below :

1. See that allthe metallic parts ofvarious equipments and appliances are properly earthed. 2. Before energising the domestic wiring its insulation must be checked.

3. The phase wire must be connected through the switch.

4. Never try to handle or operate any electrical appliances with wet hands or standing on a wet floor.

5. Make it a habit to switch offthe supply and then pull out the plugwhenever you have finished with a particular gadget.

1 1 .8 FUSES AND HRC FUSES

A fuse is a small piece ofwire connected inbetween two terminals mounted on insulated base and is connected in series with the circuit. The fuse is perhaps the cheapest and simplest form of protection and is used for protectinglow voltage equipments against overloads and/or short circuits. The fuse is expected to carry the normal working current safely without overheating and during overloads or short circuits it gets heated up to melting point rapidly. The materials used normally are tin, lead, silver, zinc, aluminium, copper etc. For small values ofcurrents an alloy oflead and tin in the ratio of 37 per cent and 63 per cent respectively is used. For currents more than 15 amperes this alloy is not used as the diameter ofthe wire will be large and after fusing, the metal released will be excessive. Silver is found to be quite satisfactory as a fuse material because it is