i) The neutral conductor of 3 phases, 4 wire a.c. electrical system and the middle conductor of two phase, 3 wire system should be earthed by not less than two separate and distinct connections with earth at the generating station and at the sub-station.
ii) Metal frame of generators, motors and other metallic parts of the equipment.
iii) All metal parts of electrical installation such as metal conduits, light fittings, iron clad main switches, iron clad distribution boards, cable sheathes etc.
iv) Earth pin of the three pin lighting and power plug sockets.
v) Steel tower, tubular poles, rail poles,-etc. used in overhead transmission line should be earthed.
vi) Stay wires provided for overhead lines. Q # 28: What is the purpose of installing circuit breakers? Answer: Circuit breakers are installed to perform the following duties:
(a) To carry full load current continuously.
(b) To open and close the circuit on no load.
(c) To make and break the normal operating current.
(d) To make and break the short circuit currents of magnitude up to which it is designed for.
(e) In indoor substations the oil filled and hard gas types of circuit breakers are usually used. Q # 29: What is the purpose by Power Transformers? Answer: Power transformers are used for stepping down the voltage for transmission at generating stations and for stepping down the voltage for further distribution at main step down transformer substations. Usually naturally cooled, oil immersed, known as on type, two windings, three phase transformers, are used up to the rating of 10 MVA. The transformers of rating higher than 10 MVA, are usually air blast cooled. For very high rating the forced oil, water cooling and air blast cooling may be used. For regu1atin the voltage the transformers used are provided with on load tap changer. Q # 30: What is wet action system?. Answer: In such system the distribution pipe work is permanently charged with water at normal system pressure. Water is contained with the piping by fusible bulb detectors fitted either to the projector nozzles (spriniders) or to detector control valves. The flow of water which results from fusing of the bulb causes the section deluge valve to life off its seat and supply a continuous flow of water to the affected projectors. The section of pipe work downstream of the detector control valve is normally dry and is fitted with open projectors covering the risk area. When the deluge v1ve operates, it allows water pressure into a recess to operate a local audible alarm and a remote “fire” or “water applied” alarm. Q # 31: Why a Fire Alarm System is compulsory in each factory and building? Answer: A fire alarm system may be required in a building for the following purposes:
i) Protection of life
ii) Protection of property
Some further aspects of the building code include:
i) Circuit design
ii) Zones
iii) Alarm trigger devices
iv) Sitting of control and indicating devices
v) Power supplies
vi) Wiring system
vii) Testing, inspection and commissioning Q # 32: What is Single-Line Telephone System? Answer: Figure gives the circuit for simple two-way call and speaking which is suitable for short distances. With the telephone hand set lifted off the cradle and station “A” button pressed, the bell will ring in station “B”. By raising the receiver at “B” both stations can communicate with each other. The battery connections are reversed so that they are connected in series for maximum voltage during the speaking session. Lifting the telephone set and pressing the push of station “B” causes the bell at station “A” to ring Q # 33: What is Smoke and heat detector? Answer: this type of detectors come with a photoelectric smoke detector and a fixed temperature heat detector. They offer a general purpose protection of a rapid fire and a smouldering fire incident. The performance of this type of detectors is better than the single detection detectors, and generally popular in both the residential and industrial applications. These are located on ceilings of risk areas and have LED indicators to identify which device has operated. Q # 34: Give the names of wiring test instruments. Answer: Names of wiring test instruments are as under:
i Continuity tester
ii. Electrician’s test lamp
iii. Polarity tester
iv. Multi-meter
v. Resistance bridge
vi. Megger Q # 35: What is Automatic Dial System? Answer: Automatic Dial System
Automatic dial system is implemented in private automatic branch exchange (PABX) systems. The individual call stations are connected each by a twisted pair of wires to the automatic exchange. This is also the termination of exchange lines and if required, the connecting lines to other private branch exchanges. The dialled connections are made automatically. Q # 36: Give the working of industrial dry chemical system of firefighting? Answer: Working of industrial dry chemical system of firefighting is as under:
1. Fire detectors sense fire heat in work area, plenum or duct.. .a signal is sent to the Control Panel.
2. The Control Panel electrically activates the Cylinder Control Head, which opens the cylinder valve and actuates the system.
3. The Control Panel sounds an alarm, notifies the fire authorities and shuts down airflow in the hazard area.
4. The fire is suppressed as dry chemical is propelled by stored pressure through the discharge pipes and nozzles into the protected area. Q # 37: What is Pole-mounted Substation? Answer: Pole-mounted Substations: Such substations are erected for mounted distribution transformers of capacity upto 250 kva. Such substations are cheapest, simple and smallest of substations. All the equipment is of outdoor type and mounted on the supporting structure of HT distribution line. Triple Pole Mechanically Operated (T.P.M.O) switch is used for switching. ‘on’ and ‘off’ HT transmission line. HT fuse unit is installed for protection of HT side. To control LT side MCCB is installed. Lightning arrestors are installed over the HT line to protect the transformer from the surges Substations are earthed at two or more places. Generally transformers up to 100 kva. are mounted double pole structure and for transformers of capacity above 100 kva but not exceeding 250 kva 4-pole structure with suitable platform is used. This type of pole mounted substation is erected in very thickly populated location.
The maintenance cost of such substations is low and by using a large number of such substations in a town possible to lay the distributors, at a lower cost. But owing to increase in number of transformers, total kva is increased, no load losses increase and the cost per kva is thus more. Economy is the main consideration when a choice is made for such substations. Q # 38: What is wiring colour scheme? Answer: Wiring colours: The standard wiring colours in the UK are the same as elsewhere in Europe, Australia, and New Zealand and follow international standard LEC 60446.
Cables of USA origin will have White as Neutral and Black as live. This can occur on IEC mains leads and dual 220V/1IOV imported equipment. Q # 39: Give the faults in wiring Answer: To identify a faulty circuit causing fuses to blow or MCBs to reset can be a tricky process. Initially try to remember what appliance was switched on, or what happened at the time the fault first occurred. The two reasons for a fault occurring are circuit overloading and a short circuit in the system.
Overloading
Overloading is a relatively easy problem to solve, it is caused by using too many appliances on one circuit exceeding the power rating for that circuit, causing the fuse to blow. The solution is-simple, never use all the appliances on that circuit at the same time.
Short Circuit
if the circuit is not being overloaded the next step is to inspect all the outlets, lights, flexes and appliances using the blown circuit. Look for damage that could have caused a short circuit and repair it. Q # 40: Give the steps of commission the wiring? Answer: Commission the wiring, follow these steps:
• Check the wiring of the alarm and floats.
• Check the pump Is are sitting right in the chamber.
• Check the power wire to the control panel.
• Check the wire of float switch control up for the pumps.
• Check the wire of tie high water alarm if extra beacon light.
• If 3-phase pumps are installed check the rotation of the pumps.
• Test all connections and test everything is work correctly.
• Once completed, issue a Wiring and Commissioning Certificate for your records. Q # 41: How fault(s) are located in the electrical composite equipment? Answer: Location of fault(s) in the electrical composite equipment:
• Electrical composite equipment and associated circuits are isolated, where necessary, in accordance with established procedures.
• Other OHS policies and procedures are followed.
• Visual checks of the electrical composite equipment and components are carried out in accordance with established procedures to detect any abnormal or obvious damage or fault.
• Safety tests and circuit continuity are progressively carried out to assure isolation, and to detect operational, electrical or other non-conformances or fault(s).
• Electrical composite equipment is dismantled and/or removed, where necessary, and components stored in accordance with established procedures to protect them against loss or damage.
• Fault(s) are confirmed and components to be replaced or adjusted are determined and details recorded in accordance with established procedures.
• On-going checks of the quality of work are undertaken in accordance with established procedures. Q # 42: How faults are rectified in the electrical composite equipment? Answer: After location fault(s) rectified as under:
• Isolation of electrical composite equipment and associated circuits is confirmed in accordance with requirements and established procedures.
• Materials and resources necessary to complete the work are obtained in accordance with established procedures and checked against job requirements.
• Adjustments are made in accordance with established procedures, where necessary, to ensure electrical composite equipment operates in accordance with intended parameters.
• Fault(s) are rectified in accordance with established procedures, where necessary.
• Approval is’ obtained in accordance with established procedures from appropriate personnel, before any contingencies are implemented.
• Tests on the electrical composite equipment are in accordance with established procedures performed to ensure safe return to service and operation of the electrical composite equipment. Q # 43: Describe the Catenary System of wiring. Answer: Catenary System
A catenary, is a system of overhead wires used to supply electricity to a locomotive, streetcar, or light rail vehicle which is equipped with a pantograph. Unlike simple overhead wires, in which the un-insulated wire is attached by clamps to closely spaced cross wires supported by poles, catenary systems use at least two wires.
The catenary or messenger wire is hung at a specific tension between line structures, and a second wire is held in tension by the messenger wire, attached to it at frequent intervals by clamps and connecting wires. The second wire is straight and level, parallel to the rail track, suspended over it as the roadway of a suspension bridge is over water.
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