***Useful Information for Apprentices***

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H & S / CSCS :
Q ) Why is it Important to Report all Accidents ?
1 ) It Might Stop them Happening Again : ( A )
2) Some Type of Accident have to be Reported to the Heath & Safety Executive : ( A )
3 ) Details have to be Entered in the Accident Book : ( A )

( A ) 4 ) All of the Other Answers ◄ ◄ ◄ -&-s ( 4 / Q in one Answers
PS : They May come Up Like This

 

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* The maximum voltage drop allowed for a lighting circuit in a consumer[FONT=Times+New+Roman1252]’s installation, supplied by a public L.V system is[/FONT]
3% (App 12)
6%
10%
5%
* The maximum Zs for a BS EN 60898 32A B Type circuit breaker protecting a domestic ring circuit is
1.44[FONT=Times+New+Roman+Bold1253]Ω ( Table 41.3 )[/FONT]
1.5[FONT=Times+New+Roman+Bold1253]Ω [/FONT]1.2[FONT=Times+New+Roman+Bold1253]Ω[/FONT]
0.72[FONT=Times+New+Roman+Bold1253]Ω[/FONT]
* Automatic disconnection of supply is used as a method of protection for :-
Indirect contact
Basic protection
Fault protection ( Definitions )
Direct contact
* BS EN 60898 circuit breaker can be used for :-
Isolation
Emergency switching
Functional switching
All of the above ( Table 53.2 ) p/117
* Devices for protection against the risk of fire include :-
RCDs not exceeding 500mA
RCDs not exceeding 300mA ( 532.1 ) p/113
Time delay RCDs
Type B circuit breakers
* The following sources for safety services are recognised
Storage batteries
Primary cells
Generator sets independent of the normal supply
All of the above ( 351.1 ) p/41
24. The minimum insulation value when testing a FELV system is
( Table 61 )[FONT=Times+New+Roman-1-1]≥ 0.5M[FONT=Times+New+Roman1253]Ω ( test Voltage 250v – dc )[/FONT][/FONT]
[FONT=Times+New+Roman-1-1]≥ 0.75M[FONT=Times+New+Roman1253]Ω [/FONT][/FONT]
[FONT=Times+New+Roman-1-1]≥ 1M[FONT=Times+New+Roman1253]Ω[/FONT][/FONT]
[FONT=Times+New+Roman-1-1]≥ 0.25M[FONT=Times+New+Roman1253]Ω[/FONT][/FONT]
25. The maximum measured value for a BS EN 60898 16A B Type circuit breaker is
2.3[FONT=Times+New+Roman+Bold1253]Ω [/FONT]2.87[FONT=Times+New+Roman1253]Ω (41.3 & App 14 )[/FONT]
1.44[FONT=Times+New+Roman1253]Ω[/FONT]
2.19[FONT=Times+New+Roman1253]Ω[/FONT]
26. The maximum Zs to ensure 30mA RCD operation is
1667[FONT=Times+New+Roman1253]Ω ( Table 41.5 ) p/50 ( R ≤ 50V ÷ 30mA = 1667Ω )[/FONT]
500[FONT=Times+New+Roman1253]Ω[/FONT]
167[FONT=Times+New+Roman1253]Ω [/FONT]300[FONT=Times+New+Roman1253]Ω[/FONT]

Remember when your Testing RCBOs / Remove the White Tail ( Earth )

 

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CSCS : Q ) Which of these is the most Accurate way to Locate Buried Services ?
A ) Trial Holes ◄ a Must
* Cable Plans
* Architects Drawings
* Survey Drawings
Q ) if you Need to dig Near Underground Services , you should Use ?
A ) a Spade or Shovel ◄ a Must
* a Jack Hammer
* an Excavator
* a Pick and Fork
Q ) if you Need to Work in a Confined Space . One Duty of the “ Top-Man !!!! is to ?
A ) Start the Rescue Plan if Needed ◄ a Must
Q ) You are Using some Equipment . it has Just been given a PROHIBITION NOTICE . What does this Mean ?
A ) You Must NOT Use it Till it is Made SAFE !!!! ◄ a Must
Q ) The whole Site has been ISSUED with a PROHIBITION NOTICE . What does this Mean ?
A ) You must STOP Work ◄ a Must
Q ) After watching you Work , an Heath & Safety Executive Inspector Issues an IMPROVEMENT NOTICE . What does this Mean ?
A ) You are NOT Working in a SAFE WAY . ◄ a Must
Q ) a PERMIT to Work Allows ?
A ) Certain JOBS to be Carried OUT Under Controlled Conditions ◄ a Must

 

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Some old Notes
1) The balancing of single-phase loads connected across three-phase-supply systems is to :
A) Reduce the overall cable size ,
2) The purpose of the National Grid is to :
A) Transmit electricity from power stations to substations .
3) The reason for transmitting voltages at high levels is to : A) Decrease Current levels .
4) Electrical energy is transmitted over long distances by means of the National “ Super Grid “ system at a nominal voltage of : A) 400kV .
5) The common supply voltages for public “ Distribution “ in Britain are : A) 230V : 400V : 11kV :
6) The Neutral conductor of four-wire three-phase and Neutral system carries the : A) Out off balance current :
7) The relationship between Line-Current ( IL ) and phase current ( IP ) in a Three-Phase Star- connected system is given by the formula : A) ( IL =IP )
8) A Star connected system has a Line voltage of 500V, what is the Phase Voltage : A) 289V
9) The relationship between Line current ( IL ) and Phase current ( IP ) in a Three-Phase Delta connected system is given by the formula :
A) ( IL = √3 x IP )
10) A Delta connected system has a Line voltage of 500V, what is the Phase Voltage : A) 500V
11) The point at which the Neutral and Earth are connected at the supply Authorities transformer is called the : A) Star-Point .
12) Each set of windings in a three-phase power generator are set apart by : A) 120° .
13) The earth neutral of Authorities supply must be fitted at the Intake position with a : A) Solid Link .
14) The electrical energy generated by power stations is transferred to the Grid by means of : A) Transformers .
15) A typical insulator for an Overhead line Distribution system is : A) Glass .
16) Power in three-phase circuits can be calculated from which of the following formula : A) P = √3 VL IL cosØ .
17) A balanced Star-connected three-phase load is fed from a 400V, 50Hz supply . The line current is 40A, Calculate the power consumed : A) 27.7kW .
18) The nominal voltage at which heavy industry in UK takes energy from the supply companies is : A) 33kV .
19) The TT System of earthing is usually used in : A) Rural Areas .
20) When measuring the line voltage in a three-phase four-wire supply system the voltmeter should be connected :
A) Between any Two-Phase conductor’s :
21) – PME ( Protective Multiple Earthing ) is mainly associated with which of the following Earthing Systems : A) TN-C-S .
22) What protective device would normally be installed with a TT System to Offer extra protection : A) Residual Current Device .
23) A supply system which uses the sheath of a SWA cable to provide an Earth connection is : A) TN-S .
24) Three-Phase step down transformers are used in Transmission and Distribution systems to step-down from 11,000V to 400 / 230V . The form of connection on Primary and Secondary windings respectively are : A) Delta / Star .
26) The greatest source of air pollution from coal-fired power station chimneys is : A) Carbon Dioxide .
27) Electrical supply are sometimes connected in the form of a Ring because : A) This system gives better supply security than more common Radial systems .
28) in TT Earthing system the connection to Earth is Achieved by connecting to : A Metal Rod drive into the Ground .
29) Electricity is generated at an alternating frequency and time period of : 50Hz , 20mS .
30 ) Which of the following methods of generating electricity uses a fossil fuel : A) Gas Fired ,

 

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Refrigeration & Air Conditioning ( RAAC ) / CSSC

Q) If you find a coloured wire sticking out of an electrical plug , which is the correct action to take ?
A) Mark the item as defective and follow your company procedure for defective items .

Q) Extension leads in use on a site should be ? : A) Located so as to prevent a tripping hazard ,

Q) what should you do if you need additional temporary wiring for your power tools whilst working on site ?
A) Stop work until an authorized supply has been installed ,

Q) When planning a lifting operation the sequence of operations to enable a lift to be carried out safely should be confirmed in ? :
A) A method statement :

Q) The safe working load ( SWL ) of lifting equipment is ? : A) The absolute maximum safe working load :

Q) Before using a ladder you must make sure that ? : A) it is secured to prevent it from moving sideways or sliding outwards ;

Q) When positioning and erecting a stepladder , which of the following is essential for its safe use ?
A) The restraint mechanism is spread to its full extent .

Q) Generally , how may working platforms should be in use at any one time on a mobile tower ? A) One ,

Q) What is the recommended maximum height for a free-standing mobile tower ? A) in accordance with the manufacturer’s recommendations

 

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Q) What is the first thing you should do after getting on the platform of a mobile tower ? A) close the access hatch to prevent falls of personnel, tools or equipment .
Q) What should be done before a mobile tower is moved ? A) All people and equipment must be removed from the platform .
Q) What must be done first before any roof work is carried out ? A) A risk assessment must be carried out .
Q) What is edge protection designed to do ? A) Prevent the fall of people and materials .
Q) Where should Liquefied Petroleum Gas ( LPG ) cylinders be positioned when supplying an appliance in a site cabin ? A) Outside the cabin .
Q) if you are working where welding is being carried out , what should be provided to protect you from “ Welding Flash “ . A) Screens
Q) how should you position the exhaust of an engine-driven generator which has to be run inside a building ? A) position the exhaust outside the building .
Q) how should cylinders containing Liquefied Petroleum Gas ( LPG ) be stored on site ? A) in a locked external compound at least 3 metres from any oxygen cylinders .
Q) when working in a riser, how should access be controlled ? A) By a permit to work system .
Q) which of the following electrical equipment does not require portable appliance testing ? A) Battery-powered rechargeable drill .
Q) what must be clearly marked on all lifting equipment ? A) the safe working load .
Q) what is the first thing you should do after getting on to the platform of a correctly erected mobile tower ? A) Close the access hatch to prevent falls of personnel, tools or equipment .
Q) when assembling a mobile tower what major overhead hazard must you be aware of ? A) Suspended electric cables .
Q) when a new piece of plant has been installed but has not been commissioned, how should it be left ? A) with all valves and switches “ locked off “
Q) who is permitted to work with refrigerant gases ? A) A competent, trained person .
Q) what is the colour of an acetylene cylinder ? A) maroon .

 

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2330 – level 3 Unit 1 . 301 / paper 3 :- some old papers

1) the power in a three-phase circuit can be calculated using which one of the following formulae :-
►P = √3VL x IL cosθ.
P = VL x IL sinθ.
P = VL x √IL cosθ.
P = √3VL x IL sinθ.
2) the difference between the synchronous speed and rotor speed in a three-phase induction motor is called the :-
a) offset. B) drag. ►C) slip. D) phase angle.
3) When someone has received an electric shock is the first action would be to :-
a) apply resuscitation.
b) locate the isolator.
►c) raise the alarm, call for help or get someone to go for help.
d) check for moisture.
4) Thyristors conduct when a trigger voltage is applied to the :-
a) drain. ► b) gate. c) base. d) junction.
5) with a TN-C-S three-phase and neutral supply an isolator must :-
► a) break all phases.
b) break all phases and neutral.
c) break the neutral only.
d) break the phase, neutral and earth conductors.
6) Semi-conductor diodes :-
► a) allow current to flow in one direction only.
b) let current through when triggered.
c) amplify current and voltage.
d) are bi-directional.
7) the total impedance of capacitors, inductors and resistors in series can be calculated using :-
► a) Z = √ [ R2 + ( XL - XC )2 ].
b) Z = √ [ R2 + XL2 - XC2 ] .
c) Z = R2 + ( XC2-XL2 ).
d) Z = √ [R2 - XL2 ].

 

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RCD - Principles of operation

An RCD is designed to operate and isolate the circuit from the supply when there is an imbalance in the phase and neutral currents above a certain value. Any imbalance must either be the result of current leakage from the phase or from the neutral conductors to earth as, in normal operation, all current should be returned via the neutral conductor. As the earth impedance is unlikely to be zero, any ‘earthed’ metalwork would rise in potential, which would lead to a risk of shock.

► The basic construction of an RCD consists of three windings on a transformer core:

► When there is an imbalance in the phase and neutral currents the magnetic flux in the core will cause a current to flow in the third winding. As the current in this third winding increases, the magnetic trip will operate and disconnect the supply from the equipment. The trip current is arranged to be that at which the desired RCD characteristic is achieved.

► The RCD will trip at a much lower current than that required to break a conventional fuse or to trip a miniature circuit breaker. ◄◄

► If a person touches the phase conductor on the load side of the RCD this will also imbalance the supply, by creating a leakage path to earth, and trip the RCD.

► A Built-In ‘ Test’ Button can be Used to Place a Load between the Output Phase Conductor and the Input Neutral Conductor, Creating an Imbalance in the Windings - this Imbalance Causes the RCD to Trip. However, the ‘ Test’ Button Only Demonstrates that the Trip Mechanism Works, Confirming Nothing Else about its Specific Operational Characteristics.

 

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H&S Legislation

The Electricity at Work Regulations 1989 specifically deal with electrical “systems” in Regulation 4:
(1) All systems shall at all times be of such construction as to prevent, so far as is reasonably practicable, danger.
(2) As may be necessary to prevent danger, all systems shall be maintained so as to prevent, so far as is reasonably practicable, such danger.
(4) Any equipment provided under these Regulations for the purpose of protecting persons at work on or near electrical equipment shall be suitable for the use for which it is provided, be maintained in a condition suitable for that use, and be properly used.

 

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► where an RCD is Used to Supply ►Band(S) ◄ Equipment, the Individual Items must be Tested for Earth Continuity to Any Metal Chassis as the RCD ◄◄ is Only the Secondary Protection Reducing the Severity◄◄◄

►For portable units the “ test “ button should be operated before every use at a new site to ensure that it is functioning , any device failing to trip must be removed from service ( and destroyed/disposed of ) immediately

Maintenance and Testing of RCDs
all RCDs are suitably maintained, and where appropriate, tested by a competent person

► an RCD is Used wherever Transportable or Trolley-Mounted Equipment is being Used which are Identified as having Added Risks Due to their Frequent Movement

► an RCD is used whenever long cables and extension leads are being used where it is unlikely that the required ►fuse disconnection time would be met◄

► all Workshop Areas are Fitted with RCDs which are Rated at 30mA / 40mS or better

Use of RCDs
RCDs MUST be Used in Conjunction with an Overload Protection Device such as an MCB. ◄◄

► If this Disconnection Time Cannot be Achieved by Fusing Alone then an RCD Must be Used. ◄◄

 

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Electrical Appliances

Earthed Equipment
In Practice, the Most Common Instances of Faulty Earthing are:-
1) Earth connections broken accidentally or corroded through age.
2) Earth connections incorrectly made.
3) Earth connections not made at all.
4) Earth connections removed for some specific purpose and not reinstated.

 

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2330 – level 3 Unit 1 . 301 / paper 3 :- some old papers 8 Flemings left hand rule can be used to determine the :-
a) emf generated in a conductor within a magnetic field.
b) movement of a conductor within a magnetic field.
c) current flowing in a conductor within a magnetic field.
d) angle of the conductor within a magnetic field.
9 Shunt wound d.c. motors and generators have shunt field coils :-
a) connected across the supply terminals.
b) connected in parallel with the armature.
c) connected in series with the armature.
d) connected in parallel with the rotor.
10 The résistance of a 2.5mm2 copper conductor of length 8m and resistivity p = 1.72 x 10-8 Ωm is :-
a) 0.055 Ω. B) 0.55 Ω. C) 0.12 Ω. D) 5.56 Ω.
11 A circuit consisting of an inductance of 0.35H connected to a 50Hz has an inductive reactance of :-
A) 9.1 mΩ. b) 0.09 Ω. C) 109.9 Ω. D) 1.099 kΩ.
12 When carrying out volt drop calculations the symbol ( Iz ) represents the :-
a) design current.
b) current rating of the protective device.
c) overload current for the circuit.
D) current carrying capacity of the cable.
13 the Star rotor windings of a three-phase wound rotor induction motor are connected to a bank of external resistors by :-
a) slip rings.
b) a commutator and slip rings.
c) slip rings and brushes.
d) an armature and brushes.
14 A series a.c. circuit of 12Ω résistance , 6Ω inductive reactance and capacitive reactance of 3Ω . the impedance is :-
a) 13 Ω. B) 3.6 Ω. C) 153 Ω. D) 12.37 Ω.
15 An eight pole-motor operating on a 50Hz supply would have a synchronous speed of :-
a) 750 rpm. b) 3000 rpm. c) 1500 rpm. d) 2400 rpm.
16 The junction voltages for silicon diodes is approximately :-
a) 0.6V. b) 0.3V. c) 0.7V. d) 0.2V.
17 A short circuit occurring in the primary winding of an auto-transformer could cause :-
a) the supply voltage to be developed across the secondary.
b) a reduced secondary voltage.
c) a reduction of the secondary current.
d) oscillations in the output voltage.
18 The formula for calculating power dissipated in a resistor is :-
A_ P = I/R. b) P = V²R. c) P = l²R. d) P = IR².
19 Fires in the workplace may be caused by :-
a) waste disposal systems.
b) an accumulation of rubbish.
c) storing materials and packaging in designated areas.
d) preventive maintenance practices.
20 Semiconductor devices are sometimes described as :-
a) accessories. B) electronic components. C) resistors. D) devices.
21 MCBs operate on which of the following principles :-
a) thermal and magnetic.
b) thermal.
c) either thermal or magnetic.
d) magnetic.
22 Whilst working in confined spaces you :-
a) should bring a fire extinguisher.
b) use low voltage equipment.
c) use amplification equipment.
d) should be accompanied.
23 Transformers are rated in :-
kWh. B) kVA. C) kW. D) kVAr.
24 Supplies connected to the stator windings in three-phase induction motors create :-
a) opposing magnetic fields.
b) stationary magnetic fields.
c) in phase magnetic fields.
d) rotating magnetic fields.

 

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The Electricity at Work Regulations 1989 came into force on 1st April 1990; their purpose is to require precautions to be taken against the risk of death or personal injury from electricity, in work activities.

In the main, the Regulations are concerned with the prevention of danger from electric shock, electric burn, electrical explosion or arcing or from fire or explosion initiated by electric energy.

All places of work covered by the Health and Safety at Work Act (shops, offices, factories, workshops, farms, garages, sports and entertainment centres, etc.) are covered under these Electricity at Work Regulations.

Electricity at Work Regulations 1989
Safe Working Practices

WHAT THE REGULATIONS COVER

Some of the regulations are absolute (see * below), which means they must be complied with regardless of costs or other considerations. If you utilise electricity in the workplace you will be affected.

Systems, work activities and protective equipment (Reg 4) (4.4)*
Strength and capability of electrical equipment (Reg 5)*
Adverse or hazardous environments (Reg 6)
Insulation, protection and placing of conductors (Reg 7)
Earthing or other suitable precautions (Reg 8)
Integrity of referenced conductors (Reg 9)
Connections (Reg 10)*
Means for protecting from excess current (Reg 11)*
Means of cutting off the supply and for isolation (Reg 12)*
Precautions for work on equipment made dead (Reg 13)*
Work on or near live conductors (Reg 14)*
Working space, access and lighting (Reg 15)*
Competence to prevent danger and injury (Reg 16)*

The regulations convey principles of electrical safety, as applied to any electrical equipment, any work activity having a bearing on electrical safety - in other words they all apply to all electrical systems and equipment, in connection with work activities, whenever manufactured, purchased, installed or taken into use, even if its manufacture or installation pre-dates the regulations.

► FIXED AND PORTABLE ELECTRICAL INSTALLATION

The Electricity at Work Regulations 1989 place a duty on the employer to maintain and conduct tests of all fixed and portable electrical equipment at reasonable intervals. Records of maintenance and electrical testing must be retained for inspection.

 

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8. Cable surrounded by thermal insulation for 400 mm or more has a derating factor of :-
a) 0.63
* b) 0.51 (Table 52.2) p/104
c) 0.55
d) 0.5

9. When automatic disconnection of supply is used as a measure of protection, additional protection by RCD shall be provided for :-
a) Mobile equipment having a rating of greater than 32A
b) Socket outlets in commercial and industrial locations
c) Only for sockets rated at 32A or less where it is reasonable to expect they may be used to supply equipment for use outdoors
* d) (i) Socket outlets with a rated current Not exceeding 20A that are for use by ordinary persons and are intended for general use , and (411.3.3) p/47

10. The maximum disconnection time for a lighting circuit in a commercial premises protected by a ( TT ) system is
0.07
0.4s
5.0s
* 0.2s (Table 41.1) ◄►( 120V < Uo ≤ 230V ) p/46

11. Socket outlets are allowed in a location containing a bath, providing
Located outside of zone 2
Located outside of zone 3
Located outside of zone 2 and protected by a 30 ma RCD
Located 3 metres from the edge of the bath and protected by 30mA RCD (701.512.3)

12. All circuits in a location containing a bath or shower shall have
a) A disconnection time of 0.4s
b) Be installed at a depth of at least 50mm
c) Be installed using earthed conduit
* d) Additional protection by a 30mA RCD (701.411.3.3) p/166

13. When considering if supplementary bonding can be omitted from a location containing a bath or shower, the maximum resistance of extraneous conductive parts connected to the Main Earth Terminal is :-
* a) 1.44[FONT=Times+New+Roman1253]Ω[FONT=Times+New+Roman+Bold1253] 41.3 [/FONT](415.2.2) [/FONT]
b) 0.05[FONT=Times+New+Roman1253]Ω[/FONT]
c) 0.5[FONT=Times+New+Roman1253]Ω[/FONT]
d) 7.6[FONT=Times+New+Roman1253]Ω[/FONT]

[FONT=Times+New+Roman1253]I picked at random ( Amps ) [/FONT]

► R ≤ 50 V/Ia ( 50 ÷ 160 = 0.3 ( I2 / trip sec : ( Type B / MCB . 32A ÷ 160A = 0.2sec ) ◄ R ≤ 50 V/Ia in A.C. ◄◄◄ systems :
► R ≤ 120 V/Ia ( in D.C. ◄◄◄ systems : p/59 !! watch for this

Max loop Impedance : ( Ief )
Zs ≤ Uo / Ia : Zs ≤ 230 ÷ 160A = ( Zs ≤ 1.437Ω )
( Formula : calculating Earth Loop Impedance > ( Zs = Ze + R1 + R2 ) -&-s

435 : Overload Current ( Fault Current Protection / p77
435.1 : Protection afforded by One Device ( RCD/MCB ) p77 ( If ◄ : Short Circuit / Current Protection ) Equal to or Greater than PSCC .

 

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1) 75% of 140 ?
a. 110
b. 115
►c. 105
d. 125

2) Output from a single phase alternator is via..?
►a. Slip Rings
b. Commutator
c. Rotor
d. Field windings

3) Temperature increase on copper has the effect of[FONT=Arial+Bold1252]…[/FONT]?
a. expands length ways
b. expands in thickness
► c. expands in all directions
d. decreases in all directions

4) A poor conductor is[FONT=Arial+Bold1252]…[/FONT]?
a. copper
b. salt water
►c. glass
d. gold

5) If power in is 100watts and power out is 75watts, What is the efficiency ?
a. 25%
► b. 75%
c. 33%
d. 100%

6) One application of the chemical effect is ?
a. magnets
►b. battery
c. lighting
d. Carbon

7) one transmission voltage ?
a. 100kV
b. 200kV
► c. 400kV
d. 500kV

8) Which materials resistance decreases when temperature increases ?
a. copper
► b. carbon
c. aluminium
d. tungsten

9) why do we use the colour yellow for extension leads on-site?
a. Good visibility
► b. denotes voltage
c. it’s a pretty colour
d. denotes length

10) In order to prove a circuit is safely isolated we[FONT=Arial+Bold1252]…[/FONT]..?
a. ask someone if its ok to start work
► b. use approved volt tester to check supply
c. check current flow by switching on
d. look for the red or green light

 

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11) The flow of electrons is basically[FONT=Arial+Bold1252]………[/FONT]..?
a. voltage
►b. current
c. resistance
d. water flow

12) The insulation [FONT=Arial+Bold1252]“[/FONT]Magnesium Oxide[FONT=Arial+Bold1252]” [/FONT]does not work well ?
a. in high ambient temperatures
b. at high operating temperatures
► c. when ingress of moisture has occurred
d. in damp conditions

 

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Remember !

Delta = 3 phase, 3 wire system ◄
Star = 3 phase, 4 wire system ( 3 phase + Neutral ) ◄

Math’s of star and delta systems

The voltages and currents that exist within a three phase system have particular relationships.

Voltages measured across any phase winding are known as ‘Phase Voltages’ and are symbolised VP.

Voltages measured between any pair of lines are known as ‘Line voltages’ and are symbolised VL.

Currents measured through any phase winding are known as ‘Phase Currents’ and are symbolised IP.

Currents flowing along any line are known as ‘Line currents’ and are symbolised IL. Indicate all line and phase voltages and all line and phase currents in the diagrams below.

Delta systems
Voltage Relationships ( VL = VP Therefore ( VP = VL

Current Relationships : IL = 1.732 x IP& IP = IL ►1.732 ◄

Star Systems
Voltage Relationships ( VL = 1.732 x VP & VP = VL ►1.732 ◄

Current Relationships ( IL = IP Therefore IP = IL

You must remember these relationships

[FONT=Comic+Sans+MS0]►Note! Rather than use 1.732 many texts 3 will use the square root of 3 which equates to 1.732. ( √3 ) ◄◄[/FONT]

Progress :
[FONT=Comic+Sans+MS0]►[/FONT]A star connected system has phase voltages of 300v and phase currents of 12A. Calculate line voltages and currents. !! now ask your tutor

[FONT=Comic+Sans+MS0]►[/FONT]If the system above was a delta connected system, calculate the line voltages and currents. !! now ask your tutor

Balanced and unbalanced systems
* Star and delta systems can also be connected as loads. Many three phase motors will be connected in delta since only three wires are required to make the connection

* Star connections are often used as a means of connecting single phase loads to a 3 phase supply, such as in the case of a street of domestic dwellings.

A delta connected load will have identical phase impedances. The impedance of each phase will therefore be identical both in terms of it’s magnitude AND phase angle. When this is the case the current that flows in each phase will be identical and the sum of the currents will be zero. The system is said to be balanced and there is no need for a 4th wire (neutral conductor ).

It is also possible to have a star connected load where the impedance of each phase is identical in terms of it’s magnitude and phase angle. As with the delta system this will lead to identical phase currents and no excess current to flow back to neutral. Again, the system is said to be balanced and there is no need for a 4th wire(neutral conductor).

It is more usual however, to connect dissimilar loads to each phase of a star system. This will result in differences in phase currents which in turn will create some ‘leftover’ current which will flow back along the 4th wire (neutral) to the star point of the supply origin. Such a system is known as an unbalanced system and usually arises as a result of connecting separate single phase loads to each phase of a three phase star connection.

Progress :
What is meant by a 3 phase balanced system ?
Is there any need for a 4th wire (neutral) in such a system ?
How does an unbalanced system differ ?
What function does the 4th wire serve in an unbalanced system.
Which type of 3 phase system is more likely to be found with an unbalanced load ?

 

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Power in three phase systems.
Each phase of a 3 phase system will result in some power consumption.

If the system you are dealing with is balanced there will be an equal power dissipation per phase and therefore it is necessary only to calculate the power dissipated in one phase and multiply the result by 3 (for the 3 phases).

When doing this however you must remember that each phase is an a.c system and as such the power factor is required in addition to phase voltage and current.

Power/phase = VpIp Cos[FONT=Arial+Bold1253]Ø[/FONT]
If the system is balanced :Total Power = 3VpIp Cos[FONT=Arial+Bold1253]Ø[/FONT]

For an unbalanced system it is necessary to calculate each phase power in turn and add them in order to find total power.

An alternative power formula exists which utilises the line voltages and currents:
Total Power = 1.732 x VLIL Cos[FONT=Arial+Bold1253]Ø [/FONT]

These formula apply to both Star and Delta connected systems equally.

Progress :
A star connected balanced load has a phase voltage of 230v and line current of 9A. The system power factor is 0.65. Calculate the total power for the system using both total power formulae. Once you have calculated total power calculate the power per phase.

A delta connected load has a phase voltage of 180v, a phase current of 22A and a phase angle per phase of 25° . Calculate the total power dissipated in the load.

Delta : L1 L2 L3 ,
Star : L1 L2 L3 ( N ◄

Stringbags , I hope this will help your apprentices , something to go on , I knock this up !!
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2330 – level 3 Unit 1 . 301 / paper 3 :- some old papers sorry I forgot to put the Answers up for this , MTF .

 

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Three Phase Electricity Supplies and Systems
The generation and supply of electricity within the U.K is achieved through the use of a 3-phase system.

Voltage – Time !!!!!!

This consists of 3 separate phase conductors along with a neutral conductor transmitting supplies to a given destination

If we took three of these generators (A,B & C) and positioned them equidistantly (120° ) apart) around a rotating magnet we would generate three separate a.c. waveforms each with a phase displacement of ( 120° ) from each other.

* A * / * B * / * C *
Such an arrangement would provide us with a 3 phase supply but we would need 6 conductors in order for it to function. However if we take each coil and interconnect it with the other coils in particular ways we can achieve a three phase supply in either a 3 wire (Delta) system, or a 4 wire (Star) system.

Delta Connection (3-phase, 3 wire system)
Using this method of connection each of the three coil windings are wound in such a way so they each have a start and a finish to their winding.

Then the finish of winding 1 is connected to the start of winding 2. The finish of winding 2 is connected to the start of winding 3. The start of winding 3 is connected to the start of winding 1.

each winding, A,B or C, then mark their starts and finishes.
The supply is then taken from the interconnection of each winding combination as shown above. This means we can achieve a 3-phase system using only 3 wires.

There is an alternative connection that may be made for the provision of a three phase supply which is known as ‘Star’ connection.

Star Connection (3-phase, 4 wire system)
The three coils are also wound so that each has a definite start and finish to their winding.

However to provide a star connection the starts of each winding are connected together at a common point known as the star point. The phase supplies are then taken from the finish of each winding to the point of utilisation. The star point may also have a fourth wire connected to it (known as the neutral conductor) which is then fed along with the three phase conductors to the point of utilisation.

Balanced and unbalanced systems
Star and delta systems can also be connected as loads. Many three phase motors will be connected in delta since only three wires are required to make the connection.

Star connections are often used as a means of connecting single phase loads to a 3 phase supply, such as in the case of a street of domestic dwellings.

A delta connected load will have identical phase impedances. The impedance of each phase will therefore be identical both in terms of it’s magnitude AND phase angle. When this is the case the current that flows in each phase will be identical and the sum of the currents will be zero. The system is said to be balanced and there is no need for a 4th wire (neutral conductor).

It is also possible to have a star connected load where the impedance of each phase is identical in terms of it’s magnitude and phase angle. As with the delta system this will lead to identical phase currents and no excess current to flow back to neutral. Again, the system is said to be balanced ◄◄and there is no need for a 4th wire(neutral conductor).

It is more usual however, to connect dissimilar loads to each phase of a star system. This will result in differences in phase currents which in turn will create some ‘leftover’ current which will flow back along the 4th wire (neutral) to the star point of the supply origin. Such a system is known as an unbalanced system and usually arises as a result of connecting separate single phase loads to each phase of a three phase star connection.

* What is meant by a 3 phase balanced system ?
* Is there any need for a 4th wire (neutral) in such a system ?
* How does an unbalanced system differ ?
* What function does the 4th wire serve in an unbalanced system.

* Which type of 3 phase system is more likely to be found with an unbalanced load ?