***Useful Information for Apprentices***

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[FONT=IIFPD I+ Gill Sans]RCDs for protecting people have a rated tripping current (sensitivity) of not more than 30 milliamps (mA). Remember: [/FONT]
* an RCD is a valuable safety device,
* if the RCD trips, it is a sign there is a fault. Check the system before using it again;
* if the RCD trips frequently and no fault can be found in the system, consult the manufacturer of the RCD;
* the RCD has a test button to check that its mechanism is free and functioning. Use this regularly.

Use of 110V or 230V Equipment and Supplies : Q/A .

For Many Years , the Use of 110V CTE , ( Centre Tap Earthed ) has been Encouraged in Harsh Environments such as Construction Sites , This was Largely a UK Initiative , but in the rest of Europe , Systems of Working with 230V have been Established . if Responsible Individuals decide to Stipulate that Reduced Voltage is to be Used it must be made Clear that the Requirement is Dependant on the Environment and is therefore a Site Decision , the Use of 110V CTE , and Other Low-Voltage Systems in the UK is NOT Compulsory Under the Law but has been Recognised as Good-Practice in Harsh Environments , Guidance on Reduced Low Voltage Systems and Extra Low Voltage Systems is Provided in BS-7671

 

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The Regulations affecting RCD protection
Rule 411.3.3
Additional protection by means of a 30mA RCD is to be provided for all socket outlets with a rated current not exceeding 20A for use by ordinary persons. The only exceptions allowed are for socket outlets for use under the supervision of ‘skilled’ or ‘instructed persons’ e.g. some commercial/industrial locations, or a specifically labelled socket provided for connection of a particular item of equipment, such as a freezer.

Rule 710.411.3.3
In specific locations such as those containing a bath or shower there is now a requirement to provide RCD protection on all circuits, including lighting and shower circuits.

Rule 314.1 & 2
Every installation should be provided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault. Designers are required to reduce the possibility of unwanted RCD tripping due to excessive protective conductor currents but not due to an earth fault.
Separate circuits may be required for parts of the installation which need to be separately controlled in such a way that they are not affected by the failure of other circuits. The appropriate subdivision should take account of any danger arising from the failure of a single circuit, for example and RCD trip causing the disconnection of an important lighting circuit.

Regulation 522.6.7
Much greater use of RCDs is required to protect wiring concealed in walls or partitions, even where this is installed in previously defined Safe Zones.
This effectively means that all concealed wiring at a depth of less than 50mm from the surface now requires protection by a 30mA RCD unless provided with earthed mechanical protection.

 

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HSE Guidance
To comply with Regulation 13 of the Electricity at Work Regulations, precautions need to be taken on equipment that has been made dead. this includes securing the means of disconnection in the OFF position, putting a notice or label at the point of disconnection, and proving dead at the point of work using proprietary voltage detectors.

To comply with Regulation 14 of the Electricity at Work Regulations, dead working should be the normal method of carrying out work on electrical equipment or circuits. Live working should only be carried out in particular circumstances where it is unreasonable to work dead, such as some fault finding and testing, where the risks are acceptable, and where suitable precautions can be taken against injury. The pressure to carry out live work is becoming more common in areas such as construction sites, high-cost manufacturing and in retail outlets operating twenty-four hours per day. The requirements of the Regulations still apply in such situations and live working should only be carried out when justified using the criteria explained

Proving Dead Isolated Equipment or Circuits
Following isolation of equipment or circuits and “ BEFORE “ starting work it should be “ PROVED “ that the parts to be worked on and those nearby, are dead. It should “ NEVER BE ASSUMED “ that equipment is “ DEAD “ because a particular isolation device has been placed in the off position.

The procedure for proving dead should be by use of a proprietary test lamp or two pole voltage detector as recommended in HSE Guidance Note GS38, Electrical test equipment for use by electricians. Non-contact voltage indicators ( VOLTAGE STICKS) and “ MULTI-METERS “ should “ NOT BE USED “ . The test instrument should be proved to be working on a known live source or proprietary proving unit before and after use. All PHASES of the SUPPLY and the NEUTRAL should be tested and proved dead.

Safe Systems of Work
The employer must ensure that all employees involved in work on electrical equipment are competent and are instructed on safe systems of work, have been issued with written rules and instructions, and have access to, and use, appropriate locking-off devices, caution notices, a proprietary voltage detector and, where appropriate for the type of voltage detector being used, a proving unit.

 

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Type of Electrical Circuits :-
There are three main types of circuits encountered in a domestic situation. They are Ring Circuits, Radial Circuits and Lighting Circuits.

 

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* Electric Shock Occurs when a Person become Part of the Electrical Circuit :

To Prevent People Receiving an Electric Shock Accidentally , all Circuits must contain Protective Devices and all Exposed Metal must be Earthed ,
- All Circuits must be Electrically Isolated before any Work is Carried Out ,
- Electrical Isolation is an Important Safety Procedure ,
* the IEE Regulations tell us that Every Circuit must be Provided with Means of Isolation ,
* the Electricity at Work Regulations tell us that before Work Commences on Electrical Equipment it must be Disconnected from the Source of Supply and that the Disconnection must be Secure ,

Three-Effects of an Electric Current :-
When an Electric Current flows in a Circuit it can have One or More of the following Three-Effects : Heating : Magnetic or Chemical
* Heating Effect :-
* The Electrons moving in the Conductor causes the Conductor to Heat Up
* The Amount of Heat Generated depends upon the :
1) Amount of Current Flowing ,
2) Dimensions of the Conductors ,
3) Type of Conductor Material Used ,

* Practical Applications of the Heating Effect of an Electric Current are :-
1) Radiant Heaters which Heat Rooms ,
2) Circuit Protection Fuse and MCBs which Cut off the Supply when an Overcurrent Flows ,

Magnetic Effect :-
* Whenever a Current Flows in a Conductor a Magnetic Field is Set Up around the Conductor like an Extension of the Insulation ,
* Increasing the Current Increases the Magnetic Field ,
* Switch the Current off Causes the Magnetic Field to Collapse ,
* Practical Applications of the Magnetic Effect are :-
1) Electric Motors which Rotate because of the Magnetic Flux Generated by the Electrical Supply door Chimes and Buzzers which ding dong or buzz because of the Magnetic Flux Generated by the Electrical Supply ,

Chemical Effect :-
* When an Electric Current Flows though a Conducting Liquid , the Liquid Separates into its Chemical Parts , a Process called Electrolysis ,
* Alternatively , if two Metals are Placed in a Conducing Liquid they React Chemically and Produce a Voltage ,
* Practical Applications of the Chemical Effect are :-
1) Industrial Processes such as Electroplating which is Used to Silver Plate Sports Trophies and Cutlery ,
2) Motor Car Batteries which Store Electrical Energy ,

Three-6Ω Resistors are Connected in Series :- ( for any Series Connection )
Resistors in Series , Rt = R1 + R2 + R3 ,
Rt = 6Ω + 6Ω + 6Ω = 18Ω
Total Current It = Vt – Rt
Therefore :- It = 12V - 18Ω = 0.67A

The Voltage Drop across ( R1 is )
V1 = It x R1
Therefore :- V1 = 0.67A x 6Ω = 4V
The Voltage Drop across ( R2 is )
V2 = It x R2
Therefore :- V2 = 0.67A x 6Ω = 4V
The Voltage Drop across ( R3 is )
V3 = It x R3
Therefore :- V3 = 0.67A x 6Ω = 4V
I1 / R1 - 6Ω
I2 / R2 - 6Ω
I3 / R3 - 6Ω
It ( Vt = 12V )

 

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Resistors in Parallel :-
For any Parallel Connection , ( 1/Rt = 1/R1 + 1/R2 + 1/R3 )
Therefore :- 1/Rt = 1/6Ω + 1/6Ω + 1/6Ω

1/Rt = 1 + 1 + 1 = 3… 6Ω
Rt = 6Ω ÷ 3 = 2Ω
Total Current It = Vt/Rt , therefore :- It = 12V ÷ 2Ω = 6A
The Current flowing through ( R1 is )
I1 = Vt/R1 , therefore :- I1 = 12V ÷ 6Ω = 2A
The Current flowing through ( R2 is )
I2 = Vt/R2 , therefore :- I2 = 12V ÷ 6Ω = 2A
The Current flowing through ( R3 is )
I3 = Vt/R3 , therefore :- I3 = 12V ÷ 6Ω = 2A

Component Parts of an Electrical Circuit :-

These Series and Parallel Resistors are Connected together to form an Electrical Circuit , so , what is an Electrical Circuit ?
An Electrical Circuit has the following Five Components :-
* a Source of Electrical Energy , this might be a Battery giving a D.C. ( direct current ) Supply or the Main Supply which is A.C. ( alternating current )
* a Source of Circuit Protection , this might be a Fuse or Circuit-Breaker which will Protect the Circuit from “ Overcurrent “
* the Circuit Conductors or Cables . these carry Voltage and Current to Power the Load ,
* a Means to Control the Circuit , this might be a simple On/Off Switch but it might also be a Dimmer or a Thermostat ,
* and a Load , this is Something which needs Electricity to make it Work , it might be a Electric Lamp , an Electrical Appliance , an Electric Motor or an i-pod

 

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A Transformer Feeds the 9.81kW Motor Driving the Mechanical Hoist , the Input Power to the Transformer was found to be 10.9kW
Find the Efficiency of the Transformer :- ?? ŋ = Power Out / Power Input x 100 ( ŋ = 9.81kW / 10.9kW x 100 = 90% )

Thus the Transformer is 90% Efficient ,
Note : that Efficiency has No Units , but is Simply Expressed as a Percentage ,

Electrical Transformers :-
A Transformer is an Electrical Machine without Moving Parts , which is Used to Change the Value of an Alternating Voltage ,
- a Transformer will Only Work on an Alternating Supply . it will NOT Normally Work from a D.C. Supply such as a Battery ,

Transformer : Consists of Two-Coils called the ( Primary and Secondary ) Coils or Windings , wound on to a Common Core , the Iron Core of the * * *Transformer is NOT Solid but Made Up of very Thin Sheets called ( Laminations ) to Improve Efficiency ,
* an Alternating Voltage Applied to the Primary Winding Establishes an Alternating Magnetic Flux in the Core ,
* the Magnetic Flux in the Core causes a Voltage to be Induced in the Secondary Winding of the Transformers ,
* the Voltage in both the ( Primary and Secondary ) Windings is Proportional to the Number of Turns ,
* this Means that if you Increase the Number of Secondary Turns you will Increase the Output Voltage , this has an Application in Power Distribution ,
* Alternatively , Reducing the Number of Secondary Turns will Reduce the Output Voltage , this is Useful for ( Low-Voltage-Supplies ) such as Domestic bell Transformers’ , because it has NO Moving Parts , a Transformer can have a Very High Efficiency , Large Power Transformers , Used on
Electrical Distribution Systems , can have an Efficiency of Better than 90% ,

These Power Transformers need ( Cooling ) to take the ( Heat ) Generated away from the ( Core ) this is Often Achieved by Totally Immersing the Core and Windings in Insulating ( Oil )

Very Small Transformers are Used in Electronic Applications , Small Transformers are Used as Isolating Transformers in ( Shaver Sockets ) and can be Used to Supply ( SELV ) separated extra low voltage , Sources , Equipment Supplied from a SELV Source may be Installed in a Bathroom or Shower room , Provided that it is Suitably Enclosed and Protected from the ( Ingress of Moisture ) this includes Equipment such as Water Heater , Pumps for Showers and Whirlpools Baths ,

 

[brucelee]

Thanks mr leaf
great work will be a great help

 

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Q : What is the Voltage of the Neutral Terminal ?
A : it stays as a Potential Close to Zero with Respect to Earth ,

Q : What should be done to Appliances with Metal Cases ?
A : They are Usually Earthed ,

Q : Describe how the Résistance of a Thermistor changes as the Temperature Increases ?
A : it Decreases ,

Q : For Components Connected in Series , what do you know about the Total Potential Difference ?
A : The Potential Difference of the Supply is Shared between the Components According to their Résistance – Bigger Résistance – Bigger Share ,

Q : What is the Voltage of the Live Terminal ?
A : The Live Terminal of the Mains Supply Alternates between Positive and Negative Potential with Respect to the Neutral Terminal ,

Q : For Components Connected in Parallel , what do you know about the Current Through Them ?
A : The Total Current through the Whole Circuit is Equal to the Sum of the Currents through the Separate Components – and the Lower the Résistance if the Component , the More the Current Flows ,

Q ; For Components Connected in Parallel , what do you know about the Potential Difference Across each Component ?
A : it is the Same ,

Q :What Type of Current do Cells and Batteries Supply ?
A : Direct Current ( d.c. )

Q : for Components Connected in Series , what do you know about the Current through each Component ?
A : it is the Same ,

Q : for Components Connected in Series , how do you Calculate the Total Résistance ?
A : The Total Résistance is the Sum of the Résistance of Each Component ,

Q : How do you know what Value of Fuse to put in an Appliance’s Plug ?
A : From the Power and the Voltage we can Calculate the Current and the Fuse it Needs ,

Q : What is Résistance ?
A : The Ratio of Potential Difference across a Component to the Current Flowing through it ,

Q : What happens when an Electrical Charge Flows through a Resistor ?
A : Electrical Energy is Transformed into Heat Energy ( it get Hot )

Q : What does the Size of the Current in a Circuit Depend on ?
A : How hard the Supply tries to Push Charge through the Circuit and how hard the Circuit Resists having Charge Pushed through it ,

Circuit Basics :-
All Electrical Circuits Require three-Elements ,
1) A Source Voltage , that is , an Electron Pump usually a Battery or Power Supply , ( Energy In )
2) A Conductor to Carry Electrons from and to the Voltage Source ( Pump ) the Conductor is often a Wire , ( Energy Transfer )
3) A load or Résistance , A Point where Energy is Extracted form the Circuit in the Form of Heat , Light , Motion , etc. ( Energy Out )

Potential Changes of Current in a Circuit :-
High Energy Current ► Résistance ( Potential Drop ) Low Energy Current ► ,
High Energy Current ◄ Voltage Source ( Potential Rise ) Low Energy Current ◄

Measureable Quantities that can be Obtained from an Electrical Circuit :-
1) Voltage Rise – Measures the Energy given to Electrons as they leave a Voltage Source , it is Measured in Volts ( + )
2) Voltage Drop - Measures the Energy lost by to Electrons when they leave a Résistance , it is Measured in Volts ( - )
3) Current - Measures the Flow Rate through a Conductor , it is Measured in Amperes ( AMPS )
4) Résistance - Measures the Opposition to Current Flow through a Conductor or Resistor , it is Measured in Ohms ( its Symbol . is Omega )

Voltage Sources and Internal Resistance :-
1) All Voltage Sources contain Internal Résistance , that is Resistance that is Part of the Voltage Producing Device itself which cannot be Eliminated ,
2) The Voltage that the Device ( Battery for Example ) could Produce if no Internal Résistance was Present is called its ( EMF ) stands for Electromotive Force – the Force that moves the Electrons ,
3) The Useable Voltage which is Available to the Circuit after the Internal Résistance Consumes its Share of the ( EMF ) is Called the Terminal Voltage ,

 

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Q: What are the advantages of star-delta starter with induction motor ?
A 1). The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting.(2). Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.