ELECTRICAL PANEL AND ITS COMPONENTS OF A COMPUTER NUMERIC CONTROL MACHINE PRESENTED

                   ABSTRACT

This deals with the control system of a computer numeric control machine and its associated interaction with electromechanically devices. Also, here we shall be looking at the various components such as circuit breakers, contactors, relays etc that makes this control system possible. This component’s description, operation, types and uses shall as well be discussed.

INTRODUCTION

The use of simple electrical components for the designing and constructing of a panel capable of controlling an electrical machine both in the forward and reverse directions has led to automation engineering and subsequently to Computer Numeric Control machine technology. The computer numeric control (CNC) panel is the automated design part of a CNC machine where electrical inputs are fed and controlled. Such control includes the spindle motor, the X,Y, motors, the coolant motors etc. It is made up of both electrical and electronics components. Here emphasis shall be laid on the electrical components as much of it does the control system.

Below is an outline of the various components used in a CNC control panel;

Circuit breaker

Control transformer

Motor protection switch

Contactors

Rectifier

Relays e. t. c.

CIRCUIT BREAKERS 

DESCRIPTION

break An early form of circuit breaker was described by Thomas Alva Edison in an 1879 patent application, although his commercial power distribution system used fuses.^[1] Its purpose was to protect lighting circuit wiring from accidental short-circuits and overloads.

OPERATION OF A CIRCUIT BREAKER

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism.                           

The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source. Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit.

Contacts are made of copper or copper alloys, silver alloys, and other materials.

Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

POLES OF A CIRCUIT BREAKER

Circuit breaker poles are usually expressed in 1p, 2p, 3p, 4p, e.t.c. this indicates the terminal(s) of the breaker.

 

TYPES OF CIRCUIT BREAKER.

Low voltage circuit breakers

Magnetic circuit breaker

Thermal magnetic circuit breaker Common trip breakers

High-voltage circuit breakers

Sulfur hexafluoride (SF₆) high-voltage circuit-breakers

Other breakers

Breakers for protections against earth faults too small to trip an over-current device:

Residual-current device (RCD, formerly known as a residual current circuit breaker) — detects current imbalance, but does not provide over-current protection.

Residual current breaker with over-current protection (RCBO) — combines the functions of an RCD and an MCB in one package. In the United States and Canada, panel-mounted devices that combine ground (earth) fault detection and over-current protection are called Ground Fault Circuit Interrupter (GFCI) breakers; a wall mounted outlet device providing ground fault detection only is called a GFI.

Earth leakage circuit breaker (ELCB) — This detects earth current directly rather than detecting imbalance. They are no longer seen in new installations for various reasons

Autorecloser — A type of circuit breaker which closes again after a delay. These are used on overhead power distribution systems, to prevent short duration faults from causing sustained outages.

Polyswitch (polyfuse) — A small device commonly described as an automatically resetting fuse rather than a circuit breaker

 

CONTROL TRANSFORMER

A control transformer is a device used to transform or "step down" a high main circuit voltage to a lower voltage which is then used to operate the control or switching components of the main circuit. These devices are commonly used in industrial starter circuits where the main circuit voltage is not suitable for use in the control circuit and where a separate control circuit feed would not be practical. For example in a starter panel designed to start a 500 volt electric motor, the contactors and relays used to switch the motor on or off would typically use electromagnetic coils rated for a far lower voltage.

 

USES OF CONTROL TRANFORMER

To supply this voltage without the need for a separate power feed, power is tapped off the main incoming 500 volt feed and passed through a control transformer which would then supply the lower control circuit voltage.

Heavy electrical machinery that starts automatically or remotely generally makes use of contactors which rely on an electromagnetic force to close them to start the machinery. This force is created by an electric coil placed in the center of a laminated steel core. These coils are typically designed to operate at fairly low voltages, ranging from 110 volts to as low as 12 volts. As these machines themselves typically run on far higher voltages, this creates the need for a separate control voltage feed.

 

Instead of having to run separate cables or install extra sets of bus bars, it is far simpler to use the main circuit voltage and step it down with a control transformer to the appropriate control voltage. Low control circuit voltages are used for various reasons including the fact that parts of the control circuit include push buttons in a remote control room, on the starter panel door, and at the machine itself. It would not be wise to have high voltages used in these applications for obvious safety reasons. It is also undesirable to have densely packed control wiring carrying very high voltages inside the starter panel either. For these reasons, lower voltages are typically used in control circuits.

Another benefit of using a control transformer is the inherent stability of the voltage supplied from a transformer as well as its ability to handle extreme peaks in demand. When the start button on a motor starter is pushed and the contactor coil energizes, there is a very brief (typically 30 to 50 milliseconds) surge in current demand known as an "inrush current". This peak can exceed 10 times the normal current flow, and transformers handle these peaks far more efficiently than a conventional supply. Using a control transformer to supply control power thus allows a lower, safer and more efficient control circuit voltage to be used in high working voltage applications. The excellent inrush current handling characteristics of transformer supplied power also makes for a more efficient power supply. Lastly, the use of lower voltages in a control circuit make for far safer use by workers using stop and start buttons in hazardous environments.

                                        Motor protection switch

Mainly used for overload and short-circuit protections of motor in AC 50/60Hz, up to 660V, 0.1 to 80A power circuit
     Full voltage starter to start and cut-off motor
     Under AC-3 load or for the overload and short-circuit protection of circuit and power equipment in power distribution network

Conform to IEC60947.2 and IEC 60947-4.1 as well as the EN60947-1 standard
     Mainly used for overload and short-circuit protections of motor in AC 50/60Hz, up to 660V, 0.1 to 80A power circuit
     Full voltage starter to start and cut-off motor
     Under AC-3 load or for the overload and short-circuit protection of circuit and power equipment in power distribution network

CONTACTOR

Contactors are electrical devices used to control electric motors, lighting, heating, and other electrical loads.

A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings.^[1] A contactor is controlled by a circuit which has a much lower power level than the switched circuit. A contactor is composed of three different items. The contacts are the current carrying part of the contactor. This includes power contacts, auxiliary contacts, and contact springs. The electromagnet provides the driving force to close the contacts. The enclosure is a frame housing the contact and the electromagnet. Enclosures are made of insulating materials like Bakelite, Nylon 6, and thermosetting plastics to protect and insulate the contacts and to provide some measure of protection against personnel touching the contacts. Open-frame contactors may have a further enclosure to protect against dust, oil, explosion hazards and weather.

 

OPERATING PRINCIPLE

Unlike general-purpose relays, contactors are designed to be directly connected to high-current load devices. Relays tend to be of lower capacity and are usually designed for both normally closed and normally open applications. Devices switching more than 15 amperes or in circuits rated more than a few kilowatts are usually called contactors. Apart from optional auxiliary low current contacts, contactors are almost exclusively fitted with normally open contacts. Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents. When current passes through the electromagnet, a magnetic field is produced, which attracts the moving core of the contactor. The electromagnet coil draws more current initially, until its inductance increases when the metal core enters the coil. The moving contact is propelled by the moving core; the force developed by the electromagnet holds the moving and fixed contacts together.

 

When the contactor coil is de-energized, gravity or a spring returns the electromagnet core to its initial position and opens the contacts .

For contactors energized with alternating current, a small part of the core is surrounded with a shading coil, which slightly delays the magnetic flux in the core. The effect is to average out the alternating pull of the magnetic field and so prevent the core from buzzing at twice line frequency.

Most motor control contactors at low voltages (600 volts and less) are air break contactors; air at atmospheric pressure surrounds the contacts and extinguishes the arc when interrupting the circuit. Modern medium-voltage motor controllers use vacuum contactors. High voltage contactors (greater than 1000 volts) may use vacuum or an inert gas around the contacts.

 

APPLICATIONS/USES

Lighting control

Contactors are often used to provide central control of large lighting installations, such as an office building or retail building. To reduce power consumption in the contactor coils, latching contactors are used, which have two operating coils. One coil, momentarily energized, closes the power circuit contacts, which are then mechanically held closed; the second coil opens the contacts.

Magnetic starter

A magnetic starter is a contactor designed to provide power to electric motors. The magnetic starter has an overload relay, which will open the control voltage to the starter coil if it detects an overload on a motor.^[

Overload relays may rely on heat produced by the motor current to operate a bimetal contact or release a contact held closed by a low-melting-point alloy. The overload relay opens a set of contacts that are wired in series with the supply to the contactor feeding the motor. The characteristics of the heaters can be matched to the motor so that the motor is protected against overload. Recently, microprocessor-controlled motor digital protective relays offer more comprehensive protection of motors.

PICTURE OF A CONTACTOR

The picture below shows the nature, appearance and connection of a typical contactor.

Motor control contactors can be fitted with short-circuit protection (fuses or circuit breakers), disconnecting means, overload relays and an enclosure to make a combination starter. Several combination starters and other switchgear and control devices can be grouped in a common enclosure called a motor control center/panel.

 

RECTIFIER

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components.

A device which performs the opposite function (converting DC to AC) is known as an inverter.

When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used

 

HALF-WAVE RECTIFICATION.

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one-phase supply, or with three diodes in a three-phase supply. The output DC voltage of a half wave rectifier can be calculated with the following two ideal equations:^[1]

FULL-WAVE RECTIFICATION.

A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient.

However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification.

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many windings are required on the transformer secondary to obtain the same output voltage compared to the bridge rectifier above.

Full-wave rectifier using a center tap transformer and 2 diodes.

 

The average and root-mean-square output voltages of an ideal single phase full wave rectifier can be calculated as

 

V_r.m.s = V_p/√2

 

V_dc,V_av - the average or DC output voltage,

V_p - the peak value of half wave,

V_rms - the root-mean-square value of output voltage.

π = ~ 3.14159

 

RELAYS

DESCRIPTION

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram.

OPERATION OF A RELAY

Relays allow one circuit to switch a second circuit which can be completely separated from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical. Relays are usually SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available.

The relay's switch connections are usually labeled COM, NC and NO:

COM = Common, always connect to this, it is the moving part of the switch.

NC = Normally Closed, COM is connected to this when the relay coil is off.

NO = Normally Open, COM is connected to this when the relay coil is on.

Connect to COM and NO if you want the switched circuit to be on when the relay coil is on.

Connect to COM and NC if you want the switched circuit to be on when the relay coil is off.

 

Protection diodes for relays

Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected 'backwards' across the relay coil to provide this protection.

Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

USES/APPLICATIONS OF RELAYS

Relays are used to switch AC and DC

Relays can switch higher voltages.

Relays are often a better choice for switching large currents (> 5A).

Relays can switch many contacts.

CONCLUSION

With the use of control panel, the production of convectional machine has become uncommon in the field of engineering and technology and subsequently makes machine automatic in operation. Today over 50% of our machines in the workshop can be operated with little or no human intervention. Therefore, for an efficient and effective maintenance of computer numeric control machines, a sound knowledge of paneling is essential.                     

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Capacitor and their usage

What do we mean by the word capacitor?

Capacitors are electric device used for storage electrical charges. This device is like a condenser. It is an important device in any electrical or electronic and in telecommunication industries. E.g. in radio, television receiver and in transmitter circuit.

Types of capacitor

There are different types of Capacitors available in the market place and each one has its own set of characteristics and applications from small delicate trimming capacitors up to large power metal can type capacitors used in high voltage power correction and smoothing circuits. Like resistors, there are also variable types of capacitors which allow us to vary their capacitance value for use in radio or "frequency tuning" type circuits. Either way, capacitors play an important part in electronic circuits so here are a few of the more "Common" types of capacitors available.

1.Dielectric
 Dielectric Capacitors are usually of the variable type such as used for tuning transmitters, receivers and transistor radios. They have a set of fixed plates and a set of moving plates that mesh with the fixed plates and the position of the moving plates with respect to the fixed plates determines the overall capacitance.

The capacitance is generally at maximum when the plates are fully meshed. High voltage type tuning capacitors have relatively large spacing's or air-gaps between the plates with breakdown voltages reaching many thousands of volts.

Variable Capacitor Symbols

2. Film Capacitors
Film Capacitors are the most commonly available of all types of capacitors, consisting of a relatively large family of capacitors with the difference being in their dielectric properties. These include polyester polystyrene,polypropylene,polycarbon-ate, metalized paper etc.

 Film type capacitors are available in capacitance ranges from 5pF to 100uF depending upon the actual type of capacitor and its voltage rating. Film capacitors also come in an assortment of shapes and case styles which include:

Rectangular & Round film capacitors are the rectangular metalized film and cylindrical film & foil types are shown below

Cylindrical  Type

3. Ceramic Capacitors
Ceramic Capacitors or Disc Capacitors as they are generally called, are made by coating two sides of a small ceramic disc with silver and are then stacked together to make a capacitor.

For very low capacitance values, a single ceramic disc of about 3-6mm is used. Ceramic capacitors have a high dielectric constant (High-K) and are available so that relatively high capacitance can be obtained in a small physical size.

They are large non-linear changes in capacitance against temperature and as a result are used as de-coupling or by-pass capacitors as they are also non-polarized devices. Ceramic capacitors have values ranging from a few picofarads to one or two microfarads but their voltage ratings are generally quite low.

For example, 10³ would indicate 10 x 10³pF which is equivalent to 10,000 pF or 0.01μF.
4. Electrolytic Capacitors
Electrolytic Capacitors are generally used when very large capacitance values are required. Here instead of using a very thin metallic film layer for one of the electrodes, a semi-liquid electrolyte

solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode). The majority of electrolytic types of capacitors are polarized, that is the voltage applied to the capacitor terminals must be of the correct polarity as an incorrect polarization will break down the insulating oxide layer and permanent damage may result.

    Electrolytic Capacitors are generally used in DC power supply circuits to help reduce the ripple voltage or for coupling and decoupling applications. Electrolyte's generally come in two basic forms; Aluminium Electrolytic and Tantalum Electrolytic capacitors.

Electrolytic Capacitor

Also tantalum capacitors although polarized, can tolerate being connected to a reverse voltage much more easily than the Aluminium types but are rated at much lower working voltages. Typical values of capacitance range from 47nF to 470uF

Aluminium & Tantalum Electrolytic Capacitor

Capacitor Characteristics
The characteristics associated with the humble capacitor so here are just a few of the more important ones.
1. Working Voltage, (Vn)
The Working Voltage (Wvdc, Wvac) is the maximum continuous voltage that can be applied to the capacitor without failure during its working life.

DC and AC values are usually not the same as the AC value refers to the r.m.s. value. Common working DC voltages are 10V, 16V, 25V, 35V, 63V, 100V, 160V, 250V, 400V and 1000V and are printed onto the body of the capacitor. 2. Tolerance, (±%)
As with resistors, Capacitors also have a tolerance rating expressed as a plus-or-minus value either in Picofarads (±pF) for low value capacitors generally less than 10pF or as a percentage (±%) for higher value capacitors generally higher than 10pF. Capacitors are rated according to how near their actual values are to the rated capacitance with coloured bands or letters used to indicated the actual tolerance. The most common tolerance for capacitors is 5% or 10% but some electrolytic capacitors are rated as high as 20%.

3. Leakage Current
The dielectric used inside the capacitor is not a perfect insulator resulting in a very small current flowing or "leaking" through the dielectric when applied to a constant supply voltage. This small current flow in the region of micro amps (μA) is called the Leakage Current. This leakage current is a result of electrons physically making their way through the dielectric medium, around its edges or across the leads. The "leakage current" of a capacitor is sometimes called the "insulation resistance" and can be found using Ohm's law.

4. Working Temperature, (T)
Changes in temperature around the capacitor affect the value of the capacitance because of changes in the dielectric. If the air or surrounding temperature becomes to hot or to cold the capacitance value of the capacitor may change so much as to affect the correct operation of the circuit. The normal working range for most capacitors is -30°C to +125°C with nominal voltage ratings given for a working temperature of no more than +70°C. Generally electrolyte's can not be used below about -10°C, as

the electrolyte jelly freezes.
6. Polarization
Polarization generally refers to the Electrolytic type capacitors but mainly the Aluminium Electrolyte's, with regards to their connection. The majority are polarized types, that is the voltage connected to the capacitor terminals must have the correct polarity, i.e. +ve to +ve and -ve to -ve. Incorrect polarization can cause the oxide layer inside the capacitor to break down resulting in very large currents flowing through the device.

The majority of electrolytic capacitors have their -ve terminal clearly marked with a black stripe or black arrows down the side to prevent any incorrect connection. Some electrolyte's have their metal can connected to the negative terminal but high voltage types.

Types of connection in  capacitors.
1. parallel and series connection.

Capacitance and Charge
We saw in the previous tutorials that a Capacitor consists of two parallel conductive plates (usually a metal) which are prevented from touching each other (separated) by an insulating material called the "dielectric". We also saw that when a voltage is applied to these plates an electrical current flows charging up one plate with a positive charge with respect to the supply voltage and the other plate with an equal and opposite negative charge. Then, a capacitor has the ability of being able to store an electrical charge Q (units in Coulombs) of electrons.

When a capacitor is charged there is a potential difference between its plates, and the larger the area of the plates and/or the smaller the distance between them (known as separation) the greater will be the charge that the capacitor can hold. The Capacitors ability to store this electrical charge (Q) between its plates is proportional to the applied voltage, V for a capacitor of known capacitance in Farads, capacitance C is always positive. The greater the applied voltage the greater will be the charge on the plates. Likewise, the smaller the applied voltage the smaller the charge. Therefore, the actual charge Q on the plates of the capacitor can be calculated as:

Capacitor Charge

where A is the area of the plates in square metres, d is the distance between them and ε (epsilon) is the value of the dielectric constant.

Parallel Plate Capacitor

The capacitance of a parallel plate capacitor is proportional to the area A and inversely proportional to the distance, d between the plates. The capacitance can be increased by inserting a dielectric which has a relative permittivity or dielectric constant greater than that of air with typical values of epsilon ε being: Air = 1, Paper = 2.5, Glass = 5, Mica = 7 etc.

Charging & Discharging a Capacitor
Consider the following circuit.

Assume that the capacitor is fully discharged and the switch connected to the capacitor has just been moved to position A. The voltage across the 100uf capacitor is zero at this point and a charging current i begins to flow charging up the capacitor until the voltage across the plates is equal to the 12v supply voltage. The charging current stops flowing and the capacitor is said to be "fully-charged".
Then, Vc = Vs = 12v. Once the capacitor is "fully-charged" in theory it will maintain its state of voltage charge even when the supply voltage has been disconnected as they act as a sort of temporary storage device.

However, while this may be true of an "ideal" capacitor, a real capacitor will slowly discharge itself over a long period of time due to the internal leakage currents flowing through the dielectric. This is an important point to remember as large value capacitors connected across high voltage supplies can still maintain a significant amount of charge even when the supply voltage is switched OFF.
If the switch was disconnected at this point, the capacitor would maintain its charge indefinitely, but due to internal leakage currents flowing across its dielectric the capacitor would very slowly begin to discharge itself as the electrons passed through the dielectric.

The time taken for the capacitor to discharge down to 37% of its supply voltage is known as its Time Constant.  If the switch is now moved from position A to position B, the fully charged capacitor would start to discharge through the lamp now connected across it, illuminating the lamp until the capacitor was fully discharged as the element of the lamp has a resistive value. The brightness of the lamp and the duration of illumination would ultimately depend upon the capacitance value of the capacitor and the resistance of the lamp (t = CxR). The larger the value of the capacitor the brighter and longer will be the illumination of the lamp as it could store more charge.

Example No1.
Calculate the then the charge on the capacitor is 1.2 millicoulombs.


The Farad
We now know that the ability of a capacitor to store a charge gives it its capacitance value C, which has the unit of the Farad, F. But the farad is a extremely large unit on its own making it impractical to use so submultiples or fractions of the standard Farad unit are used instead. The prefixes used in charge in the above capacitor circuit.

electronic formulas and component values are the micro (μ), nano (n) and the pico (p). For example;
Sub-units of the Farad
Convert the following capacitances a) 22nF to uF, b) 0.2uF to nF, c) 550pF to uF a)  22nF = 0.022uF
b)  0.2uF = 200nF
c)  550pF = 0.00055uF
Energy When a capacitor charges up from the power supply connected to it, energy in Joules is stored in the capacitor and is given by the formula;

REFERENCES

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Terrell Croft "Library of Practical Electricity"(1917)

 J. Ho, T. R. Jow, S. Boggs, Historical Introduction to Capacitor Technology

 Adam Marcus Namisnyk (23 June 2003). "A Survey of Electrochemical Supercapacitor Technology" (PDF). Retrieved 2011-06-24.

 WIMA, Characteristics of Metallized Film Capacitors in Comparison with Other Dielectrics [1]

Film Capacitors, TDK Epcos, General technical information

 AVX, Dielectric Comparison Chart

 Holystone, Capacitor Dielectric Comparison, Technical Note 3

  P. Bettacchi, D. Montanari, D. Zanarini, D. Orioli, G. Rondelli, A. Sanua, KEMET Electronics Power Film Capacitors for Industrial Applications

 S. P. Murarka, Moshe Eizenberg, A. K. Sinha (2003) (in German), Interlayer dielectrics for semiconductor technologies, Academic Press, pp. 338–339, ISBN 9780125112215

  General technical information of (RFI/EMI)Noise suppression capacitors on AC mains

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PRINCIPLE OF CONDUIT WIRING

DEFINITION OF CONDUIT WIRING

Is the type of wiring whereby insulated cables are drawn into metal or plastic tube. Plastic conduit is now mostly used in domestic installation. 

  Advantages of conduit wiring
1. It provides mechanical protection for the cable.
2. Provide efficient earth continuity 3. Minimize fire risk
4. It is durable

Disadvantages
1. It is expensive 
2. It is liable to corrosion  
3. Difficult to conceal

 Types of conduit
 1.  Light gauge conduit
2. Heavy guage conduit
3. Flexible metallic conduit
4. Aluminium conduit

Light gauge conduit
Light gauge is a product made from strip steel which is form into a tube. It has an open seam and is only used for small installation at or below 250v. 

 It cannot easily be bent because of it light weight and it also unsuitable for used in damp situation, and does not provide a very high degree of mechanical protection.
Type of light gauge conduit  .

  (A)     Close joint- this type has the edges butter together without a mechanical joint. close joint cannot be bent or set as the seam tend to open.

 (B)      Brazed or welded joint – the seam, is mechanically joined by brazing or welding.  

          FITTINGS-
 Grip fitting are mostly used to ensure electrical and mechanical continuous at all joints. 

 This is achieved by the use of lug grip which can be tightened to the end of the conduit by the screw .The electrical continuity may properly be made by removing paint or enamel from the end of the conduit.

HEAVY GUAGE SCREWED CONDUIT-This type of conduit is heavy, all joint to fitting are made of screwed threads.

   There are two types of heavy gauge conduit.
 A.   welded joint 

  B. solid drawn

In each case both ends of length of conduit are screwed electrical thread.                                                

 a.   The heavy gauge welded conduit is almost universally employed for general work.   

  b.   The solid drawn is used mostly in situations where explosive gases are present and flameproof installation e.g garage 

 MATERIALS FOR FIXING CONDUIT.
Distance saddle- they are used to keep dust from collecting between the conduit and the wall.
Ordinary saddle- they are used to hold the conduit firmly against the wall.
Space bar saddle – are used to hold the conduit away from damp plaster.

CONDUIT  TERMINATION.
Conduit run are always terminated at switches, sockets and ceiling outlets by using an appropriate box. Conduit run and terminations must be electrically and mechanically continuous throughout the installation. It is also essential that the end of the conduit are properly butted to ensure electrical and mechanical continuity and also to allow the free passage for the drawing in cables.

 INSTALLING CONDUIT. 

      W hen installing conduit the following steps should be taken 

   1   planning the layout-Use the architects drawing to determine whether the entire requirement for the conduit installation has been meet by consumer.                                                        

   2 Marking out-This is done for the proper routing of conduit pipe for all outlets with the aid of a chalk line. 

 Preparing the conduit –Here cutting threading and bending of the conduit pipe take place.  

PROCEDURE FOR THREADING A PIPE                
1 Cut the conduit pipe with fine hacksaw blade.                                                                                       

  2Taper the end of the conduit with a flat file.                                                                                             

 3At the end of the cutting, the end of the conduit should be smoothed with either a reamer or flat file. 

 4Wipe the threaded ends with a clean rag to remove excess lubricant or metal filings.                  
 5 Lubricate the part to be threaded using appropriate lubricant such as mineral oil or tallow.  I.E.E 

REGULATION REQUIRES THAT                                                                                                             

 1 Conduit must be securely fixed and protected from mechanical damage.                                        

 2 Extra- low voltage and low voltage cables must not be run in the same conduit. 

 3 The conduit system must be completed before cables are drawn in.
4 Inspection boxes, draw in boxes, should be so situated that they remain accessible throughout the life of the installation.                                                                                                                            

 5 Where conduit pass through floor walls, partition or ceiling the boxes should be made good with cement or similar material.  

References

Cauldwell, Rex (2002). Wiring a House (For Pros By Pros). Newtown, CT, USA: Taunton Press.

  R.K. Clidero Applications of Electrical Construction, General Publishing Co., Don Mills Ontario Canada, 1975, 

 R. S. Means Electrical Cost Data 22nd Edition,  pages 106-107 table 16 200 Conduits 

 John H. Chiang, (Ed), RS Means Electrical Cost Data 30th Annual Edition, RSMeans Construction Publishers, Kingston MA USA, 2007,

                                           
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LASER TECHNOLOGY AND IT`S APPLICATIONS

ABSTRACT

      

This paper emphasis on the principle of operation of LASER technology, the simulated emission that generate the signal, it`s uses and role in information technology, it`s safety measures and warnings signs.

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LIGHTING SYSTEM (FLUORESCENT LAMP)

ABSTRACT

This paper shows an over view of lighting system (fluorescent lamp), the types of fluorescent lamp (circling and tubular fluorescent fixtures), the features of fluorescent lamp such as glass tube, choke or ballast and starter, the working principle of fluorescent lamp, installation of  fluorescent lamp fixtures and fluorescent troubleshoot.

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EFFECT OF ELECTROMAGNETIC RADIATIONS ON HUMAN

ABSTRACT

The man of 21st century is a man of comfort, totally depending and cannot spend a second without technology. Various equipment such as wireless phones, electrical/electronics equipment etc. has become an integral part of his life. Large numbers of these devices generates electromagnetic radiations. Also these electromagnetic devices have various uses in domestic, industries and medicine. In spite of all these important applications the electromagnetic fields, imposes great danger to the human body.

Electromagnetic radiation is a form of energy that is produce by oscillating electric and magnetic disturbance, or by the movement of electrically charged particles travelling through vacuum of matter. When something in the environment is called a pollutant, it implies that it is somehow harmful to nature and to human beings. This paper discusses the effect of the electromagnetic pollution on the human body and also the various sources responsible for electromagnetic pollution and safety guide to EMRs. This paper throws light on the other side of the emerging technology.

                                         INTRODUCTION

Electromagnetic radiation itself is a form of energy that is produce by oscillating electric and magnetic disturbance, or by the movement of electrically charged particles travelling through vacuum of matter. Electromagnetic radiations can be classified into two types ionizing radiation and non-ionizing radiation. They are called so based on whether they are capable of ionizing atoms and breaking covalent bonds or not. Ultra violet and higher frequency radiations, such as X-rays or gamma rays are ionizing.

…INTRODUCTION CONTD.

infrared waves, visible light, ultraviolet light, x-rays and gamma rays. Wave frequency differentiates one class of radiation from another. Figure 1 is a graphical representation of the spectrum of electromagnetic energy or radiation in ascending frequency (decreasing wavelength). Electromagnetic pollution is due to frequencies which are oscillating slower than visible light waves. But x-rays and gamma rays (which oscillate faster than visible light) are highly dangerous but they are rarely present at our dwelling places and workplaces. Electromagnetic pollution is everywhere.

…INTRODUCTION CONTD.

These pose their own special effects on the life of human beings. Non-ionizing radiation is associated with two major potential hazards that are electrical and biological. Moreover, induced electric current caused by radiation can generate sparks and create a fire or explosive hazard. The electromagnetic spectrum includes several different classes of radiation: low frequency, radio waves, microwaves,

 

SOURCES OF ELECTROMAGNETIC RADIATIONS

In the last six years, researchers has made a lot of breakthroughs in understanding how electromagnetic fields affect living systems, not just humans, but all living systems. Electro-magnetic fields (EMFs) come from everything that uses electricity in our world today. Fluorescent lights the most used form of light in NEDDI or even in our homes have transformers. Another large source of EMF in our environment is motors. Electric shaver is an example. In fact, studies have shown that just a few minutes exposure to that kind of a field will affect you for more than 24 hours.

The biggest sources of electromagnetic radiation in our environment are transformers and electric motors.

…SOURCES OF EMRS CONTD

Other devices responsible for electromagnetic pollution are as follows:

•   Computers and related equipment

•   Cellular (mobile) phones

•   Information networks

•   Electrical appliances

•   Electronic equipment

•   Cell phone masts

•   Microwave ovens

•   Cables and conductors

•   High and low voltage power lines and many others.

ELECTROMAGNETIC RADIATIONS (EMRs)

Low Frequency EMRs

Strong electromagnetic fields (EMFs) of low frequency about 50 to 60 cycles per second (hertz, or Hz) are very harmful. It is possible to shield a house from electric field generated by a nearby power lines but is difficult to provide shielding from magnetic field generated by them. The magnetic field can be shielded by using the underground transmission system but the cost is much higher than the overhead transmission line system. The long-term exposure of low frequency EMFs may give rise to various health problems especially fatigue, irritability, aggression, hyperactivity, sleep disorders and emotional instability.

 

Large numbers of individuals are becoming hypersensitive to EMR. EMR exists around power lines, power tools, boilers, electric stoves, heaters, freezers and television sets when in use using an electric iron or an electric keyboard or working with handheld power tools can quickly drain our energies. Stray currents and radiating fields can be present around us even if appliances are switched off. AC electric fields do not disappears when an appliance is switched off, only AC magnetic fields disappears.

High Frequency EMRs

High frequency electromagnetic field is mainly generated by the cellular phones, microwaves and antennas. High frequency EMFs is due to radio frequency energy in the frequency range of low, medium, high, very high, ultrahigh frequencies or microwaves and is often referred to as radio energy. The term radio frequency energy is used for all the frequencies between 30kHz and 300GHz. Biological effects of radio frequency (RF) energy are:

 

The RF energy heats up the tissues in a similar manner a microwave oven heats the food and it can be dangerous in case of prolong exposure.

Tissues can get damaged if exposed to RF energy because they are not capable of dissipating large amount of heat generated. This can lead to skin burns, deep burns and heat strokes.

Eyes are most affected by the RF energy because the lack of blood flow to cool the cornea can lead to cataract.

RADIO FREQUENCY RADIATIONS

•               Cellular Phones EMRs. Cellular phones work by emitting radio frequency radiations that are transmitted through the antenna in the phone. While using the cellular phone, the device and thus its antenna end up right next to the head and the radiation from it easily affect the brain.

•               Cordless Phones EMRs Swedish scientists have found that cordless phones give rise to the risk of cancer. Researchers studied malignant brain tumor patients on the basis of their usage of cell phones and cordless phones and they found that cancer risk was increased for those who used cordless phones and the user which use both have even higher risk.

 

HAZARDS OF ELECTROMAGNETIC POLLUTION

Electromagnetic pollution has various hazards like electrical hazards, fire hazards, biological hazards and DNA fragmentation.

•               Fire Hazard

Extremely high power electromagnetic radiation can cause sparks (electrical arcs). When an induced voltage exceeds the breakdown voltage of the surrounding medium (e.g. air). These sparks can then ignite flammable materials or gases, possibly leading to an explosion. This can be a particular hazard in the vicinity of explosives or pyrotechnics, since an electrical overload might ignite them.

 

This risk is commonly referred to as Hazards of Electromagnetic Radiation to Ordnance (HERO). On the other hand, the risk related to fuelling is known as Hazards of Electromagnetic Radiation to Fuel (HERF).

•               Biological Hazard

The most understood and discussed biological effect of electromagnetic fields is dielectric heating. For example, touching or standing around an antenna while a high-power transmitter is in operation can cause severe burns. These are exactly the kind of burns that would be caused inside a microwave oven. This heating effect varies with the power and the frequency of the electromagnetic energy. A measure of the heating effect

 

is the Specific Absorption Rate (SAR), which has units of watts per kilogram (W/kg). The IEEE^[5] and many national governments have established safety limits for exposure to various frequencies of electromagnetic energy based on SAR, mainly based on the International Commission on Non-Ionizing Radiation Protection (ICNIRP) Guidelines.

•               Electrical Hazard

Strong radiation can induce current capable of delivering an electric shock to persons or animals. It can also overload and destroy electrical equipment. The induction of currents by oscillating magnetic fields is also the way in which solar storms disrupt the operation of electrical and electronic systems,

 

causing damage to and even the explosion of power distribution transformers, blackouts and interference with electromagnetic signals (e.g. radio, TV, and telephone signals).

•               DNA Fragmentation

In 2009 the study at the University of Basel in Switzerland found that intermittent (but not continuous) exposure of human cells to a 50Hz electromagnetic field at a flux density of 1mT induced a slight but significant increase of DNA fragmentation in the Comet assay. However this level of exposure is already above current established safety exposure limits.

SAFETY GUIDELINES

There are two major sources of electromagnetic pollution and their effects are different on human body, so different protection measures are required for both low frequency and high frequency electromagnetic pollution.

Low Frequency EMP:

•             It must be tried to minimize electromagnetic pollution, especially while sleeping when the pineal gland is most susceptible.

•             Also one should sleep in the dark or at least with a dim lamp to protect the immune-stimulating hormone melatonin.

 

•             Preferably all power points in the bedroom should be switched off and all electric leads with 2-prong plugs should be unplugged before going to sleep.

•             If the head faces a wall with power-points or other electric wiring inside the wall close to the bed, then bed should be moved towards the middle of the room.

•             While using electric blankets, the bed should be warmed beforehand and the plug should be removed before going to bed.

•             It should be tried not to habitually remain within a few meters of a working electric appliance.

High Frequency EMP

•             Human beings should spend less time on the cellular phones.

•             Cellular phones with a lower specific absorption rate (SAR) should be used. Use of cell phones should be limited in case of children below 10 years.

•             It should be avoided to make calls with a low signal and low battery as the cell phone will generate more radiation in an effort to compensate for it.

CONCLUSION

In conclusion, I want to say that electromagnetic fields are harmful and can have adverse effect on human body depending upon the intensity and frequency of the electromagnetic field exposed to. It is always a good idea to avoid unnecessary exposure to electromagnetic fields whenever possible. Though it is true that technology has made life very comfortable but it is at the expense of our health, it is our first duty to save our lives. Thus we should use technology wisely so that we can save ourselves as well as mother earth. Remember Health is wealth and wisdom is profitable to direct.

REFERENCES

Ø Ali Zamanian and Cy Hardiman, “Electromagnetic Radiation and Human Health: A Review of Sources and Effects”, Summit Technical Media, July 2005.

Ø Focke F, Schuermann D, Kuster N, Schar P, “DNA Fragmentation in Human Fibroblasts Under Extremely Low Frequency Electromagnetic Field Exposure”, Mutation Research 683(1-2), November 2009.

Ø Professor John E. Moulder, Ph.D., Electromagnetic Fields and Human Health.

Ø Ankur Mahajan, “Human Health and Electromagnetic Radiations”, June, 2013.

Ø "Standard for Safety Level with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3KHz to 300GHz". IEEE STD (IEEE) C95.1. Oct 2005.

Ø International Commission on Non-Ionizing Radiation Protection, “Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz)”, Health Physics 74 (4), April 1998.

Ø http://image.gsfc.nasa.gov/poetry/workbook/stroms.html.

 

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