Ammeter

Contents:

1. Equipment needed for electrical maintenance Ammeter Voltmeter Multimeter Tong tester Energy meter Fuses Relays Circuit breaker

AMMETER: An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes,

It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The voltage drop across the coil is kept to a minimum to minimize resistance across the ammeter in any circuit into which it is inserted.To measure larger currents, a resistor called a shunt is placed in parallel with the meter. Most of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure large currents.

Voltmeter: A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an Analog to digital converter.

Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Small portable instruments, usually equipped with facilities to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed

on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant.

Analog voltmeter

A moving coil galvanometer of the d'Arsonval type. Wire carrying current to be measuredRestoring springN and S are poles of magnet

A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

One of the design objectives of the instrument is to disturb the circuit as little as possible and hence the instrument should draw a minimum of current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance.

The sensitivity of such a meter can be expressed as 'ohms per volt", the number of ohms resistance in the meter circuit divided by the full scale value. For example a meter with a sensitivity of 1000 ohms per volt would draw 1 milliampere at full scale voltage; if the full scale was 200 volts, the resistance at the instrument's terminals would be 200,000 and at full scale the meter would draw 1 milliampere from the circuit under test. For multi-range instruments, the input resistance varies as the instrument is switched to different ranges.

Moving-coil instruments respond only to direct current; measuremnent of AC voltage requires a rectifier in the circuit so that the coil deflects in only one direction. Moving-

coil instruments are also made with the zero position in the middle of the scale instead of at one end; these are useful if the voltage reverses its polarity.

Voltmeters operating on the electrostatic principle use the mutual repulsion between two charged plates to deflect a pointer attached to a spring. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts and work with either alternating or direct current.

Vacuum Tube Voltmeter (VTVM)

The sensitivity and input resistance of a voltmeter can be increased if the current required to deflect the meter pointer is supplied by an amplifier instead of the circuit under test. A once- popular form of voltmeter of this type was the vacuum tube voltmeter, frequently referred to as a VTVM. Today these instruments use a solid-state amplifier using field-effect transistors. The electronic amplifier between input and meter gives two benefits; a rugged moving coil instrument can be used, since its sensitivity need not be high, and the input resistance can be made high, reducing the current drawn from the circuit under test. Amplified voltmeters often have an input resistance of 1, 10, or 20 megohms which is independent of the range selected.

Digital voltmeters

Two digital voltmeters. Note the 40 microvolt difference between the two measurements, an offset of 34 parts per million.

The first digital voltmeter was invented and produced by Andrew Kay of Non-Linear Systems (and later founder of Kaypro) in 1954.

Digital voltmeters usually employ an electronic circuit that acts as an integrator, linearly ramping output voltage when input voltage is constant (this can be easily realized with an opamp). The dual-slope integrator method applies a known reference voltage to the integrator for a fixed time to ramp the integrator's output voltage up, then the unknown voltage is applied to ramp it back down, and the time to ramp output voltage down to zero is recorded (realized in an ADC implementation). The unknown voltage being measured is the product of the voltage reference and the ramp-up time divided by the ramp-down time. The voltage reference must remain constant during the ramp-up time, which may be difficult due to supply voltage and temperature variations.

Digital voltmeters necessarily have input amplifiers and like vacuum tube voltmeters generally have a constant input resistance of 10 megohms regardless of set measurement range.

Mulimeter: A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several functions in one unit. A standard multimeter may include features such as the ability to measure voltage, current and resistance. There are two categories of multimeters, analog multimeters (or analogue multimeters in British English) and digital multimeters (often abbreviated DMM.)

A multimeter can be a hand-held device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as batteries, motor controls, appliances, power supplies, and wiring systems.

Safety

Some multimeters include a fuse, which will sometimes prevent damage to the multimeter if it is overloaded. However the fuse often only protects the highest current range on the multimeter. A common error when operating a multimeter is to set the meter to measure resistance or current and then connect it directly to a low-impedance voltage source; meters without protection are quickly damaged by such errors, and can sometimes explode causing injury to the operator.

Quantities measured

Contemporary multimeters can measure many quantities. The common ones are:

Voltage in volts. Current in amperes. Resistance in ohms.

Additionally, multimeters may also measure:

Capacitance in farads. Frequency in hertz Duty cycle as a percentage. Temperature in degrees Celsius or Fahrenheit. Conductance in siemens. Inductance in henrys Audio signal levels in decibels.

Digital multimeters may also include circuits for:

Continuity that beeps when a circuit conducts. Diodes and Transistors

Various sensors can be attached to multimeters to take measurements such as:

light level Acidity/Alkalinity(pH) Wind speed Relative humidity

Digital Multimeters (DMM)

Modern multimeters are often digital due to their accuracy, durability and extra features.

In a DMM the signal under test is converted to a voltage and an amplifier with an electronically controlled gain preconditions the signal.

A DMM displays the quantity measured as a number, which prevents parallax errors.

Tong tester: A clamp meter (clamp-on ammeter) is a type of ammeter that measures electrical current without the need to disconnect the wiring through which the current is flowing.

Clamp meters are also known as tong testers or Amprobes (after one of the first vendors of such devices).

The most common forms of clamp meter are:

A probe for use with a multimeter. A self-contained unit. A built-in part of a specialized multimeter used by electricians.

A multimeter with a built in clamp facility.Pushing the large button at the bottom opens the lower jaw of the clamp, allowing the clamp to be placed around a conductor (wire).

In order to use a clamp meter, the probe or clamp is opened to allow insertion of the wiring, and then closed to allow the measurement. Only one conductor is normally passed through the probe; if more than one conductor were to be passed through then the measurement would be a vector sum of the currents flowing in the conductors and could be very misleading depending on the phase relationship of the currents. In particular, if the clamp were to be closed around a mains extension or similar cord, no current will be measured at all as the current flowing in one direction will cancel that flowing in the other direction.

In practice, nearly all clamp meters are used by electricians and the meters often include additional circuitry to allow the reading of voltage and, sometimes, resistance. The meters also often contain a mechanical pointer-locking device so that a reading can be taken in locations where the meter pointer can't be seen, the pointer then locked, and the meter brought out to a more-convenient place for reading. For the meter shown in the picture, the white push-button marked "lock" provides this function.

Energy meter:

An electric meter or energy meter is a device that measures the amount of electrical energy supplied to or produced by a residence, business or machine.

The most common type is more properly known as a kilowatt hour meter or a joule meter. When used in electricity retailing, the Utilities record the values measured by these meters to generate an invoice for the electricity. They may also record other variables including the time when the electricity was used.

Unit of measurement

The most common unit of measurement on the electricity meter is the kilowatt hour, which is equal to the amount of energy used by a load of one kilowatt over a period of one hour

Types of enegy meter:

: The most common type of electricity meter is the Thomson or electromechanical induction watt-hour meter, invented by Elihu Thomson in 1888.

Technology

The electromechanical induction meter operates by counting the revolutions of an aluminium disc which is made to rotate at a speed proportional to the power. The number of revolutions is thus proportional to the energy usage. It consumes a small amount of power, typically around 2 watts.

The metallic disc is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees using a lag coil. [1]This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current and voltage. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc - this acts as a brake which causes the disc to stop spinning when power stops being drawn rather than allowing it to spin faster and faster. This causes the disc to rotate at a speed proportional to the power being used.

The type of meter described above is used on a single-phase AC supply. Different phase configurations use additional voltage and current coils.

Solid state meters

Some newer electricity meters are solid state and display the power used on an LCD, while newer electronic meters can be read automatically.

In addition to measuring electricity used, solid state meters can also record other parameters of the load and supply such as maximum demand, power factor and reactive power used etc. They can also include electronic clock mechanisms to compute a value, rather than an amount, of electricity consumed, with the pricing varying of by the time of day, day of week, and seasonally.

Solid state electricity meter used in a home in Holland.Technology

Multiple tariff (variable rate) meters

Electricity retailers may wish to charge customers different tariffs at different times of the day. This is because there is generally a surplus of electrical generation capacity at times of low demand, such as during the night (see supply and demand).

Some multiple tariff meters use different tariffs for different amounts of demand. These are usually industrial meters

Domestic usage

Domestic variable-rate meters normally only permit two tariffs ("peak" and "off-peak") and in such installations a simple electromechanical time switch may be used. They are commonly used in conjunction with electrical storage heaters.

Multiple tariffs are made easier by time of use (TOU) meters which incorporate or are connected to a time switch and which have multiple registers.

Switching between the tariffs may happen via a radio-activated switch rather than a time switch to prevent tampering with a sealed time switch to obtain cheaper electricity.

FUSES: In electronics and electrical engineering a fuse (short for fusible link), is a type of overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows, which breaks the circuit in which it is connected, thus protecting the circuit's other components from damage due to excessive current.

A practical fuse was one of the essential features of Thomas Edison's electrical power distribution system. An early fuse was said to have successfully protected an Edison installation from tampering by a rival gas-lighting concern.

Fuses (and other overcurrent devices) are an essential part of a power distribution system to prevent fire or damage. When too much current flows through a wire, it may overheat and be damaged, or even start a fire. Wiring regulations give the maximum rating of a fuse for protection of a particular circuit. Local authorities will incorporate national wiring regulations as part of law. Fuses are selected to allow passage of normal currents, but to quickly interrupt a short circuit or overload condition.

Automotive fuses

Plug-in type fuses come in three physical sizes: mini, ATO and maxi

Automotive fuses protect the wiring and electrical equipment for vehicles. They are generally rated for circuits no higher than 24 volts direct current, but there exists

brands of mini and maxi fuses that is modifed to be able to work in the 42 volt environment.

Blade type

Plug-in fuses (also called blade or spade fuses), with a plastic body and two prongs that fit into sockets, are used in automobiles. These types of fuses come in three different physical dimensions: mini (or minifuse), ATO (or ATC) and maxi (or maxifuse).The physical dimensions, including the connector, of the fuses are as follows (LxWxH) (ampere ratings in the parenthesis):

mini: 10.9x3.6x16.3 mm (2A, 3A, 4A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A) ATO (Automotive Technology Organization): 19.1x5.1x18.5 mm (1A, 2A, 3A,

4A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A, 40A) maxi: 29.2x8.5x34.3 mm (20A, 30A, 40A, 50A, 60A, 70A, 80A)

It is possible to replace[3] an ATO-type plug-in fuse with a circuit breaker that has been designed to fit in the socket of an ATO-sized fuse holder. These circuit protectors are more expensive than a regular fuse.

Colour-coding

Blade fuses use a colour-coding standard. [4] The Mini and ATO style fuses use the same colour-coding system, while the larger maxi fuses use a different system, with only some colours representing the same current ratings.

Mini and ATO Colour-coding:

Colour Current (A)

black* 1

grey 2

violet 3

pink 4

orange/tan 5

brown 7.5

red 10

aqua/blue 15

yellow 20

clear/natural 25

green 30

blue green* 35

amber* 40

* = available in ATO fuses only

Maxi Colour-coding:

Colour Current (A)

yellow 20

grey 25

green 30

brown 35

orange 40

red 50

blue/aqua 60

tan 70

clear/natural 80

Bosch type

Bosch type fuse (used in older cars)

Bosch type fuses are used in old (often European) automobiles. The physical dimension of this type of fuse is 6x25 mm with conical ends. Bosch type fuses usually use the same color coding for the rated current. The DIN standard is 72581/1

Color Ampere

yellow 5A

white 8A

red 16A

blue 25A

Lucas type

Lucas type fuses are used in old British made or assembled automobiles. The physical length of this type of fuse is either 1" or 1.25" with conical ends. Lucas type fuses usually use the same color coding for the rated current. Lucas fuses have three ratings; the continuous current they are designed to carry, the instantaneous current at which they will fuse, and the continuous current at which they will also fuse as well. The figure found on Lucas fuses is the continuous fusing current which is twice the continuous amp rating that the system should be using; this can be a source of confusion when replacing Lucas fuses with non Lucas fuses.

Color Continuous amps Instantaneous fusing amps

Blue 1.5A 3.5A

Yellow 2.25A 5A

Red on Yellow 2.5A 6A

Green 3A 7A

Nut Brown 4A 10A

Red on Green 5A 12A

Green on Black 5A 12A

Red on Brown 6A 14A

Light Brown 7.5A 18A

Pink 12.5A 30A

White 17.5A 40A

Purple on Yellow 25A 60A

Yellow on Red 30A 75A

80A

High voltage fuses

Fuses are used on power systems up to 115,000 volts AC. High-voltage fuses are used to protect instrument transformers used for electricity metering, or for small power transformers where the expense of a circuit breaker is not warranted. For example, in distribution systems, a power fuse may be used to protect a transformer serving 1-3 houses. A circuit breaker at 115 kV may cost up to five times as much as a set of power fuses, so the resulting saving can be tens of thousands of dollars. Pole-mounted distribution transformers are nearly always protected by a fusible cutout, which can have the fuse element replaced using live-line maintenance tools.

Large power fuses use fusible elements made of silver, copper or tin to provide stable and predictable performance. High voltage expulsion fuses surround the fusible link with gas-evolving substances, such as boric acid. When the fuse blows, heat from the arc causes the boric acid to evolve large volumes of gases. The associated high pressure (often greater than 100 atmospheres) and cooling gases rapidly extinguish (quench) the resulting arc. The hot gases are then explosively expelled out of the end(s) of the fuse. Other special High Rupturing Capacity (HRC) fuses surround one or more parallel connected fusible links with an energy absorbing material, typically silicon dioxide sand. When the fusible link blows, the sand absorbs energy from the arc, rapidly quenching it, creating an artificial fulgurite in the process.

CIRCUIT BREAKER: A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

A 2 pole miniature circuit breaker

TYPES OF CIRCUIT BREAKER: Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.

Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, include:

MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.

MCCB (Moulded Case Circuit Breaker)—rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.

Low voltage power circuit breakers can be mounted in multi-tiers in LV switchboards or switchgear cabinets.

THERMOMAGNETIC CIRCUIT BREAKER

MAGNETIC CIRCUIT BREAKER

Common trip breakers

Three pole common trip breaker for supplying a three-phase device. This breaker has a 2 A rating

When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two pole common trip breakers are common on 120/240 volt systems where 240 volt loads (including major appliances or further distribution boards) span the two live wires. Three-pole common trip breakers are typically used to supply three-phase electric power to large motors or further distribution boards.

High-voltage circuit breakers

A 1200 A 3-pole 115,000 V breaker at a generating station in Manitoba, Canada.

Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of "high voltage" varies but in power transmission work is usually thought to be 72,500 V or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protection relay scheme can be complex, protecting equipment and busses from various types of overload or ground/earth fault.

RELAY: A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier

TYPES OF RELAYSo 2.1 Latching relay o 2.2 Reed relay o 2.3 Mercury-wetted relay o 2.4 Polarized relay o 2.5 Machine tool relay o 2.6 Contactor relay o 2.7 Solid state contactor relay o 2.8 Buchholz relay o 2.9 Forced-guided contacts relay o 2.10 Solid-state relay o 2.11 Overload protection relay

Operation

When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.

If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.[1]

By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an optocoupler can be used which is a light-emitting diode (LED) coupled with a photo transistor

Applications

Relays are used:

to control a high-voltage circuit with a low-voltage signal, as in some types of modems or audio amplifiers,

to control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile,

to detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays),

A DPDT AC coil relay with "ice cube" packaging to isolate the controlling circuit from the controlled circuit when the two are at

different potentials, for example when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors in an effort to conserve energy,

to perform logic functions. For example, the boolean AND function is realised by connecting NO relay contacts in series, the OR function by connecting NO contacts in parallel. The change-over or Form C contacts perform the XOR (exclusive or) function. Similar functions for NAND and NOR are accomplished using NC contacts. The Ladder programming language is often used for designing relay logic networks.

o Early computing. Before vacuum tubes and transistors, relays were used as logical elements in digital computers. See ARRA (computer), Harvard Mark II, Zuse Z2, and Zuse Z3.

o Safety-critical logic. Because relays are much more resistant than semiconductors to nuclear radiation, they are widely used in safety-

critical logic, such as the control panels of radioactive waste-handling machinery.

to perform time delay functions. Relays can be modified to delay opening or delay closing a set of contacts. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening release time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly. The time period can be varied by increasing or decreasing the flow rate. For longer time periods, a mechanical clockwork timer is installed

The main objectives of earthing are as follows -

To ensure safety of life and property from hazards of electric shock and electric fires.

To ensure that system voltages on healthy lines remain within reasonable limits under fault Conditions thereby preventing insulation breakdowns.

To provide a low impedance path to facilitate the satisfactory operation of protective devices under fault conditions.

To minimize arcing burn downs as in an earthed system arcing fault would produce a current in ground path thereby providing an easy means of detecting and tripping against phase to earth arcing fault breakdowns.

To provide an equipotential platform on which electronic equipments can operate.

To provide an alternative path for induced current and minimize the electrical noise in cables.