Services 3rd Yr.

8/9/2019 Services 3rd Yr.

1/7

Earthing

In electricity supply systems, an earthing system defines the electrical potential of the

conductors relative to that of the Earth's conductive surface. The choice of earthing system has

implications for the safety and electromagnetic compatibility of the power supply. Note that

regulations for earthing (grounding) systems vary considerably among different countries.

Aprotective earth (PE) connection ensures that all exposed conductive surfaces are at the

same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a

person touches a device in which an insulation fault has occurred. It ensures that in the case of

an insulation fault (a "short circuit"), a very high current flows, which will trigger an overcurrent

protection device (fuse, circuit breaker) that disconnects the power supply.

A functional earth connection serves a purpose other than providing protection against electrical

shock. In contrast to a protective earth connection, a functional earth connection may carry a

current during the normal operation of a device. Functional earth connections may be required

by devices such as surge suppression and electromagnetic interference filters, some types

ofantennas and various measurement instruments. Generally the protective earth is also used

as a functional earth, though this requires care in some situations

The earth is made up of materials that is electrically conductive. A fault current will flow to

'earth' through the live conductor, provided it is earthed . This is to prevent a potentially live

conductor from rising above the safe level . All exposed metal parts of an electrical installation

or electrical appliance must be earthed .

The main objectives of the earthing are to :

1) Provide an alternative path for the fault current to flow so that it will not endanger the user

2) Ensure that all exposed conductive parts do not reach a dangerous potential

3) Maintain the voltage at any part of an electrical system at a known value so as to prevent

over current or excessive voltage on the

appliances or equipment .

The qualities of a good earthing system are :

1) Must be of low electrical resistance

2) Must be of good corrosion resistance

3) Must be able to dissipate high fault current repeatedly
http://en.wikipedia.org/wiki/Ground_(electricity)http://en.wikipedia.org/wiki/Electricity_supplyhttp://en.wikipedia.org/wiki/Ground_(electricity)http://en.wikipedia.org/wiki/Electrical_potentialhttp://en.wikipedia.org/wiki/Safetyhttp://en.wikipedia.org/wiki/Electromagnetic_compatibilityhttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Electrical_shockhttp://en.wikipedia.org/wiki/Electrical_shockhttp://en.wikipedia.org/wiki/Surge_suppressionhttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Antenna_(radio)http://en.wikipedia.org/wiki/Ground_(electricity)http://en.wikipedia.org/wiki/Electricity_supplyhttp://en.wikipedia.org/wiki/Ground_(electricity)http://en.wikipedia.org/wiki/Electrical_potentialhttp://en.wikipedia.org/wiki/Safetyhttp://en.wikipedia.org/wiki/Electromagnetic_compatibilityhttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Electrical_shockhttp://en.wikipedia.org/wiki/Electrical_shockhttp://en.wikipedia.org/wiki/Surge_suppressionhttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Antenna_(radio)


2/7

Types of electric wires

Whether you're building a new house or remodeling an old one, you can use different electrical

wiring methods. The methods are differentiated by the type of wire used and how the wires are

connected, and the decision on which one to use depends on the electrical needs of the home.

Types

1. The most common type of house wiring method involves using copper wire to connect

wall outlets and fixtures to junction boxes, which are then connected to the house's main

electrical panel and circuit breaker box by copper wire. Variations to this method can be

made be changing the type of wire used to either aluminum wire or silver wire as well as

bypassing the junction box and connecting the outlet or fixture directly to the circuit

breaker. In rare situations, mostly those in older homes, wires are routed and connected

into a fuse instead of a circuit breaker.

Effects

2. The type of house wiring method used will greatly effect the performance of the house'selectrical system. Copper wire is a better conductor of electricity than aluminum wire,

which has about 60 percent of the conductivity of copper. However because of its

lightweight property, aluminum wire is better to run over long distances than copper wire,

which is heavier. Also, aluminum wiring is cheaper than copper wire and usually delivers

better performance. Silver wire can be used, but it is expensive compared to copper and

aluminum. It should only be used when an extremely high level of conductivity is

required. The use of junction boxes will also effect the electrical system. Using them

allows you to connect several fixtures and outlets into a single circuit, which is a more

cost-effective method because it reduces the number of circuit breakers and the amount

of wire needed. However better performance will be achieved when fixtures and wall

outlets are wired directly into their own circuit because it decreases the chance of

overloading and tripping the circuit. But the benefit comes with the increased costs,

mainly extra wire.

Lumen method

Also known as the zonal cavity method, the lumen method is a series of calculations that uses

horizontal illuminance criteria to establish a uniform luminaire layout in a space. In its simplest

form, the lumen method is merely the total number of lumens available in a room divided by the

area of the room[1]. In order to perform this calculation, many factors, coefficients, lamp lumendata and other quantities must be gathered. Despite the scientific impression of the lumen

method equations, there are inaccuracies and assumptions built into the method. Therefore, the

lumen method should not typically be used as a standalone, final solution; it should be used as

a tool in particularly uniform settings of lighting design if a simple, rough technique of

illuminance quantification is desired.

The lumen method was described in a 1908 paper presented to the Illuminating Engineering

Society. [3]The method was described as a simplification from the point-by-point method which

required much calculation. Since the average illumination is calculated, the lumen method alone
http://en.wikipedia.org/wiki/Illuminancehttp://en.wikipedia.org/wiki/Luminairehttp://en.wikipedia.org/wiki/Lumen_method#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Lumen_method#cite_note-2%23cite_note-2http://en.wikipedia.org/wiki/Lumen_method#cite_note-2%23cite_note-2http://en.wikipedia.org/wiki/Illuminancehttp://en.wikipedia.org/wiki/Luminairehttp://en.wikipedia.org/wiki/Lumen_method#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Lumen_method#cite_note-2%23cite_note-2


3/7

gives no estimate of the uniformity of the light between dimmest and brightest parts of the room,

nor does it model daylight contributions nor obstacles within the room.

Method

The lumen method in brief consists of calculation of the "cavity ratios" of the upper, middle, and

lower volumes of the space to be lighted. The lower cavity is from the floor to the working

height, the upper cavity is from the lower edge of the luminaires to the ceiling, and the middle

cavity is the volume between these planes.

The effective reflectance of ceiling, floor, and walls are estimated from tabular data. A coefficient

of utilization, representing the fraction of light that is directed to the working plane, is supplied by

manufacturers for each luminaire design for the various calculated room cavity ratios.

Some of the light produced by the lamps is lost due to non-ideal lamp operating conditions, dirt

on the luminaires, dirt on the room surfaces. A light loss factor is calculated for all these, based

on tabulated empiracal factors.

Given the usual lighting problem of obtaining an average lighting level at the working plane, the

number of luminaires can be caculated based on the effective amount of useful light that each

luminaire has been calculated to emit.

Since the zonal cavity method only gives an average lighting level, manufacturers tabulate

recommended spacing to mounting height ratios that must not be exceeded if uniform

illumination is desired.

Usage

The lumen method can be manipulated to solve for a particular variable. This is valuable

because certain numbers are needed at different times in the design process. Number of

luminaires is important because this number can be used to estimate costs and layout the

spacing of luminaires in a computer lighting calculation program

ELCB

Purpose

Many electrical installations have a relatively high earth impedance. This may be due to the use

of a local earth rod (TT systems), or to dry local ground conditions.

These installations are dangerous cccccand a safety risk if a live to earth fault current flows.

Because earth impedance is high:

1. not enough current exists to trip a fuse orcircuit breaker, so the condition

persists uncleared indefinitely

2. the high impedance earth cannot keep the voltage of all exposed metal to a safe

voltage, all such metalwork may rise to close to live conductor voltage.
http://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Riskhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Riskhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Circuit_breaker


4/7

These dangers can be drastically reduced by the use of an ELCB orResidual-current

device (RCD).

The ELCB makes such installations much safer by cutting the power if these dangerous

conditions occur. This approach to electrical safety is called EEBAD. In Britain EEBAD domestic

installations became standard in the 1950s.

In non-technical terms if a person touches something, typically a metal part on faulty electrical

equipment, which is at a significant voltage relative to the earth, electrical current will flow

through him/her to the earth. The current that flows is too small to trip an electrical fuse whichcould disconnect the electricity supply, but can be enough to kill. An ELCB detects even a small

current to earth (Earth Leakage) and disconnects the equipment (Circuit Breaker).

[edit]History

ELCBs were mainly used on TT earthing systems. Nowadays, ELCBs have been mostly

replaced by Residual-current devices (RCDs). However many ELCBs are still in use.

Early ELCBs responded to sine wave fault currents, but not to rectified fault current. Over time,

filtering against nuisance trips has also improved. Early ELCBs thus offer a little less safety and

higher risk of nuisance trip. The ability to distinguish between a fault condition and non-risk

conditions is called discrimination.

Types

voltageoperated and,

current operated.

Voltage-operated ELCBs were introduced in the early 20th century, and provided a major

advance in safety for mains electrical supplies with inadequate earth impedance. v-ELCBs have

been in widespread use since then, and many are still in operation.

Current-operated ELCBs are generally known today as RCDs (residual current device). These

also protect against earth leakage, though the details and method of operation are different.

When the term ELCB is used it usually means a voltage-operated device. Similar devices that

are current operated are called Residual-current devices. Cudis CPN offer Electronic and

Current operated types http://www.cudis.co.uk

Connection

The earth circuit is modified when an ELCB is used; the connection to the earth rod is passed

through the ELCB by connecting to its two earth terminals. One terminal goes to the installation

earth CPC (circuit protective conductor, aka earth wire), and the other to the earth rod (or

sometimes other type of earth connection). Thus the earth circuit passes through the ELCB's

sense coil.
http://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/w/index.php?title=Earth_leakage_circuit_breaker&action=edit&section=2http://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/Sine_wavehttp://en.wikipedia.org/wiki/Rectifierhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Residual-current_devicehttp://www.cudis.co.uk/http://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Earthinghttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/w/index.php?title=Earth_leakage_circuit_breaker&action=edit&section=2http://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/Sine_wavehttp://en.wikipedia.org/wiki/Rectifierhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Residual-current_devicehttp://www.cudis.co.uk/http://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Earthinghttp://en.wikipedia.org/wiki/Earthing_system


5/7

Operation

An ELCB is a specialised type of latching relay that has a building's incoming mains power

connected through its switching contacts so that the ELCB disconnects the power in an earth

leakage (unsafe) condition.

The ELCB detects fault currents from live (hot) to the earth (ground) wire within the installation it

protects. If sufficient voltage appears across the ELCB's sense coil, it will switch off the power,

and remain off until manually reset. An ELCB however, does not sense fault currents from liveto any other earthed body.

Advantages

ELCBs have one advantage over RCDs: they are less sensitive to fault conditions, and

therefore have fewer nuisance trips. (This does not mean they always do, as practical

performance depends on installation details and the discrimination enhancing filtering in the

ELCB.) Therefore by electrically separating cable armour from cable CPC, an ELCB can be

arranged to protect against cable damage only, and not trip on faults in downline installations.

Disadvantages

ELCBs have some disadvantages:

They do not detect faults that don't pass current through the CPC to the earth rod.

They do not allow a single building system to be easily split into multiple sections with

independent fault protection, because earthing systems are usually bonded to pipework.

They may be tripped by external voltages from something connected to the earthing

system such as metal pipes, a TN-S earth or a TN-C-S combined neutral and earth. As with RCDs, electrically leaky appliances such as some water heaters, washing

machines and cookers may cause the ELCB to trip.

ELCBs introduce additional resistance and an additional point offailure into the earthing

system

Armoured cable
http://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Fault_currenthttp://en.wikipedia.org/wiki/Cablehttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Bodyhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Buildinghttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Pipeworkhttp://en.wikipedia.org/wiki/TN-Shttp://en.wikipedia.org/wiki/Water_heaterhttp://en.wikipedia.org/wiki/Washing_machinehttp://en.wikipedia.org/wiki/Washing_machinehttp://en.wikipedia.org/wiki/Cookerhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Failurehttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Fault_currenthttp://en.wikipedia.org/wiki/Cablehttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Bodyhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Buildinghttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Pipeworkhttp://en.wikipedia.org/wiki/TN-Shttp://en.wikipedia.org/wiki/Water_heaterhttp://en.wikipedia.org/wiki/Washing_machinehttp://en.wikipedia.org/wiki/Washing_machinehttp://en.wikipedia.org/wiki/Cookerhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Failurehttp://en.wikipedia.org/wiki/Earthing_systemhttp://en.wikipedia.org/wiki/Earthing_system


6/7

Armoured cable or SWA (steel wired armoured) cable is

the accepted standard for underground installations. This

wire can be buried in the ground or can be fastened to

exterior walls using cleats. The galvanised steel wire

protects the wire from accidental damage caused by

digging etc.

Armoured cable comes in several different variables but

for domestic wiring it will normally range from 2 core to 4core. The conductor cores vary from 1.5mm for lighting

circuits up to 16mm for heavy duty loads.

The galvanised steel wire can be used as an additional

earth and sometimes is used as the only earth, although

this is not recommended.

When the armoured cable terminates at an appliance or

junction box a gland is used. You must get the correct size

and type of gland for the job at hand. If it is outside the gland

must be waterproof.

The gland serves two purposes, firstly it holds the wire and

prevents it from being pulled out of the appliance and

secondly it acts as a secondary earth.

Armoured cable does not require any special tools, it

can be cut with a pair of quality wire cutters like these

in the picture or with a hacksaw.

A pair of side cutters are useful for cutting the

individual cores of the galvanised steel wire and two

spanners of the correct size are needed for theeffective fitting of glands.

Below are the components for a external gland


7/7

1. Nut

2. Earth Tag

3. Nozzle

4. Collar

5. Collar nut

6. Nut with rubber seal

7. Cover

Services 3rd Yr.

Documents

Transcript of Services 3rd Yr.