section 12 electrical_installation_practice_2

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Section 12

Electrical installation practice 2

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Earth fault loop

A load test or earth-fault-loop impedance test is applied to an installation to ensure continuity and effectiveness of the earthing arrangement.

The earth fault loop is the circuit path followed by a fault current due to the effect of a low impedance occurring between the active conductor and a protective earth system.


Earth fault loop


Earth fault loop

If a person in direct contact was the cause of the earth fault then the severity of the electric shock received depends entirely on the impedance of the earth fault loop.

Electrocution is the highest hazard that is associated with low levels of fault current (longer time for the protective device to operate) and not the highest fault levels.


Protective devices for overload and fault currents

A protective device must be placed at the point where a reduction of the cross-sectional area (CSA) of the circuit conductors or another change causes alteration in the characteristics of the installation.


Protective devices for overload and fault currents An installation with some protective devices in series

is considered a selective coordination arrangement when, in the event of short-circuit or overload, the installation is interrupted only by the device that is immediately upstream of the fault point.

The term ‘upstream’ refers to proximity to the origin of the installation and ‘downstream’ refers to proximity to the load.

Selectivity is ensured when the time–current characteristic of the upstream MCB is above the time–current characteristic of the downstream MCB. Selectivity may be total or partial.



Protective devices must be able to interrupt any over-current including the prospective short-circuit current at the location where the devices are installed. Interrupting rating is defined as the prospective short-circuit current at rated operating voltage that a device is intended to interrupt under load conditions.

Where fuses or circuit breakers that don’t have effective interrupting ratings are used, they can rupture aggressively with consequent hazards to equipment and personnel.



Series rated

Series rated is a combination of two short-circuit protective devices that have been tested under the manufacturer’s test conditions to work together to clear a fault.

Only tested combinations can be used.



Series rated is a combination of circuit breakers or fuses and circuit breakers that can be used at short-circuit levels above the interrupting rating of the load-side (protected) circuit breaker, but not above the interrupting rating of the main or supply-side device.


Devices for automatic disconnection of supply

Circuit protection devices

The continuous load ratings of all electrical equipment are based on thermal considerations, the limiting factor being the maximum temperature at which the insulation can be stressed without any deterioration occurring.

Circuit protection devices are designed to protect the insulating materials by stopping current flow or opening the circuit.



There are two over-current protection devices: fuses and circuit breakers.

There is another protection device called the core balance earth leakage device. Its role is to protect small appliances used in earthed situations and indirectly this device can protect people.



Fuses

The word fuse comes from the Latin word fusus meaning ‘to melt’. A simple fuse is only a short length of wire called the element which is designed to melt and separate when a fault current flows.

If this happens we say that the fuse has ‘blown’.



Fuse time–current characteristic curves

Time–current characteristics determine how fast a fuse responds to over-currents. All fuses have an inverse time characteristic by which fusing time decreases as the value of over-current increases.

When properly rated, fuses provide both overload and short-circuit protection to an installation’s conductors, cables and connected components.



Fuse time–current characteristic curves

Each curve is exclusive for one fuse rating and provides the typical opening time for the amperage rating selected.

Time–current characteristic curves are extremely useful in defining a fuse, since fuses with the same current rating can be represented by considerably different time–current curves.

The time–current characteristic curve is used to check whether a particular fuse will cope with the operating demands of the protected circuit.



Fault current limiters

A fault current limiter (FCL) is a circuit protective device (isolator) which limits the prospective fault current when a fault occurs.

Current limiters fit into three basic categories: circuit breakers, fuses and positive-temperature-coefficient (PTC) devices.




When selecting suitable FCLs for an installation the following three parameters are considered:

1. The short-circuit current available from the supply

2. The ratings and characteristics of the connected equipment

3. The rating and characteristics of the protective devices




Fuses are the oldest and most common current limiters. The detailed design aspects of current-limiting fuses for fault-current-limiting applications are somewhat different from those used for typical load applications.

In particular, when used as an FCL the rating of the fuse element is significantly smaller than the normal load current expected to be carried by the FCL, thus ensuring rapid operation.



Circuit breakers

Circuit breakers are electromechanical devices extensively used in domestic, commercial and industrial electrical installations to protect circuits against over-current faults.

The circuit breaker is a reliable over-current protective device that opens and closes like a switch.


Devices for automatic disconnection of supply An overload is a circuit condition whereby the

load device draws more current from the supply than the conductor’s rated operating current. For example, a circuit can be 50% or even 100% or more overloaded. Therefore the larger the overload the shorter is the time required to operate the breaker. This characteristic is known as the inverse time characteristic because as the current increases the time decreases. At high temperatures it should trip faster than at low temperatures. This is known as a temperature-sensitive characteristic as the tripping time will depend upon the ambient (air) temperature.



Tripping releases

Circuit breakers are equipped with bi-metallic-based, inverse-time-delayed overload releases and with instantaneous over-current releases (electromagnetic short-circuit releases).



Miniature circuit breaker

A miniature circuit breaker (MCB) is a protective device that is installed primarily to protect final circuits against overload and short-circuits.

As a current-limiting device MCBs protect conductors, cables and circuit components from damage by reducing the peak let-through current, which causes damaging magnetic forces, and peak let-through energy, which generates heat.



MCB time–current characteristic curves

The time–current (tripping) characteristic defines the MCB’s speed of response (trip time) to various levels of over-current.

The time–current characteristic enables the device to be optimally matched to the application.

For example, water heater and stove circuits that can tolerate moderate over-currents only are best protected by MCBs with Type B trip characteristics.



Key selection criteria for MCBs are the trip-time zones determined at a particular design temperature.

Upper and lower limits of the curves show minimum and maximum adjustment tolerances.

Unless otherwise stated by the various manufacturers, all thermal and thermal–magnetic circuit breakers will carry 100% rated current continuously and trip within one hour at 140% rating.



The environmental conditions in which an MCB is required to operate must be considered when selecting a suitable circuit protection device.

Extreme temperature, humidity, vibration and shock can cause undesirable performance characteristics in some types of MCBs.

For instance the thermal-release element can be less reliable when it is hot than when it is cold.


Protection against over-voltage and under-voltage

Over-voltage

Over-voltage can be caused by:

Rapid reduction in power loads by heavy equipment due to faulty switching operations.

Transient switching operations that control inductive circuits such as motors and transformers (incorrect tap settings).

Lightning discharges on or near an electrical system.

Energising a capacitor bank.



Over-voltages are also called swells or spikes. When swells or spikes are short-duration increases above nominal voltage (above 253 V for a 230 V system) they are referred to as surges.

Swells and spikes are defined by their voltage gradient and time characteristic. Swells are modest voltage increases lasting from nanoseconds to one second.

Spikes are very large voltages lasting only microseconds.



Transient switching operations

Transient switching operations (for example, motors and transformers) create over-voltages because the switching action does not operate in sync with the zero point of the ac waveform.

This means that in many switching operations there is a sudden change of current value, from a high value to zero such as when a circuit breaker operates to clear a fault.


Protection against over-voltage and under-voltageOther over-voltage situations include:

Voltage unbalance (single-phase fault on a three-phase system).

A neutral-earth voltage rise that is caused by poor workmanship.

A breakdown of insulation between electrical circuits or systems of different voltages.

Electrostatic over-voltages in unearthed electrical systems as a result of atmospheric discharges (a discharging cloud) between an overhead line and the ground.



Insulation breakdown

Over-voltages are voltages that exceed electrical system design values.

Voltage ratings determine the dielectric strength of the insulation protecting the conductors, cables and equipment.

For example, insulation rated at 3 kV may be 2 mm to 2.8 mm thick. However, the thickness of insulation varies depending on the type of conductor, cable or electrical equipment.



Methods of over-voltage protection

Over-voltages are prevented from reaching conductors, cables and equipment in an installation by short-circuiting (extinguishing them before a dangerous value has been reached).

To prevent over-voltages an installation must have installed quick-reaction over-voltage protection arrestors that can respond in nanoseconds.



Over-voltage protection arrestors must be able to withstand very high current surges because a short-circuited over-voltage from an energy source such as a lightning strike can produce several thousand amperes.

An effective over-voltage arrestor must also remove any remaining over-voltage (residual voltage) that is still present during the current surge.



Over-voltage protection by the different types of arrestor devices should have the following properties:

rapid response times

a short earth conductor path to the ground (soil)

high current-carrying capacity

containment of residual voltage

good resetting time

long service life



Types of over-voltage arrestors

When a lightning event occurs near or strikes a transmission or distribution line, a high-voltage impulse waveform appears on the line.

If it reaches a transformer or an installation protected by lightning surge diverters, the impulse, if large enough, will travel through the diverter to the ground, impressing on the transformer or installation only that part of the voltage waveform that occurred before it conducted.



There are four types of over-voltage arrestors:

Valve-type arrestor (spark gap arrestor)

Varistors (voltage-dependent resistors)

Gas-filled arrestors

Suppression diodes



Internal over-voltages are those voltages generated inside electrical equipment. Typical causes of these are:

switching on and off motors and current transformers

switching on and off photocopiers and phase-controlled modulators

switching on switch-mode power supplies

tripping a fuse, a relay or an interrupter and electrical power outages of any kind



Solid-state equipment protectors

Some solid-state arrestors consist of zener diodes, which exhibit voltage-limiting characteristics.

When a transient over-voltage occurs along the line with a voltage exceeding the reverse-biased voltage rating of the zener diode, the zener will conduct and the transient will be clamped at the zener voltage.



The over-voltage protection device’s equipotential bonding conductors must be connected by the shortest possible route to the main earth terminal of the installation.

Longer routes reduce the efficiency of the over-voltage protector.



Typical lightning and surge protection specifications on an installation

All mains-operated electrical equipment should be protected from lightning and power surges.

The protection system must consist of surge protection devices and earthing systems.

The protection system should not affect the operation of equipment under normal operating conditions.



Solutions to under-voltage include:

Adding voltage regulators to improve the voltage profile.

Adding shunt capacitors or static Var (volts-amp reactive) compensators to reduce the line current.

Increasing the size of the transformer, adding series capacitors or increasing the size of line conductors to reduce the system impedance.


END

section 12 electrical_installation_practice_2

Education

fault currentscopyright

fault point

earth fault loopcopyright

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highest fault levels

protective earth system

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