Physiological Effects V2

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Physiological effects

The State of Queensland (TAFE Queensland) 2008

20, October, 2008

Version 01 Revision 1

flexiblelearning.net.au


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Australian Flexible Learning Framework i

The views expressed herein do not necessarily represent the views of the Commonwealth of Australia.

Commonwealth of Australia 2008

This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproducedwithout prior written permission. However, permission is given to trainers and teachers to make copies byphotocopying or other duplicating processes for use within their own training organisation or in a workplace where thetraining is being conducted. This permission does not extend to the making of copies for use outside the immediatetraining environment for which they are made, nor the making of copies for hire or resale to third parties. Requestsand inquiries concerning other reproduction and rights should be directed in the first instance to the Director, ICTPolicy Section, Department of Education, Employment and Workplace Relations, GPO Box 9880, Canberra, ACT,2601.


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Australian Flexible Learning Framework ii

Table of contents

Table of contents ..................................................................................................................... ii Table of figures ....................................................................................................................... ii iList of tables ............................................................................................................................ iv Physiological effects ............................................................................................................... 1

Introduction............................................................................................................................ 1Physiological effects of electrical current .............................................................................. 1Circuit protection ................................................................................................................... 4AS3000 safety requirements................................................................................................. 5Protective devices................................................................................................................. 6Residual current devices (RCDs) ....................................................................................... 10


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Australian Flexible Learning Framework iii

Table of figures

Figure 1: Circuit protect ion .................................................................................................... 4

Figure 2: Direct contact .......................................................................................................... 5

Figure 3: Indi rect con tact ........................................................................................................ 6

Figure 4: Types of circuit breakers ........................................................................................ 7

Figure 5: Residual current devices ........................................................................................ 8

Figure 6 : RCS's how they work ............................................................................................. 9

Figure 7: Fault path ............................................................................................................... 10

Figure 8: RCD......................................................................................................................... 11


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Australian Flexible Learning Framework iv

List of tables

Table 1: Effects of electrical cur rent...................................................................................... 2

Table 2: RCD classes .............................................................................................................. 8

Table 3: Portable RCDs .......................................................................................................... 8


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Australian Flexible Learning Framework 1


Introduction

Electricity is dangerous!

Electricity is useful!

Compare the two statements above and you should realise that electricity, whenhandled properly is part of our life. Although it is extremely dangerous, we cannot dowithout it. We need to understand what effect Electrical shock will have on a bodywhen it comes in contact with a live conductor. These may include but are notlimited to:

Contraction of muscles, causing the victim to be unable to free himself.

Burns at the point of contact.

Unconsciousness and signs of shock.

Heart failure caused by the shock impulse disturbing the heart beat.

Death caused by incapability to breath.

Physiological effects of electrical current

A common misconception is that larger voltages are more dangerous than smallerones. However, this is not quite true. The danger to living things comes not from thepotential difference, but rather the current flowing between two points.

Electric shock is not the only ill effect caused by electrical accidents. The heatgenerated by electric arcs can be over 3000 C and have enough energy to melt

metal switchboards. It can certainly cause a great deal of pain to humans burnt bythe heat. In generating such heat within a confined space, explosions of hot air andmetal vapour can burst switchboards open like fire crackers. You might imaginewhat effects that would have on human bodies.

Electric shocks caused by electrical equipment occur without warning and are oftenserious. The average worker is frequently involved in a dangerous electricalsituation through not realising that voltages are as low as 32 V AC and 114 V dc canbe just a lethal as much higher voltages.

There are three factors which determine the severity of the physiological effect ofcurrent on the human body.

The amount of electrical current (DC, AC, Wave shape, Frequency andDirection of current flow)

The path the current follows (hand to hand, hand to foot etc) - resistance ofbody

The duration of the electrical shock. (The longer the current is allowed to flowthe greater the effect.)

The following table represents the effects of an electrical current passing through anaverage human body.


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Table 1: Effects of electrical current

Current level

(Approx.)Effect upon the human body

0 2 mA Barely perceptible, slight tingling sensation may be felt

2 8 mA Sensation becomes greater and more painful

8 12 mA Pain increases and muscle spasms begin to occur

12 20 mA Muscles tighten and can not be controlled, victim maynot be able to let go of conductor

20 50 mA If current passes through chest, muscle around lungsbegin to tighten and breathing becomes difficult or evenimpossible. Reduction of oxygen transfer from lungs toblood capalaries results in insufficient oxygen rich bloodgetting to brain. This results in severe blackout or brain

damage after three minutes.

50 100 mA If current passes through heart, ventricular fibrillationoccurs resulting in low or no blood circulation. As bloodcarries oxygen to the brain, the lack of oxygen richblood to the brain ceases and brain damage can occurafter about three minutes.

100 200 mA Heart stops beating, blood circulation ceases

More than 200 mA Severe burning especially in the areas of contact withthe electrical current.


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Circuit protection

Overload / over current

When the load current on a circuit exceeds the designed circuit current, there will be

an over current. Same causes of over current are: Motor-starting currents which can be seven times higher than the normal

motor running current.

Sub circuit over current - Some causes are: too many appliances are usedon one circuit, Faults in appliances or wiring, wrong design of circuit cablesize etc.

Insulation failure: Faulty insulation on cables and appliances.

Short circuits

A short circuit has a conducting path of negligible resistance and therefore thecurrent will take the path of least resistance. This will cause excessive high currentsto flow.

Fault currents

A Fault current is often caused by short circuits. This will cause that the fault currentwill have a current that is higher that the circuits rated or designed value

Figure 1: Circuit protection

Protection

In AS 3000/2000 it outlines the requirements for protection against the effects ofcurrent.


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AS3000 safety requi rements

Protection for safety general

The requirements of this Standard are intended to ensure the safety of persons,

livestock and property against dangers and damage that may arise in thereasonable use of electrical installations.

In electrical installations the two major types of risk are as follows:

1. Shock current arising from contact with parts which are live in normal service(direct contact) or parts which become live under fault conditions (indirectcontact).NOTES:a) A shock current is an electric current of sufficient magnitude and durationto cause an electric shock. AS 3859 provides further information on theeffects of shock current through the human body.b) Direct contact and indirect contact are defined and illustrated in Clauses

1.4.31 and 1.4.32.2. Excessive temperatures likely to cause burns, fires and other injurious

effects.

1.4.31 Contact - direct

Contact with a conductor or conductive part which is live in normal service.

1.4.32 Contact - indirect

Contact with a conductive part which is not normally live but has become live under

fault conditions (due to insulation failure or some other cause).

1.4.33 Cord - flexible

A flexible cable, no wire of which exceeds 0.31 mm diameter and no

1.7.2 Protection

Protection against both direct and indirect contact by use of extra-low voltage.Persons and livestock shall be protected against dangers that may arise fromcontact with parts which are live in normal service (direct contact) or exposedconductive parts which may become live under fault conditions (indirect contact).

Figure 2: Direct contact


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Figure 3: Indirect contact

Protective devices

There are numerous types of protection devices in general use. In this section wewill only look at those in use for protection against Electrical shock, fault currents,over current.

Fuses

The function of a fuse is to detect over currents or short currents and thenautomatically disconnect the faulty equipment or circuit from the supply.

The simplest form of overload and short circuit protection is a fuse. When theelement of a fuse melts close to an over current or short circuit it is said that the fusehas blown

Definition of a fuse

A fuse is a device designed to open a circuit by melting off its element when thecurrent through it exceeds its rated value for a certain time.

Many fuses are equipped to give visual evidence that the fuse element has melted.These are called indication fuses.

Fuses are manufactured in a number of forms. The rewritable and high rupturingcapacity (HRC) types are the most common types of fuses used.

Circuit breakers

A Circuit-breaker is a device used to protect circuits against overloads and fault andshort circuit currents by automatically opening its contact points. The methods usedto open the contact points under such conditions vary according to the type of circuitbreaker used.

Definition of a circuit breaker

A circuit breaker is a protective device suitable for opening a circuit automatically asa result of short-circuit or overload.

Because of the many advantages circuit-breakers have over fuses they are moreand more generally used. Their construction is such that they never needreplacement, they are safe and cannot be tampered with. The switch on the circuit

breaker is designed in such a way that the contacts cannot be held in the closedposition under abnormal conditions.

There are two main types of Circuit breakers:


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The thermal type

The thermal circuit breakers are equipped with bi-metal strips, and their operationalprincipal is the heat effect of an electrical current flow.

Magnetic type

The operating principle of this circuit breaker is to trip if the magnetic flux producedby the current Exceeds a preset limit.

Figure 4: Types of circuit breakers

Protection

If however, an earth fault current develops, a path parallel to the neutral point of thetransformer is formed through the earth. This will result in an imbalance (the oneconductor carries more current that the other).

As a result, the magnetic fluxes set up by the two primaries no longer neutraliseeach other, and the resultant flux induces and emf in the secondary winding(transformer action).

When the earth fault current exceeds the predetermined value (between 15 mA and30 mA the emf induced in the secondary winding will trigger the tripping circuit in theprinted circuit board (PCB). The tripping coil (Shunt trip) is energised and the circuitis disconnected from the supply.

For added safety RCD switches both the active and neutral conductor.

Residual current devices

AS3000/2000 defines a residual current device as a device intended to isolatesupply to protected circuits, socket-outlets or electrical equipment in the event of acurrent flow to earth which exceeds a predetermined value.


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Figure 5: Residual current devicesResidual current devices are inserted into a circuit not to protect the circuitconductors but to protect people from electric shock. An RCD uses the principle thata current travelling in a conductor generates a magnetic field around the conductor.

The greater the current the greater the magnetic field. An RCD also uses theprinciple that magnetic fields of opposite polarity will tend to cancel each other out.

A residual current device uses a type of transformer called a toroid through whichthe active and neutral conductors pass through. If the current in the active andneutral are equal, the magnetic field within the toroid will be equal and oppositethereby cancelling each other out. If a fault to earth occurs, the fault current will

travel through the active conductor but not the neutral. This out-of-balance will bedetected by the RCDs electronic circuit and will cause the RCD to operate openingboth the active and neutral conductors.

RCDS are classed by their residual current rating and wether or not they employ aselective tripping-time delay.

Table 2: RCD classes

Type I residual current rating not exceeding 10mA

Type II residual current rating exceeding 10mA but not exceeding 30mA

Type III residual current rating exceeding 30mA but not exceeding 300mAwithout selective tripping-time delay

Type IV residual current rating exceeding 30mA but not exceeding 300mA withselective tripping-time delay

Portable RCDs are divided into two basic classifications:

Table 3: Portable RCDs

Class L Single-phase domestic useClass H General industrial use


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For further information on residual current devices see Electrical Wiring PracticesVol. 2 p. 60 to 78

The following figures provide a brief description about how an RCD saves lives andthe theory of body protection.

Figure 6 : RCS's how they work


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Residual current devices (RCDs)

AS 3000/2000 states the following

1.7.5 Protection by use of residual current devices (RCDs)

1.7.5.1 Where required RCDs shall be installed for the protection of persons andlivestock as required by this Standard. (See Clause 2.5)

NOTE: Attention is drawn to the additional provisions made for the installation ofRCDs in

other relevant Standards, including AS 3001, AS 3002, AS/NZS 3012; and

additional requirements and regulations, such as Occupational Health andSafety legislation.

Additional protect ion against direct contact

RCDs are not recognized as a sole means of protection against direct contact (innormal service) but may be used to augment one of the means set out in Clause1.7.3.2.

Protection against indirect contact

RCDs are recognized as a means of providing automatic disconnection of supply inaccordance with Clause 1.7.4.3.

Definition of a residual current device

AS3000/2000 defines a residual current device as a device intended to isolate

supply to protected circuits, socket-outlets or electrical equipment in the event of acurrent flow to earth which exceeds a predetermined value.

Purpose of an RCD

The purpose of an RCD to protect the person if the unit is to detect an earth faultcurrant (Residual current) and to automatically disconnect the circuit from the supplywhen it exceeds a specified or predetermined value (sensitivity) within a set timelimit.

Figure 7: Fault path


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Figure 8: RCDThe standard trip sensitivity of an RCD is set at 30 milliamps which will give a victim

a severe electrical shock however the trip time is so fast that the chance of the heartgoing into ventricular ventilation is negligible.

Operation principle of an RCD

When the load circuit is faultless the current through the active and neutralconductors (primaries) in the core are equal in magnitude but opposite in direction.

The magnetic flux set up by the two conductors (primaries) neutralise each other,thus no emf is induced in the secondary winding.


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For more information contact:

Austral ian Flexible Learning Framework

Phone: (07) 3307 4700

Fax: (07) 3259 4371

Email: [email protected]

Website: flexiblelearning.net.au

GPO Box 1326

Brisbane QLD 4001

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Physiological Effects V2