1. 2 Electrical safety Content Physiological Effects of Electricity Basic Electrical Theory Human...

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Electrical safety

Content

• Physiological Effects of Electricity

• Basic Electrical Theory

• Human susceptibility to electric shock

• Codes and Standards.

• Equipment Design

• Electrical Safety Testing Procedure

• Specific Hazards & Personal Safety

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Introduction

Everyone should have some knowledge of the every day source of energy known as electricity

Everyone should also know about the potential hazards & how to avoid them

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Are you aware• Electrocutions are the 5th leading

cause of Accidental death in the U.S.

• More than 700 people lose their lives every year because of accidents associated with electricity and electrical products

• 40,000 residential electrical fires occur annually.

• More than $2 billion is lost on property damage.

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• Our main objectives are to

• understand the possible electrical

hazards and fault scenarios and

• learn how to improve deigns of

medical instruments.

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Cautions

• It would appear that the young & old are more prone to electric shock

• Perhaps this is due in the first instance due to a lack of appreciation of the dangers

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Electricity in the body

• Muscles

– Muscles control all the body movements

Including & importantly those that keep us alive

- Breathing and Heart

– The brain controls voluntary muscles using Current pulses along nerves

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External current through the body

• Many people have received an electric shock at some stage or other through carelessness

• Main effects:– Neuromuscular (stimulation of nerves & muscles)

-- Loss of muscle control

– Spasms & Involuntary movement

– Inability to let go

– Burns

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External current through the body

• external & internal Muscle cramps

• Respiratory arrest

• Ventricular fibrillation

(50 microamps can interfere with cardiac cycle)

• Electrolysis

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Basic Electrical Theory

• Voltage [driving force] causes current [e - ] to flow• AC / DC - from safety perspective - negligible difference

• Single Phase / Three Phase.

• Circuit / loop is necessary for current to flow

a start point - a route - an end point

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Voltage, Current and Resistance

• Voltage increases => Current increases • Resistance decreases => Current increases

Voltage = Current / Resistance - Ohms Law

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The complete circuit A complete Circuit or loop is

necessary for current to flow

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A complete circuit complete Circuit or loop

is necessary for current to flow

Current takes the path of least resistance

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Hospital Safety• Now days the patient environment contains a large

amount of medical equipment providing various functions to aid the care of patients

• Many items of medical equipment may be connected to the patient at the same time

• The hazards are greater due to the involvement of sick people

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Human susceptibility to electric shock

• Current• Ferequency• Body and skin Resistances• Duration• Body weight• Point- of- entry

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Physiological Response to Electricity• Threshold of Perception 1 mA• Let - go – Current 5 mA• Muscles Contraction 10 mA• Pain and respiratoryParalysis 50 mA • Ventricular Fibrilation 100-300 mA• Burns 1 A

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Current vs. Frequency• Difference in DC vs AC shocks

• Very high threshold/low sensitivity at high frequencies used in many medical applications

• Diathermy F = 27,12 MHz

T = 10 -6 s• Nerve communication T =10-5

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Duration/Body Weight Susceptibility

• Strength-duration curve is a very important plot, applicable to shocks, pacemaker/defibrillator type devices, etc.

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Micro- vs. Macroshock Hazards Points of Entry

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Safety Standards

• Modern medical equipment is now manufactured in such a way that the risks of electric shock are minimal

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The ‘Liverpool’ Safety Tester 1973

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Hazards• Water & electricity• Overloaded circuits• Damaged insulation• Misuse of extension cables• Exposed wiring• Power cords in walkways

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Precautions

• If equipment gets wet, get it checked

• If cabling damaged /exposed, get it checked

• Do not replace fuses on medical equipment

• If equipment damaged, get it checked

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Fuses/Circuit Breakers– The Fuse is very thin piece of wire.

• The wire has a quite low melting point. As current flows through the wire it heats up.

• If too large a current flows it melts, thus breaking the circuit

• Use appropriate fuse size/rating

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How Can Users Assist?

• Do NOT carry out any repairs yourself

• Check equipment for damage before use

• Ensure there is no damage to mains plug

• Ensure there is no damage to mains cable

• Do understand how to use the equipment safely

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Electrical cables & plugs Mains cable

– Brown Live - power

– Blue Neutral – Green/yellow Earth

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Electrical cables & plugs

Mains cable

– Brown Live power

– Blue Neutral – Green/yellow Earth

L

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Can you spot the problems?

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What’s the problem?

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Summary

Your responsibility to take care of

yourself and others