electrical safety training

Basics of Electrical Safety TestingElectrical Safety Testing of Medical Equipments

Mehaboob RahmanBarq Consulting EngineersHealthcare Technology Management, KFMC

Mehaboob Rahman

TABLE OF CONTENTS

1 Introduction

2Hazards

Common Hazards on Medical Equipments

3Basics of Electrical Safety

Physiological Effects of Electricity on the Body

4

Electrical Safety TestingWhy do we need Electrical Safety Test

Terminologies of EST

Class of Medical Equipments

Types of Medical Equipments

5International Electrotechnical commission

IEC 60601

IEC 62353

6

Electrical Safety Testing ProcedureHow Testing is performed

Documentation

Visual Inspection

Earth Resistance Test

Insulation Test

Leakage Current Test

7 Conclusion

2 | P a g e E l e c t r i c a l s a f e t y t e s ti n g o f M e d i c a l E q u i p m e n t s

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Introduction

Medical technology has substantially improved health care in all medical specialties and has reduced morbidity and mortality for critically ill patients.

However, the increased complexity of medical devices and their utilization in more procedures result in about 10,000 device- related patient injuries in USA.

Most of these injuries are attributable to improper use of devices as a result of inadequate training and lack of experience. Medical personnel rarely read user manuals until a problem has occurred. Furthermore, medical devices eventually fail, so engineers must develop fail-safe designs.


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HAZARDSA hazard is any biological, chemical, mechanical, environmental or physical agent that is reasonably likely to cause harm or damage to humans, other organisms, or the environment in the absence of its control.

Most hazards are dormant or potential, with only a theoretical risk of harm; however, once a hazard becomes "active", it can create an emergency. A hazardous situation that has come to pass is called an incident. Hazard and possibility interact together to create risk

Hazards on Medical Equipments

Medical electrical equipment can present a range of hazards to the patient, the user, or to service personnel.Many such hazards are common to many or all types of medical electrical equipment, whilst others are peculiar to particular categories of equipment. The root causes for injures involving medical equipment include Human Error, Faulty Equipment Design & Poor Maintenance. However, It is unwise to assume anything until a through investigation is made and failure analysis is performed on the equipment.

Common Hazards of Medical Equipments.

Mechanical Hazards

All types of medical electrical equipment can present mechanical hazards.These can range from insecure fittings of controls to loose fixings of wheels on equipment trolleys. The former may prevent a piece of life supporting equipment from being operated properly, whilst the latter could cause serious accidents in the clinical environment.

The EnclosureThe enclosure of the device must be sufficiently strong to retain its integrity under conditions of normal wear and tearHandles of portable equipment are tested with a force of four times the weight of the product. If there is more than one handle, this weight is distributed between the handles.

Moving Parts Moving parts which could produce a safety hazard must be suitable guarded to prevent access, unless exposure is essential to the operation of the equipment.If movement of the equipment, or parts of the equipment can cause injury to the patient, this movement can only be achieved by continuous operation of the control by the operator.Any electrically controlled mechanical movement must have an emergency switch.

Sharp Edges - The device must not have sharp edges, corners, etc.

Stability - Medical devices must not overbalance when tilted to an angle of 10°.


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Risk of Fire or Explosion

All mains powered electrical equipment can present the risk of fire in the event of certain faults occurring such as internal or external short circuits. In certain environments such fires may cause explosions. Although the use of explosive anesthetic gases is not common today, it should be recognized that many of the medical gases in use vigorously support combustion.

Medical devices typically contain a number of electro-mechanical and chemical systems and power sources. Power can be supplied to an actuating mechanism, or fluids and gases can be handled through compression, dispersion or valving. The devices typically contain items that include foamed padding and/or structural plastics. All of these things in combination present an energy source for ignition, fuel and oxidizer – good conditions for fire ignition and propagation.

Absence of Function

Since many pieces of medical electrical equipment are life supporting or monitor vital functions, the absence of function of such a piece of equipment would not be merely inconvenient, but could threaten life

This recommend the use of proper test equipments to verify the correct operation of the equipment.

Excessive or insufficient output

In order to perform its desired function equipment must deliver its specified output. Too high an output, for example, in the case of surgical diathermy units, would clearly be hazardous. Equally, too low an output would result in inadequate therapy, which in turn may delay patient recovery, cause patient injury or even death.This highlights the importance of correct calibration procedures

Infection

Medical equipment that has been inadequately decontaminated after use may cause infection through the transmission of microorganisms to any person who subsequently comes into contact with it.

Clearly, patients, nursing staff and service personnel are potentially at risk here.

Misuse5 | P a g e E l e c t r i c a l s a f e t y t e s ti n g o f M e d i c a l E q u i p m e n t s

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Misuse of equipment is one of the most common causes of adverse incidents involving medical devices. Such misuse may be a result of inadequate user training or of poor user instructions. Do not modify or alter devices, unless in the instructions for use it is clear that the manufacturer sanctions the modification or alteration.

Radiation Hazard

The medical use of ionizing radiations, whether for diagnosis or therapy, not only results in the irradiation of the patient but may also result in some degree of exposure of radiologists, radiographers, other workers of the department.

Although many patients benefit from radiation’s ability to destroy cancer cells or capture real-time images of the human body, radiation can harm healthy cells wherever it enters the body. It is well documented that ionizing radiation can cause damage ranging from uncontrollable cell replication to cell death.

Risk of exposure to spurious electric currents

All electrical equipment has the potential to expose people to the risk of spurious electric currents. In the case of medical electrical equipment, the risk is potentially greater since patients are intentionally connected to such equipment and may not benefit from the same natural protection factors that apply to people in other circumstances.

Whilst all of the hazards listed are important, the prevention of many of them require methods peculiar to the particular type of equipment under consideration. For example, in order to avoid the risk of excessive output of surgical diathermy units, knowledge of radio frequency power measurement techniques is required.

However, the electrical hazards are common to all types of medical electrical equipment and can minimized by the use of safety testing regimes which can be applied to all types of medical electrical equipment.


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BASICS OF ELECTRICAL SAFETY

Electrical Safety

Electrical safety is very important in hospitals as patients may be undergoing a diagnostic or treatment procedure where the protective effect of dry skin is reduced. Also patients may be unattended, unconscious or anaesthetized and may not respond normally to an electric current. Further, electrically conductive solutions, such as blood and saline, are often present in patient treatment areas and may drip or spill on electrical equipment.

Electric Current

Injuries received from electric current are dependent on the magnitude of current, the pathway that it takes through the body and the time for which it flows.

The nature of electricity flowing through a circuit is analogous to blood flowing through the circulatory system within the human body. In this analogy the source of energy is represented by the heart, and the blood flowing through arteries and veins is analogous to current flowing through the conductors and other components of the electric circuit.

The application of an electric potential to an electric circuit generates a flow of current through conductive pathways. This is analogous to the changes in blood pressure caused by contraction of cardiac muscle that causes blood to flow into the circulatory system. For electric current to flow there must be a continuous pathway from the source of potential through electrical components and back to the source.

Physiological Effects of Electricity on the Body

What will happen when current flows through biological tissue?

Human body can easily bear electrical current of 1 milliampere passing through its body without appreciable risk or damage. However, as the amount of current increases the body may suffer different type of damages like. Fibrillation, Burns to parts of the body due to heat generated by electricity, Damage to nervous system causing loss of nervous control.

When the current passes through brain it can lead to unconsciousness and permanent damage to the brain. including death or electrocution

The physiological effects of electrical shock include the following.

Electrolysis (mainly near d.c.) Neuromuscular effects (mainly 10-100Hz) Heating (mainly 100KHz-30Mhz)


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Electrolysis

The movement of ions of opposite polarities in opposite directions through a medium is called electrolysis.

Electrolysis can be made to occur by passing DC current through body tissues or fluids.

If a DC current is passed through body tissues for a period of minutes, ulceration begins to occur. Such ulcers, while not normally fatal, can be painful and take long periods to heal.

The formation of sodium atoms at the negative electrode and chlorine atoms at the positive electrode causes local chemic al actions which kills the cells.

Neuromuscular Effects

Macroshock

Macroshock is the most common type of shock received and occurs when the human body becomes a conductor of electric current passing by means other than directly through the heart. This effect can readily occur with the use of medical electrical equipment as the natural resistance of the skin to current flow is often reduced or bypassed by electrodes and electorde paste or by invasion into mucous membrane.

Large current passing through the skin - a small proportion may pass through the heart

Macroshock has the potential for both burns and cardiac arrhythmias. Currents pass through the extremities mostly through the muscles. A current flowing from arm to arm, or arm to leg, must pass through the thorax. In the thorax the current is split between the chest wall and the great vessels, which obviously deliver the current directly to the myocardium.


Na+Na+Na+Cl- Cl- Cl-

_ +

Sodium atomscreated at electrode

Chlorine atomscreated at electrode

Ionic Current

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Microshock

Microshock is an electric shock risk that is present for hospital patients with externally protruding intracardiac electrical conductors, such as external pacemaker electrodes, or saline filled catheters

Microshock refers to currents delivered directly to the heart via intracardiac electrodes or catheters. Because the current is delivered to a very small area, only a very small current is required to reach the fibrillation threshold.

Micro-shock" is an otherwise imperceptible electric current applied directly, or in very close proximity, to the heart muscle of sufficient strength, frequency, and duration to cause disruption of normal cardiac function.

The currently accepted minimum current is 10 mA (microamps = 1/1000 of milliamps

Effect of frequency on neuro-muscular stimulation

The amount of current required to stimulate muscles is dependent on some range of frequency.

Referring to figure 1, it can be seen that the smallest current required to prevent the release of an electrically live object occurs at a frequency of around 50 Hz. Above 10 kHz the neuro-muscular response to current decreases almost exponentially.


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Burns

When an electric current passes through any substance having electrical resistance, heat is produced. The amount of heat depends on the power dissipated (I2R).

Electrical burns often produce their most marked effects near to the skin.

Muscle cramps

When an electrical stimulus is applied to a motor nerve or a muscle, it contracts. The prolonged involuntary contraction of muscles (tetanus) caused by an external electrical stimulus is responsible for the phenomenon where a person who is holding an electrically live object can be unable to let go (at frequency of 50 Hz, e.g. power supply).

Respiratory arrestThe muscles between the ribs (intercostal muscles) need to repeatedly contract and relax in order to facilitate breathing. Prolonged tetanus of these muscles can therefore prevent breathing.

Cardiac arrest

The heart is a muscular organ, which needs to be able to contract and relax repetitively in order to perform its function as a pump for the blood. Tetanus of the heart musculature will prevent the pumping process.


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Ventricular fibrillation

Ventricular fibrillation can be triggered by very small electrical stimuli.

A current as low as 70 mA flowing from hand to hand across the chest, or 20 µA directly through the heart may be sufficient.

It is for this reason that most deaths from electric shock are attributable to the occurrence of ventricular fibrillation.

Natural protection factors

Reflex and automatic contraction of muscles on receiving an electrical stimulus often acts to disconnect the person from the source of the stimulus.

A patient under anesthesia is relatively unprotected by these reflex mechanisms.

Normally, the skin has a high electrical resistance compared to the moist body tissues below, and hence serves to reduce the amount of current that would otherwise flow.

A patient skin resistance may have been lowered in order to allow good connections of monitoring electrodes to be made or, in the case of a patient undergoing surgery, there may be no skin present in the current path.

The absence of natural protection factors as described above highlights the need for stringent electrical safety specifications for medical electrical equipment and for routine test and inspection regimes aimed at verifying electrical safety


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ELECTRICAL SAFETY TESTING

Why do we do electrical safety .?

√ Ensure patient safety

Protect against macroshock

Protect against microshock

√ Test for electrical internal breakdown / damage to power cord, AC mains feed,

etc.

√ Meet codes & standards

AAMI, IEC, UL, NFPA, etc.

√ Protect against legal liability

In case of a patient incident

Terminologies of EST

Classes and Types L1 - Hot L2 - Neutral Earth - Ground Mains Line - Voltage Enclosure/Case - Chassis Protective Earth -Ground Wire Earth Leakage Current Leakage in Ground Wire Enclosure Leakage - Chassis Leakage Patient Leakage - Lead Leakage Patient Auxiliary - Leakage between Patient Leads Mains on Applied Parts - Lead Isolation Insulation Resistance - Dielectric Strength or Insulation Resistance between Hot and

Neutral to Ground Earth Resistance - Ground Wire Resistance


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Applied Parts -

A part of the equipment which in normal use:

Necessarily comes into physical contact with the patient for the equipment to perform its function; or can be brought into contact with the patient; or needs to be touched by the patient

Accessible Part

Part of equipment which can be touched without the use of a tool.

• EXAMPLE 1: Illuminated push-buttons

• EXAMPLE 2: Indicator lamps

• EXAMPLE 3: Recorder pens

• EXAMPLE 4 : Parts of plug-in modules

• EXAMPLE 5: Batteries

Leakage currents

Current that is not functional.

Several different leakage currents are defined according to the paths that the currents take.

• Earth Leakage Current

• Enclosure Leakage Current

• Patient Leakage Current

• Patient auxiliary current

Causes of Leakage currents

If any conductor is raised to a potential above that of earth, some current is bound to flow from that

conductor to earth. The amount of current that flows depends on:

1- the voltage on the conductor.

2- the capacitive reactance between the conductor and earth.

3-the resistance between the conductor and earth.


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DRIFT

As components age and equipment undergoes changes in temperature or humidity or sustains mechanical stress, performance gradually degrades. This is called drift. When this happens your test results become unreliable and both design and performance quality suffers.

While drift cannot be eliminated, it can be detected and either corrected or compensated for through the process of calibrationLeakage Current

Calibration: process of comparing an unknown against a reference standard within defined limits, accuracies and Uncertainties

Verification: process of comparing an unknown against a reference standard at usually one

MEDICAL EQUIPMENT CLASS & TYPE

Equipment Class I, II, III - method of protection against electric shock

Equipment Type B, BF, CF - degree of protection

CLASSES MEDICAL ELECTRICAL EQUIPMENT

All electrical equipment is categorised into classes according to the method of protection against electric shock that is used. For mains powered electrical equipment there are usually two levels of protection used, called "basic" and "supplementary" protection. The supplementary protection is intended to come into play in the event of failure of the basic protection.

CLASS I - (Protection relying on fault currents to Earth)

Class I equipment has a protective earth. The basic means of protection is the insulation between live parts and exposed conductive parts such as the metal enclosure. In the event of a fault that would otherwise cause an exposed conductive part to become live, the supplementary protection (i.e. the protective earth) comes into effect.

Class I equipment is fitted with a three core mains cable containing a protective earth wire. Exposed metal parts on class I equipment are connected to this earth wire

Fault Current means the electrical current that flows through a circuit during an electrical fault condition. A fault condition occurs when one or more electrical conductors contact ground and/or each other. Types of faults include phase to ground, double-phase to ground, three-phase to ground, phase-to-phase, and three-phase. A Fault Current is several times larger in magnitude


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than the current that normally flows through a circuit.

Class I medical electrical equipment should have fuses at the equipment end of the mains supply lead in both the live and neutral conductors, so that the supplementary protection is operative when the equipment is connected to an incorrectly wired socket outlet. Further confusion can arise due to the use of plastic laminates for finishing equipment. A case that appears to be plastic does not necessarily indicate that the equipment is not class I. There is no agreed symbol in use to indicate that equipment is class I. Where any doubt exists, reference should be made to equipment manuals. The symbols below may be seen on medical electrical equipment adjacent to terminals.

CLASS II

The method of protection against electric shock in the case of class II equipment is either double insulation or reinforced insulation. In double insulated equipment the basic protection is afforded by the first layer of insulation. If the basic protection fails then supplementary protection is provided by a second layer of insulation preventing contact with live parts.

Reinforced insulation is defined in standards as being a single layer of insulation offering the same degree of protection as double insulation.

Class II medical electrical equipment should be fused at the equipment end of the supply lead in either mains conductor or in both conductors if the equipment has a functional earth.

The symbol for class II equipment is two concentric squares indicating double insulation as shown.

CLASS III

Class III equipment is defined in some equipment standards as that in which protection against electric shock relies on the fact that no voltages higher than safety extra low voltage (SELV) are present. SELV is defined in turn in the relevant standard as a voltage not exceeding 25V ac or 60V dc. In practice such equipment is either battery operated or supplied by a SELV transformer.


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If battery operated equipment is capable of being operated when connected to the mains (for example, for battery charging) then it must be safety tested as either class I or class II equipment. Similarly, equipment powered from a SELV transformer should be tested in conjunction with the transformer as class I or class II equipment as appropriate.

It is interesting to note that the current IEC standards relating to safety of medical electrical equipment do not recognise Class III equipment since limitation of voltage is not deemed sufficient to ensure safety of the patient. All medical electrical equipment that is capable of mains connection must be classified as class I or class II. Medical electrical equipment having no mains connection is simply referred to as "internally powered".

TYPES OF MEDICAL EQUIPMENTS

Each classification has differing requirements from the point of view of Protection against electrical shock

As described above, the class of equipment defines the method of protection against electric shock. The degree of protection for medical electrical equipment is defined by the type designation. The reason for the existence of type designations is that different pieces of medical electrical equipment have different areas of application and therefore different electrical safety requirements. For example, it would not be necessary to make a particular piece medical electrical equipment safe enough for direct cardiac connection if there is no possibility of this situation arising.

All medical electrical equipment should be marked by the manufacturer with one of the type symbols.

Type Symbol Definition

B

Equipment providing a particular degree of protection against electric shock, particularly regarding allowable leakage currents and reliability of the protective earth connection (if present).

BFAs type B but with isolated or floating (F - type) appliedpart or parts.

CF

Equipment providing a higher degree of protection against electric shock than type BF, particularly with regard to allowable leakage currents, and having floating applied parts.


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INTERNATIONAL ELECTROTECHNICAL COMMISSION

The International Electrotechnical commission (IEC) is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electro technology".

IEC 60601-1

IEC 60601, “Medical Electrical Equipment—General Requirements for Safety” was introduced in 1977 and put forth a set of requirements for the manufacturers of medical equipment. These requirements were designed to detect and eradicate any potential electric hazards presented by the equipment being produced (e.g. leakage currents, protective grounding, etc.). Assuming the equipment was utilized properly for the duration of its lifespan, these tests were meant to curb possible defects later on. They were further utilized on equipment already in service as a means of routine tests and after repair tests; however, this practice presented some unforeseen difficulties. For example, IEC 60601 outlines type-testing in laboratory conditions, but often those conditions are not available or applicable once the device is already in use

60601 is a widely accepted benchmark for medical electrical equipment and compliance with IEC60601-1 has become a requirement for the commercialization of electrical medical equipment in many countries. Many companies view compliance with IEC 60601-1 as a requirement for most markets

• Design and Type-Test standard (Mandatory)• Electrical and Mechanical Safety and Compliance of Medical Electronic Equipment• Ensure Safety of Patient / User and Environment

IEC 60601-1 (type) Tests

• Earth Bond Test (high current)• Insulation Test• Leakage Tests (SFC’s)

o Earth o Enclosure / Toucho Patient (AC / DC) o Patient Auxiliary (AC / DC)o Patient type F

Warning : These tests are aimed to stress the equipment and can be destructiveIEC 62353


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IEC 62353, “Medical Electrical Equipment—Recurrent Test and Test After Repair of Medical Electrical Equipment” emerged to accommodate the needs of in-service devices.

IEC 62353 is specifically designed around testing equipment in the field, and is therefore more practical, more effective, and safer than using IEC 60601 in those particular circumstances.

• World wide validation of the new standard the 16 march 2007. • Has been published, world wide, the 15th of May 2007.• Has been published in France in March 2008, Italy in early 2009. Etc...

IEC 62353 Application area

It’s a standard designed for the field:

• Applicable to all the Medical devices and Systems, before the first use, during the maintenance, inspection, regular tests and tests after repair.

• Not Mandatory• Not a replacement of IEC 60601-1-1• Not a replacement of Local Standard• Does not describe Mechanical Safety • Does not describe Electro Magnetic Compatibility

IEC 62353 Tests

• Earth Bond Test• Insulation Test• Leakage Test

Equipement Direct, 2 modes (OE, ROE) Alternative, 1 mode Differential, 2 modes (Nor,Rev) Applied Part Direct 2 modes (Nor, Rev) Alternative


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How is testing performed?

Before testing, accompanying documentation must be examined, and accordingly, manufacturer recommendations of maintenance and repair taken into account. Whenever and wherever possible, the device must be disconnected from the mains supply power; otherwise, special measures must be implemented for the prevention of hazards resulting from working on live devices.

Documentation

All tests carried out must be documented in depth. Testing documents must at the very least contain the following entries:

· Designation of the test location (e.g. company, department, authority) · Name of the person(s) who performed and evaluated the test· Designation of the tested device and accessories (e.g. type, serial number, inventory

number)


Electrical Safety Testing ProcedureVisual Inspection

Earth Resistance Test

Insulation Test

Leakage Current Test

Earth Leakage Current

Touch Current Patient Leakage Current

One of the main differences is in ground bond testing.

IEC 62353 proposes a minimum test current of 200 mA instead of the 25 A required in IEC 60601-1.

This means that, provided sufficient consideration is given to potential contact resistance, test equipment can be smaller and more lightweight compared with current practices

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· Executed tests including measured values, measuring methods and utilized measuring instruments

· Function test · Final evaluation · Date and ignature of the person who prepared the evaluation · Identification of the tested device (if required by the operating service provider)

The methods of measurement used before initial start-up and the results of those measurements should be documented for the purpose of comparison with the results of later measurements. This comparison is recommended if the measured value amounts to more than 90% of the limit value. With regard to systems, initial start-up testing must be performed every time the system is altered (such as modified configuration or replacement of components), and the changes and new measurements must be documented as well.

Visual Inspection

This is a fairly easy and very effective portion of the procedure; the human eye must not be forgotten as a crucial tool that an operator of testing has available. The legibility of safety-relevant labeling is inspected, as well as the device’s compliance with the manufacturer’s specifications.

Protective Conductor Resistance Measurement

Protective conductor resistance measurements are performed on Class I devices to ensure that all accessible conductive parts (which, in the event of a fault, may become live) are appropriately secured to the protective conductor terminal. All devices must conform to the following protective conductor connection limit values:


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The resistance of the protective earth conductor is measured between the earth pin on the mains plug and a protectively earthed point on the equipment enclosure (see figure 6). The reading should not normally exceed 0.2Ω at any such point.

Devices with a removable mains power cable (measurement without mains power cable) 0.2 Ω

Devices including mains power cable 0.3 Ω

Mains power cable (all available mains power cables are tested) 0.1 Ω

Systems with multiple electrical outlets 0.5 Ω

Connector cables such as data transmission lines and functional earth cables may simulate protective conductor connections, and should be disconnected if possible before testing is started.

The disconnection of protective conductors is not called for with permanently connected devices.

Insulation TestsInsulation resistance is a helpful measurement to find insulation faults caused by dust, wetness or pollution but the measurement may be forbidden by some manufacturers to avoid damage on sensitive parts. Furthermore, there is no limit value specified in IEC 62353, but the following values can serve as a reliable guideline:

HEI 95 and DB9801 recommended that for class I equipment the insulation resistance be measured at the mains plug between the live and neutral pins connected together and the earth pin. Whereas HEI 95 recommended using a 500V DC insulation tester, DB 9801 recommended the use of 350V DC as the test voltage.

HEI 95 further recommended for class II equipment that the insulation resistance be measured between all applied parts connected together and any accessible conductive parts of the equipment. The value should not normally be less than 50MΩ


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Leakage Current Measurement

Leakage current measurement requirements only apply universally to AC components; DC leakage current measurements must be determined and explicitly expounded upon in accompanying documentation by manufacturers (complying also with IEC 60601 DC limit values).

The measured value must be corrected to the value which corresponds to the measurement at nominal line voltage.

Current that is not functional. several different leakage currents are defined according to the paths that the currents take. Earth Leakage Current Enclosure Leakage Current Patient Leakage Current Patient auxiliary current

The following leakage currents are measured:

EARTH LEAKAGE CURRENTcurrent flowing from the MAINS PART through or across the insulation into the PROTECTIVE EARTH CONDUCTOR

Under normal conditions, a person who is in contact with the earthed metal enclosure of the equipment and with another earthed object would suffer no adverse effects even if a fairly large earth leakage current were to flow. This is because the impedance to earth from the enclosure is much lower through the protective earth conductor than it is through the person. However, if the protective earth conductor becomes open circuited, then the situation changes. Now, if the impedance between the transformer primary and the enclosure is of the same order of magnitude as the impedance between the enclosure and earth through the person, a shock hazard exists.


Protection class I(LN to PE)

Protection class II(LN to accessible conductive part or type BF application part)

(LN to type CF application part)

2 MΩ 7 MΩ 70 MΩ

http://www.601help.com/Disclaimer/glossary.html



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Enclosure leakage current /Touch current

LEAKAGE CURRENT flowing from the ENCLOSURE to earth or to another part of the ENCLOSURE through a conductor other than the protective earth conductor.

Patient leakage current

Patient leakage current is the leakage current that flows through a patient connected to an applied part or parts.

It can either flow from the applied parts via the patient to earth or from an external source of high potential via the patient and the applied parts to earth.

Patient auxiliary current

The patient auxiliary current is defined as the current that normally flows between parts of the applied part through the patient, which is not intended to produce a physiological effect


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Device LeakageCurrent

Device Leakage Current (Continued)

Device leakage current is the sum of all possible leakage currents whichcould flow over the user or the patient in the event of an interrupted protective earth conductor. (For this reason, current in the protective conductor, as well as from the application parts and accessible conductive parts, must be acquired during measurement.)In the IEC 60601-1 standard, this measurement corresponds to earth leakage current with grounded application parts and housing components.In the case of protection class II devices, current corresponds to contact current.This leakage current will be designated housing leakage current in the second edition of IEC 60601-1.


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Leakage Currentfrom the Application Part

Testing is only performed on type F application parts.(Testing is usually not required for type B application parts, because it’s included in device leakage current. However, the manufacturer may require an additional leakage current measurement for type B application parts.) Depending upon device layout, testing can be performed by means of direct measurement (mains to application part) or alternative measurement.If alternative measurement is used, test voltage equal to nominal line voltage is applied between the application part to be measured and all mains power cables which are connected to each other (L, N and PE).In the case of direct measurement, test voltage equal to nominal linevoltage is applied between the application part to be measured and PE while the test object is being supplied with power from the mains.Application parts of identical type can be connected to each other during measurement, or the manufacturer’s instructions must be followed. If different application parts are included, they must be connected and measured individually, one after the other. Application parts which are not included in the measurement are not connected.This leakage current will also be designated “patient leakage current withsingle fault condition mains on application part in IEC 60601.

On some equipment it is required to measure patient leakage current on Type B applicatrion parts according to IEC 60601. This current is measured from the application part to ground and DC components will also be taken into consideration.

Permissible Values for Leakage Current Measurements:

Device Leakage Current

Direct or differential measurement: alternative measurement:

At protectionclass I parts0.5 mA1.0 mA

At protectionclass II parts0.1 mA0.5 mA

Leakage Current from the Application PartType BF: Type CF:

5.0 mA0.05 mA

Patient leakage current according to IEC 60601 Type B, BF or CF (normal condition) 0.1 mA


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Because cables and wiring (such as mains power cables, measurement cables, and data transmission lines) have an enormous influence on the leakage current measurements, they must be arranged in such a way as to minimize their interference. Permanently connected devices need not be tested for leakage current measurements if the location is in accordance with IEC 60364-7-710 (“Medical locations”), and is regular tested according to this standard.

The following table summarizes the leakage current limits (in mA) specified by IEC60601-1 (second edition)

There are three methods to choose from for measuring leakage current; selection of which should be based on the design of the device:

Alternativemethod

Cannot be used for devices for which insulation in the power pack is notincluded in the measurement (e.g. due to a relay which is only closed in the operating state).If the measured value resulting from equivalent measurement exceeds 5 mA

Direct method

Cannot be used in IG (Isolated Ground) systems.This method may not be used if the device under test cannot be isolated from ground.The protective conductor is interrupted when this measuring method is used, for which reason it’s essential not to come into contact with accessible conductive parts during testing, because danger of electrical shock wouldotherwise exist.


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DifferentialCurrent method

Cannot be used in IG systems.Measuring instrument specifications must be observed when measuring small leakage currents with this method. As a rule, the method is only conditionally suitable for current values of less than 100 µA.

Function Test

Function tests regarding safety will be specified by the manufacturer, and should be executed accordingly.

Function tests also fall under “essential performance characteristics” and “special requirements” of IEC 60601. Most of the time, additional test instruments will be required (such as with infusion pumps and defibrillators).

Restoration to Operable State

Once testing is complete, the device must be restored to an operable state.

This includes reconnection of power cables, data transmission lines and alarm devices, and general setup, etc., so that the device is in the same state as it was prior to the testing.


Frequency of Safety Tests

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Conclusion

The Electrical Safety Tests and the standards are designed to protect us from the natural proclivity of complex systems to eventually go awry.

Adherence to these is a crucial element in the operation of these electrical medical devices. At the end of the day, all of this testing and these standards are not about product quality or thoughtless conformity to mandated procedures; what it’s really about is what the whole medical industry is about: The value of human health and safety.
