医疗器械漏电流测试示意图

Safety Testing of Medical Electrical Equipment
1 Hazards of Medical Electrical Equipment
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. Listed below are various types of common hazards.
1.1 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.
Such hazards may seem too obvious to warrant mentioning, but it is unfortunately all too common for such mundane problems to be overlooked while more exotic problems are addressed.
1.2 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 anaesthetic gases is not common today, it should be recognised that many of the medical gases in use vigorously support combustion.
1.3 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.
1.4 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.
1.5 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.
1.6 Misuse
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.
1.7 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 minimised by the use of safety testing regimes which can be applied to all types of medical electrical equipment. For these reasons, it is the electrical hazards that are the main topic of this session.
2 Physiological effects of electricity
2.1 Electrolysis
The movement of ions of opposite polarities in opposite directions through a medium is called electrolysis and 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.
2.2 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). Whether or not the heat produces a burn depends on the current density.
Human tissue is capable of carrying electric current quite successfully. Skin normally has a fairly high electrical resistance while the moist tissue underneath the skin has a much lower resistance. Electrical burns often produce their most marked effects near to the skin, although it is fairly common for internal
electrical burns to be produced, which, if not fatal, can cause long lasting and painful injury.
2.3 Muscle cramps
When an electrical stimulus is applied to a motor nerve or a muscle, the muscle does exactly what it is designed to do in the presence
of such a stimulus i.e. 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.
2.4 Respiratory arrest
The 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.
2.5 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.
2.6 Ventricular fibrillation
The ventricles of the heart are the chambers responsible for pumping blood out of the heart. When the heart is in ventricular fibrillation, the musculature of the ventricles undergoes irregular, uncoordinated
twitching resulting in no net blood flow. The condition proves
fatal if not corrected in a very short space of time.
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.
2.7 Effect of frequency on neuro-muscular stimulation
The amount of current required to stimulate muscles is dependent to
some extent on 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.
Figure 1. Current required to prevent release of a live object.
2.8 Natural protection factors
Many people have received electric shocks from mains potentials and
above and lived to tell the tale. Part of the reason for this is the existence of certain natural protection factors.
Ordinarily, a person subject to an unexpected electrical stimulus is protected to some extent by automatic and intentional reflex actions.
The automatic contraction of muscles on receiving an electrical
stimulus often acts to disconnect the person from the source of the stimulus. Intentional reactions of the person receiving the shock
normally serve the same purpose. It is important to realise that a
patient in the clinical environment who may have electrical equipment
intentionally connected to them and may also be anaesthetised are relatively unprotected by these mechanisms.
Normally, a person who is subject to an electric shock receives the shock through the skin, which 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. Again, a patient does not necessarily enjoy the same degree of protection. The resistance of the skin may intentionally 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.
3 Leakage currents
3.1 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. This is true even of conductors that are well insulated from earth, since there is no such thing as perfect insulation or infinite impedance. The amount of current that flows depends on:
a.the voltage on the conductor.
b.the capacitive reactance between the conductor and earth.
c.the resistance between the conductor and earth.
The currents that flow from or between conductors that are insulated from earth and from each other are called leakage currents, and are normally small. However, since the amount of current required to produce adverse physiological effects is also small, such currents must be limited by the design of equipment to safe values.
For medical electrical equipment, several different leakage
currents are defined according to the paths that the currents take.
3.2 Earth leakage current
Earth leakage current is the current that normally flows in the earth conductor of a protectively earthed piece of equipment. In medical electrical equipment, very often, the mains is connected to a transformer having an earthed screen. Most of the earth leakage current finds its way to earth via the impedance of the insulation between the transformer primary and the inter-winding screen, since this is the point at which the insulation impedance is at its lowest (see figure 2).
Figure 2. Earth leakage current path.
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, then a shock hazard exists.
It is a fundamental safety requirement that in the event of a single fault occurring, such as the earth becoming open circuit, no hazard should exist. It is clear that in order for this to be the case in the above example, the impedance between the transformer primary and the enclosure needs to be high. This would be evidenced when the
equipment is in the normal condition by a low earth leakage current. In other words, if the earth leakage current is low then the risk
of electric shock in the event of a fault is reduced.
3.3 Enclosure leakage current
Enclosure leakage current is defined as the current that flows from an exposed conductive part of the enclosure to earth through a conductor other than the protective earth conductor. However, if a protective earth conductor is connected to the enclosure, there is little point in attempting to measure the enclosure leakage current from another protectively earthed point on the enclosure since any measuring device used is effectively shorted out by the low resistance of the protective earth. Equally, there is little point in measuring the enclosure leakage current from a protectively earthed point on the enclosure with the protective earth open circuit, since this would give the same reading as measurement of earth leakage current as described above. For these reasons, it is usual when testing medical electrical equipment to measure enclosure leakage current from points on the enclosure that are not intended to be protectively earthed (see figure 3). On many pieces of equipment, no such points exist. This is not a problem. The test is included in
test regimes to cover the eventuality where such points do exist and to ensure that no hazardous leakage currents will flow from them.
Figure 3. Enclosure leakage current path.
3.4 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. Figures 4a and 4b illustrate the two scenarios.
Figure 4a. Patient leakage current path from equipment.
Figure 4b. Patient leakage current path to equipment.
3.5 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 (see figure 5).
Figure 5. Patient auxiliary current path.
6 Electrical Safety Tests
6.1 Normal condition and single fault conditions
A basic principle behind the philosophy of electrical safety is
that in the event of a single abnormal external condition arising
or of the failure of a single means of protection against a hazard, no safety hazard should arise. Such conditions are called "single fault conditions" (SFC's) and include such situations as the interruption of the protective earth conductor or of one supply conductor, the appearance of an external voltage on an applied part, the failure of basic insulation or of temperature limiting devices.
Where a single fault condition is not applied, the equipment is said to be in "normal condition" (NC). However, it is important to understand that in this condition, the performance of certain tests may compromise the means of protection against electric shock. For example, if earth leakage current is measured in normal condition, the impedance of the measuring device in series with the protective earth conductor means that there is no effective supplementary protection against electric shock.
Many electrical safety tests are carried out under single fault conditions since these represent the worst case and will give the most adverse results. Clearly the safety of the equipment under test may be compromised when such tests are performed. Personnel carrying out electrical safety tests should be aware that the normal means for protection against electric shock are not necessarily operative during testing and should therefore exercise due precautions for
their own safety and that of others.
6.2 Protective Earth Continuity
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 0.2 O at any such point. The test is obviously only applicable to class I equipment.
In IEC60601, the test is conducted using a 50Hz current between 10A and 25A for a period of at least 5 seconds. Although this is a type test, some medical equipment safety testers mimic this method. Damage to equipment can occur if high currents are passed to points that are not protectively earthed, for example, functional earths. Great care should be taken when high current testers are used to ensure that the probe is connected to a point that is intended to be protectively earthed.
HEI 95 and DB9801 Supplement 1 recommend that the test be carried out at a current of 1A or less for the reason described above. Where the instrument used does not do so automatically, the resistance of the test leads used should be deducted from the reading.
If protective earth continuity is satisfactory then insulation
tests can be performed.
Applicable to Class I, all types
Limit: 0.2
DB9801 Yes, at 1A or
less.
recommended?:
HEI 95 recommended?: Yes, at 1A or less.
Ensure probe is on a protectively earthed
Notes:
point
Figure 8. Measurement of protective earth continuity.
6.3 Insulation Tests
IEC 60601-1, clause 17, lays down specifications for electrical
separation of parts of medical electrical equipment compliance to
which is essentially verified by inspection and measurement of
leakage currents. Further tests on insulation are detailed under
clause 20, "dielectric strength". These tests use AC sources to test
equipment that has been pre-conditioned to specified levels of
humidity. The tests described in the standard are type tests and are
not suitable for use as routine tests.
HEI 95 and DB9801 recommend that for class I equipment the
insulation resistance is measured at the mains plug between the live
and neutral pins connected together and the earth pin. Whereas HEI
95 recommends using a 500V DC insulation tester, DB 9801 recommends
the use of 350V DC as the test voltage. In practice this last
requirement could prove difficult and it is acknowledged in a
footnote that a 500 V DC test voltage is unlikely to cause any harm.
The value obtained should normally be in excess of 50M Ω but may be less in exceptional circumstances. For example, equipment containing
mineral insulated heaters may have an insulation resistance as low as
1MΩ with no fault present. The test should be conducted with all
fuses intact and equipment switched on (see figure 9).
Applicable to Class I, all types
Limits: Not less than 50M Ω
DB9801
Yes
recommended?:
HEI 95
Yes
recommended?:
Equipment containing mineral insulated heaters
Notes: may give values down to 1M Ω . Check equipment
is switched on.
Figure 9. Measurement of insulation resistance for class I equipment
HEI 95 further recommends 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 50M Ω (see figure 10). DB9801 Supplement 1 does not recommend any form of insulation test be
applied to class II equipment.
Applicable to Class II, all types having applied parts Limits: not less than 50M Ω.
DB9801 recommended?: No
HEI 95 recommended?: Yes
Notes: Move probe to find worst case.
Figure 10. Measurement of insulation resistance for class II equipment.
Satisfactory earth continuity and insulation test results indicate
that it is safe to proceed to leakage current tests.
6.4 Leakage current measuring device
The leakage current measuring device recommended by IEC 60601-1 loads
the leakage current source with a resistive impedance of about 1 kO
and has a half power point at about 1kHz. The recommended measuring
device was changed slightly in detail between the 1979 and 1989
version but remained functionally very similar. Figure 11 shows
suitable arrangements for the measuring device. The millivolt meter
used should be true RMS reading and should have an input
impedance greater than 1 M Ω . In practice this is easily achievable with most good quality modern multimeters. The meter in the
arrangements shown measures 1mV for each Aμof leakage current.
Figure 11. Suitable arrangements for measurement of leakage currents.
6.5 Earth Leakage Current
For class I equipment, earth leakage current is measured as shown in
figure 12. The current should be measured with the mains polarity
normal and reversed. HEI 95 and DB9801 Supplement 1 recommend that
the earth leakage current be measured in normal condition (NC) only.
Many safety testers offer the opportunity to perform the test under a
single fault condition such as live or neutral conductor open circuit.
Applicable to Class I equipment, all types
0.5mA in NC, 1mA in SFC or 5mA and 10mA Limits: respectively for permanently installed
equipment.
DB9801
Yes, in normal condition only. recommended?:
HEI 95
Yes, in normal condition only. recommended?:
Measure with mains normal and reversed. Notes:
Ensure equipment is switched on.
Figure 12. Measurement of Earth Leakage Current.
6.6 Enclosure leakage current
Enclosure leakage current is measured between an exposed part of the equipment which is not intended to be protectively earthed and true earth as shown in figure 13. The test is applicable to both class I and class II equipment and should be performed with mains polarity both normal and reversed. HEI 95 recommends that the test be performed under the SFC protective earth open circuit for class I equipment and under normal condition for class II equipment. DB9801 Supplement 1 recommends that the test be carried out under normal condition only
for both class I and class II equipment. Many safety testers also allow the SFC's of interruption of live or neutral conductors to be selected. Points on class I equipment which are likely not to be protectively earthed may include front panel fascias, handle assemblies etc.
Applicable to Class I and class II equipment, all types. Limits: 0.1mA in NC, 0.5mA in SFC
DB9801
Yes, NC only
recommended?:
HEI 95 Yes, class I SFC earth open circuit, class II
NC.
recommended?:
Ensure equipment switched on. Normal and Notes:
reverse mains. Move probe to find worst case.
Figure 13. Measurement of Enclosure Leakage Current.
6.7 Patient leakage current
Under IEC 60601-1 and HEI 95, for class I and class II type B and BF equipment, the patient leakage current is measured from all applied parts having the same function connected together and true earth (figure 14). For type CF equipment the current is measured from each applied part in turn and the leakage current leakage must not be exceeded at any one applied part (figure 15).
DB9801 Supplement 1 recommends that patient leakage current be measured from each applied part in turn for all types of equipment, although the recommended leakage current limits have not been
revised to take into account the changed test method for B and BF equipment.
Great care must be taken when performing patient leakage current measurements that equipment outputs are inactive. In particular,
outputs of diathermy equipment and stimulators can be fatal and can damage test equipment.
Applicable to
All classes, type B & BF equipment having
applied parts.
Limits: 0.1mA in NC, 0.5mA in SFC.
DB9801
No
recommended?:
HEI 95 Yes, class I SFC earth open circuit, class II
recommended?: normal condition.
Notes:
Equipment on but outputs inactive. Normal
and reverse mains.
Figure 14. Measurement of Patient Leakage Current with applied parts connected
Applicable to
together.
Class I and class II, type CF (B & BF for DB9801 only) equipment having applied parts.
Limits: 0.01mA in NC, 0.05mA in SFC.
DB9801
Yes, all types, normal condition only. recommended?:
HEI 95 Yes, type CF only, class I SFC earth open circuit,
recommended?: class II normal condition.
Notes:
Equipment on but outputs inactive. Normal and
reverse mains. Limits are per electrode.
Figure 15. Measurement of patient leakage current for each applied part in turn
6.8 Patient auxiliary current
Patient auxiliary current as defined in section 3.5 is measured
between any single patient connection and all other patient
connections of the same module connected together. It is not usual to
test all possible combinations since together with all possible
single fault conditions this would give an exceedingly large amount
of data of questionable value.
All classes and types of equipment having
Applicable to
applied parts.
Type B & BF - 0.1mA in NC, 0.5mA in SFC.
Limits:
Type CF - 0.01mA in NC, 0.05mA in SFC.
DB9801
No.
recommended?:
HEI 95
No.
recommended?:
Ensure outputs are inactive. Normal and
Notes:
reverse mains.
Figure 16. Measurement of patient auxiliary current.
6.9 Mains on applied parts
By applying mains voltage to the applied parts, the leakage current
that would flow from an external source into the patient circuits
can be measured. The measuring arrangement is illustrated in figure
18.
Although the safety tester normally places a current limiting
resistor in series with the measuring device for the performance of
this test, a shock hazard still exists. Therefore, great care should
be taken if the test is carried out in order to avoid the hazard presented by applying mains voltage to the applied parts.
Careful consideration should be given as to the necessity or
usefulness of performing this test on a routine basis when weighed
against the associated hazard and the possibility of causing problems
with equipment. The purpose of the test under IEC 60601-1 is to
ensure that there is no danger of electric shock to a patient who for some unspecified reason is raised to a potential above earth due to
the connection of the applied parts of the equipment under test. The standard requires that the leakage current limits specified are not exceeded. There is no guarantee that equipment performance will not
be adversely affected by the performance of the test. In particular, caution should be exercised in the case of sensitive physiological measurement equipment. In short, the test is a "type test".
Class I & class II, types BF & CF having applied Applicable to
parts.
Limit: Type BF - 5mA; type CF - 0.05mA per electrode.
DB9801
No.
recommended?:
HEI 95 No
recommended?:
Ensure outputs are inactive. Normal and reverse Notes: mains. Caution required, especially on
physiological measurement equipment.
Figure 17. Mains on applied parts measurement arrangement.
6.10 Leakage current summary
The following table summarises the leakage current limits (in mA)
specified by IEC60601-1 for the tests most commonly performed as
routine tests. Limits for DB9801 recommended tests are underlined.
Limits for HEI 95 recommended tests are given in bold type.
Type B Type BF Type CF Leakage current
NC SFC NC SFC NC SFC Earth 0.5 1 0.5 1 0.5 1
Earth for fixed
5 10 5 10 5 10 equipment
Enclosure 0.1 0.5 0.1 0.5 0.1 0.5 Patient 0.1 0.5 0.1 0.5 0.01 0.05
Mains on applied
- - - 5 - 0.05 part
Patient auxiliary 0.1 0.5 0.1 0.5 0.01 0.05
*For class II type CF equipment HEI95 recommends a limit for
enclosure leakage current of 0.01mA as per the 1979 edition of BS
5724.
Table 2. Leakage current limits summary.
6.11 Comparison of HEI 95 and DB 9801 Supplement 1
recommendations
Test HEI 95
Use test current of 1A Earth continuity or less
Limit 0.2ohm
Measure between L
and N connected Insulation for Class 1
together and E using
500v DC tester. equipment
Limit > 50M Ω .
Investigate lower
values
Measure between
applied parts and
accessible conductive Insulation for Class II parts of the
equipment equipment.
Limit > 50M Ω .
Investigate lower
values
Measure in normal
Earth leakage current condition
Limit < 0.5mA
Measure in SFC, earth
open circuit
for
Enclosure leakage
Class-1, NC for
Class-II
current
Limit <0.5 mA for
Class1
<0.1 mA for class II
Measure from all
applied parts
connected together for
B & BF equipment and
Patient leakage from each applied part in turn for type CF.
current Measure under SFC,
eart open circuit for
Class 1, NC for classII.
Limits :
Class I, B& BF <
DB9801
Supplement 1
Use test current of 1A or less
Limit 0.2ohm
Measure between L and N connected together and E using 350v DC tester. Limit > 20M Ω . Investigate lower values
No recommendation.
Measure in normal
condition
Limit < 0.5mA
Measure in NC only
Limit < 0.1 mA
Measure from each
applied part in turn,
for all types of
equipment
Measure under NC
only
Limits
Type B & BF
<0.1 mA per
electrode
Type CF < 0.01。