Category: Electrical Safety
The following paragraphs and diagrams describe the electrical safety tests commonly available on medical equipment safety testers. Please note that although HEI 95 and DB9801 are no longer current, they are referred to in the text since many medical electronics departments have used them as a basis for local acceptance testing and even routine testing protocols. Protocols based on both sets of guidance are also available on many medical equipment safety testers.
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" (SFCs) 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 even 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 various single fault conditions in order to verify that there is no hazard even should these conditions occur in practice. It is often the case that single fault conditions 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. In particular the equipment under test should not be touched during the safety testing procedure by any persons.
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. 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 recommended 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|
|DB9801 recommended?:||Yes, at 1A or less.|
|HEI 95 recommended?:||Yes, at 1A or less.|
|Notes:||Ensure probe is on a protectively earthed point|
Figure 8. Measurement of protective earth continuity.
IEC 60601-1 (second edition), 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 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. In practice this last requirement could prove difficult and it was 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 50MΩ 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 where mechanical on/off switches are present (see figure 9).
|Applicable to||Class I, all types|
|Limits:||Not less than 50MΩ|
|HEI 95 recommended?:||Yes|
|Notes:||Equipment containing mineral insulated heaters may give values down to 1MΩ. Check equipment is switched on.|
Figure 9. Measurement of insulation resistance for class I equipment
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 (see figure 10). DB9801 Supplement 1 did 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 50MΩ.|
|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.
The leakage current measuring device recommended by IEC 60601-1 loads the leakage current source with a resistive impedance of about 1 kΩ and has a half power point at about 1kHz. The recommended measuring device was changed slightly in detail between the 1979 and 1989 editions of the standard but remained functionally very similar. Figure 11 shows the arrangements for the measuring device. The millivolt meter used should be true RMS reading and should have an input impedance greater than 1 MΩ. In practice this is easily achievable with most good quality modern multimeters. The meter in the arrangements shown measures 1mV for each µA of leakage current.
Figure 11. Arrangements for measurement of leakage currents.
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 recommended that the earth leakage current be measured in normal condition (NC) only. Many safety testers offer the opportunity to perform the test under single fault condition, neutral conductor open circuit. This arrangement normally gives a higher leakage current reading.
One of the most significant changes with regard to electrical safety in the 2005 edition of IEC 60601-1 is an increase by a factor of 10 in the allowable earth leakage current to 5mA in normal condition and 10mA under single fault condition. The rationale for this is that the earth leakage current is not, of itself, hazardous.
Higher values of earth leakage currents, in line with local regulation and IEC 60364-7-710 (electrical supplies for medical locations), are allowed for permanently installed equipment connected to a dedicated supply circuit.
|Applicable to||Class I equipment, all types|
|Limits:||0.5mA in NC, 1mA in SFC or 5mA and 10mA respectively for equipment designed to IEC60601-1:2005.|
|DB9801 recommended?:||Yes, in normal condition only.|
|HEI 95 recommended?:||Yes, in normal condition only.|
|Notes:||Measure with mains normal and reversed. Ensure equipment is switched on.|
Figure 12. Measurement of earth 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 recommended 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 recommended 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.
The term "enclosure leakage current" has been replaced in the new edition of the IEC 60601-1standard by the term "touch current", bringing it into line with IEC 60950-1 for information technology equipment. However, the limits for touch current are the same as the limits for enclosure leakage current under the second edition of the standard, at 0.1 mA in normal condition and 0.5 mA under single fault condition.
In practice, if a piece of equipment has accessible conductive parts that are protectively earthed, then in order to meet the new requirements for touch current, the earth leakage current would need to meet the old limits. This is due to the fact that when the touch current is tested from a protectively earthed point with the equipment protective earth conductor disconnected, the value will be the same as that achieved for earth leakage current under normal condition.
Hence, where higher earth leakage currents are recorded for equipment designed to the new standard, it is important to check the touch current under single fault condition, earth open circuit, from all accessible conductive parts.
|Applicable to||Class I and class II equipment, all types.|
|Limits:||0.1mA in NC, 0.5mA in SFC|
|DB9801 recommended?:||Yes, NC only|
|HEI 95 recommended?:||Yes, class I SFC earth open circuit, class II NC.|
|Notes:||Ensure equipment switched on. Normal and reverse mains. Move probe to find worst case.|
Figure 13. Measurement of enclosure leakage current
Under IEC 60601-1, 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).
HEI 95 adhered to the same method, however, DB9801 Supplement 1 recommended that patient leakage current be measured from each applied part in turn for all types of equipment, although the recommended leakage current limits were not 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.|
|HEI 95 recommended?:||Yes, class I SFC earth open circuit, class II normal condition.|
|Notes:||Equipment on, but outputs inactive. Normal and reverse mains.|
Figure 14. Measurement of patient leakage current with applied parts connected together
|Applicable to||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 recommended?:||Yes, all types, normal condition only.|
|HEI 95 recommended?:||Yes, type CF only, class I SFC earth open circuit, class II normal condition.|
|Notes:||quipment 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
Patient auxiliary current is measured between any single patient connection and all other patient connections of the same module or function connected together. Where all possible combinations are tested together with all possible single fault conditions this yields an exceedingly large amount of data of questionable value.
|Applicable to||All classes and types of equipment having applied parts.|
|Limits:||Type B & BF - 0.1mA in NC, 0.5mA in SFC. Type CF - 0.01mA in NC, 0.05mA in SFC.|
|HEI 95 recommended?:||No.|
|Notes:||Ensure outputs are inactive. Normal and reverse mains.|
Figure 16. Measurement of patient auxiliary current.
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".
Most medical equipment safety testers refer to this test as "mains on applied parts", although this is not universal. One manufacturer refers to the test simply as "Patient leakage - F-type". In all cases there should be a hazard indication visible where the test is selected.
|Applicable to||Class I & class II, types BF & CF having applied parts.|
|Limit:||Type BF - 5mA; type CF - 0.05mA per electrode.|
|HEI 95 recommended?:||No|
|Notes:||Ensure outputs are inactive. Normal and reverse mains. Caution required, especially on physiological measurement equipment.|
Figure 17. Mains on applied parts measurement arrangement
The following table summarises the leakage current limits (in mA) specified by IEC60601-1 (second edition) for the most commonly performed tests. Most equipment currently in use in hospitals today is likely to have been designed to conform to this standard, but note that the allowable values of earth leakage current have been increased in the third edition of the standard as discussed above.
The values stated are for d.c. or a.c. (r.m.s), although later amendments of the standard included separate limits for the d.c. element of patient leakage and patient auxiliary currents at one tenth of the values listed below. These have not been included in the table since, in practice, it is rare that there is a problem solely with d.c. leakage where that is not evidenced by a problem with combined a.c and d.c. leakage.
|Earth for fixed equipment||
|Mains on applied part||
* 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.
|Test||HEI 95||DB9801 Supplement 1|
|Earth continuity||Use test current of 1A or less Limit 0.2ohm||Use test current of 1A or less Limit 0.2ohm|
|Insulation for Class 1 equipment||Measure between L and N connected together and E using 500v DC tester. Limit > 50MΩ. Investigate lower values||Measure between L and N connected together and E using 350v DC tester. Limit > 20MΩ. Investigate lower values|
|Insulation for Class II equipment||Measure between applied parts and accessible conductive parts of the equipment. Limit > 50MΩ. Investigate lower values||No recommendation.|
|Earth leakage current||Measure in normal condition Limit < 0.5mA||Measure in normal condition Limit < 0.5mA|
|Enclosure leakage current||Measure in SFC, earth open circuit for Class-1, NC for Class-II Limit <0.5 mA for Class1 <0.1 mA for class II||Measure in NC only Limit < 0.1 mA|
|Patient leakage current||Measure from all applied parts connected together for B & BF equipment and from each applied part in turn for type CF. Measure under SFC, eart open circuit for Class 1, NC for classII. Limits :
||Measure from each applied part in turn, for all types of equipment Measure under NC only Limits