Sikkerhed og certifisering for sundhedsteknologisk udstyr

Transcription

1 Sikkerhed og certifisering for sundhedsteknologisk udstyr Temadag på Sundhedsteknologiuddannelsens 6. semester Kim Dremstrup Nielsen, IST, AAU

2 Program Elsikkerhed, IEC mm. Lektor Kim Dremstrup Nielsen Sikkerhedsprocedurer på AS, Medikoteknisk chef Hans Henrik Pedersen, AS Certificeringsprocedurer i en medikoteknisk virksomhed Projektleder Lars Nilsson, Judex Datasystemer Sikkerhed i laboratorier, Lektor Michael Voigt Diskussion SMI - IST KDN/2002

3 Hazards by medical equipment Medical electrical equipment can present a range of hazards to the patient, the user, or to service personnel. 1 Mechanical Hazards Insecure fitting of controls to loose wheels on equipment trolleys. 2 Risk of fire or explosion Internal or external short circuits Many medical gases support combustion. 3 Absence of function The absence of function could threaten life. 4 Excessive or insufficient output E.g surgical diathermy units, anaestethic equipment etc. Calibration procedures are crucial. 5 Risk of exposure to spurious electric currents All electrical equipment may potentially expose people to the risk of unintentional exposure to electricity. In the case of medical electrical equipment the risk is potentially greater since patients are intentionally connected to such equipment.

4 Physiological effects of electricity Electrical Current can do following on human and animal tissue: AC can stimulate muscles and nerves DC and AC can burn skin and and other tissue depending on amplitude, frequency and duration DC can give electrochemical etching of skin underneath electrodes etc.

5 Physiological effects of electricity Electrical Current thus can have following effects on the human body: 1 Electrolysis 2 Burns 3 Muscle cramps 4 Respiratory arrest 5 Cardiac arrest 6 Ventricular fibrillation

6 Physiological effects of electricity Electrical Current thus can have following effects on the human body: 1 Electrolysis 2 Burns 3 Muscle cramps 4 Respiratory arrest 5 Cardiac arrest The movement of ions of opposite polarities in opposite directions through a medium is called electrolysis and can be made to occur be made to occur by passing DC currents 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. 6 Ventricular fibrillation

7 Physiological effects of electricity Electrical Current thus can have following effects on the human body: 1 Electrolysis 2 Burns 3 Muscle cramps 4 Respiratory arrest 5 Cardiac arrest 6 Ventricular fibrillation 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 currents 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 easy for internal electrical burns to be produced which if nor fatal can cause long lasting injury.

8 Physiological effects of electricity Electrical Current thus can have following effects on the human body: 1 Electrolysis 2 Burns 3 Muscle cramps 4 Respiratory arrest 5 Cardiac arrest When an electrical stimulus is applied to a motor nerve or 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 external electrical stimulus is responsible for the phenomenon where a person who is holding an electrically live object can be unable to let go. 6 Ventricular fibrillation

9 Physiological effects of electricity Electrical Current thus can have following effects on the human body: 1 Electrolysis 2 Burns 3 Muscle cramps The muscles between the ribs (intercostal muscles) need to repeatedly contract and relax in order in order to facilitate breathing. Prolonged tetanus of these muscles can therefore prevent breathing. 4 Respiratory arrest 5 Cardiac arrest 6 Ventricular fibrillation

10 Physiological effects of electricity Electrical Current thus can have following effects on the human body: 1 Electrolysis 2 Burns 3 Muscle cramps 4 Respiratory arrest The heart is a muscular organ which needs to 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. 5 Cardiac arrest 6 Ventricular fibrillation

11 Physiological effects of electricity Electrical Current thus can have following effects on the human body: 1 Electrolysis 2 Burns 3 Muscle cramps 4 Respiratory arrest 5 Cardiac arrest 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 70mA flowing from hand to hand across the chest or 20microamps directly through the heart may be enough. It is for this reason that most deaths from electric shock are attributable to the occurrence of ventricular fibrillation.

12 Electrical Safety Exposition = Current x Time - Duration [msec] vs. Current[mA] 1. Below sensation (up til 1mA ) 2. Sensation but no risc (10mA over longer time can give pain) 3. Risk for muscle cramp 4. < 50% s risk for cardiac fibrillation(small burns can occur) 5. > 50 % s risk for cardiac fibrillation (paralysis of breathing and burns) 50 kg bodymass at 50 Hz

13 Electrical Safety Let-go Current NB! Largest sensitivity at 50Hz

14 Prevention of accidents Design of own electro medical equipment : Use the regulatives and the recommandations from the International Electrotechnical Commission IEC standards (IEC and substandards) Of the shelf / Bougth equipment: Must comply to these standards

15 Regulations in Denmark Stærkstrømsbekendtgørelsen af 1/5-1993, former stærkstrømsreglementet, generel part, and part: Elektromedicinske apparater afsnit 135, is a translation of: DS/EN A1 + A2 + A11 + A12 + A13 and corrections Which again is an adaptation to IEC and sub-standards Shall, Should and May

16 Classification of electro medical equipment Classes I,II and III Types B, BF, and CF

17 Class I product: Classification of electro medical equipment A product that is provided with a reliable protective earth such that all accessible metal parts can t become live in the event of a failure of Basic Insulation and therefore will provide protection against electric shock in the case of the failure of Basic Insulation. Class II product: A product without a protective earth and where Double or Reinforced Insulation (typically a plastic enclosure) is relied upon to provide the protection against electric shock. Class III product: For internally powered source, such as a battery.

18 Classification of electro medical equipment - Class I Class 1 equipment GROUNDING Class 1 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 which would otherwise cause an exposed conductive part to become live, the supplementary protection (i.e. protective earth) comes into effect. A large fault current flows from the mains part to earth via the protective earth conductor which causes a protective device (usually a fuse) in the mains circuit to dissconnect the equipment from the supply. 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 1. There is no agreed symbol in use to indicate that equipment is class 1.

19 Classification of electro medical equipment Class II Class II equipment Double Insulation 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 basic protection fails then supplementary protection is afforded by a second layer of insulation preventing contact with live parts. 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.

20 Classification of electro medical equipment Class III Class III equipment is defined 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 relevent 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. 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 1 or class II equipment. Similarly, equipment powered from a SELV transformer should be tested in conjunction with the transformer as class 1 or class II equipment as appropriate. The standard relating to medical electrical equipment does not recognise class III equipment since limitation of voltage is not sufficient to ensure safety of the patient, but use: Internally Powered Equipment

21 Classification of electro medical equipment - Typing Type B Sufficient protection of patient, regarding leakage currents and grounding if applied (Type I) Type BF Type B equipment with FLOATING/ISOLATED patient connection Type CF Lower leakage currents than BF, but also floating patient connection

22 Leakage currents Causes of leakage currents If any conductor is raised to a potential above earth potential, then 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 resistance. The amount of current that flows depends on: The voltage on the conductor The capacitive reactance between the conductor and earth The resistance between the conductor and earth The current which flows from or between conductors which are normally insulated from earth and each other are called leakage currents. Such currents must be limited by the design of the equipment to safe values.

23 Leakage currents For medical electrical equipment, several different leakage currents are defined according to the paths that the leakage currents flow through. Earth leakage current Enclosure leakage current Patient leakage current Patient auxilliary current

24 Earth leakage current Earth leakage current is the current which normally flows in the earth conductor of a protectively earthed piece of equipment. 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. If the earth leakage current is low then the risk of electric shock in the event of a fault is reduced

25 Enclosure leakage current Enclosure leakage current is described as the current that flows from an exposed conductive part of the conductor to earth through a conductor other than the protective earth conductor. It is usual when testing medical electrical equipment to measure enclosure leakage current from points on the enclosure which are not protectively earthed. On many pieces of equipment no such points may exist. This is not a problem. The test included in the Figure 2. Enclosure leakage path test regimes to cover the eventuality where no such points do exist and ensure no hazard is created by them.

26 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.

27 Patient auxilliary current The patient auxilliary current is defined as the current which normally flows between parts of the applied part through the patient which is not intended to produce a physiolgical effect, also called meassurement current.

28 Patient leakage current limits IEC 60601

29 Patient leakage current Type B BF CF Cond N SF N SF N SF Pat leak All currents in mikroamp (10E-6A) Also limits for ground leakage, capsulation leakage, measurement current

30 Grounding All type I equipments must have a ground lead In the supply wire. This ensures that current from e.g. broken mains leads inside the capsulation will go to ground and not to the subject.

31 Safety Transformers. In heterogeneous setups as often seen in labs: Use SAFETY TRANSFORMERS

32 Safety precautions Equipment must be safe Type B, BF og CF Connect equipment to a safe supply source Use safety transformers, only ONE source Use the equipment in a safe manner Remove lamps, coffe machines etc. Use common sense

33 Further reading Stærkstrømsbekendtgørelsen afs. 135 IEC and substandards Webster J.G., Medical Instrumentation, 3rd edt, Chapter 14 EL-sikkerhed, Råd om elektromedicinsk udstyr. Projektrapport 83.01, Dansk Sygehus Institut 1983