Medical Device Link.

Originally Published IVD Technology June 2003

PROCESSING TECHNOLOGIES

Electrical equipment presents hazards that IVD manufacturers should be aware of.

Steli P. Loznen

A large range of IVD electrical equipment has been developed to automate clinical laboratory processes, analyze body fluids and tissues, and report the results to clinicians. As with all other electric products, laboratory electrical equipment presents hazards against which proper design and manufacturing can protect. Hence there is a growing awareness of the necessity to issue safety certifications on such equipment.

While the IVD Directive is primarily concerned with reagents, calibrators, markers, and control materials, the final performance of the test and the quality of the result may depend on the performance and design of the electrical equipment that is used to run an assay. The electrical equipment associated with these biochemical elements is recognized as an inseparable part of the overall system performance and is subject to the compliance regulations in the directive.

For electrical equipment used in the laboratory, the harmonized European Union general safety standard is EN 61010-1:2001.¹ At the same time, another standard was elaborated specifically for IVD equipment, EN 61010-2-101:2002, which will remain valid until December 2004.²

A ground continuity test is performed on the hydropneumatic system of the Synchron LXi 725 by Beckman Coulter (Fullerton, CA). (click to enlarge)

This standard specifies the tests that must be run on an electrical product to determine if it can meet the requirements of the standard. The purpose of this standard is to ensure that the design and manufacturing can adequately protect the operator and the surrounding area against hazards that can occur during the use of IVD medical electrical equipment. Such hazards include electric shock, burn, mechanical hazards, excessive temperature, spread of fire, hazards from fluids, biohazards, hazards from chemical substances, effects of radiation, liberated gases, explosion, and implosion. This article discusses various safety factors that IVD manufacturers should consider when designing their electrical equipment to make sure that these hazards do not occur.

Basic Construction Requirements

The design of IVD electrical equipment should primarily protect against electric shock by using adequate insulation (i.e., basic insulation, protective impedance, enclosures, barriers, shields) and proper construction (i.e., segregation of low-voltage circuits from hazardous voltages). The two separate and independent means that provide protection against electric shock are basic insulation during normal use and a supplementary insulation in case of failure of the basic insulation. These two methods together form a double, or reinforced, insulation.

It should be noted that lacquer, enamel, oxides, anodic films, nonimpregnated hygroscopic materials (e.g., paper, fibers) do not provide suitable insulation. Moreover, additional protection, such as protective grounding, double insulation, and protective impedance, should be provided to prevent accessible parts from becoming hazardous live parts in single-fault conditions.

Normal Conditions

The following should be determined to evaluate whether protection from electric shock is adequate under normal conditions: the state of the accessible parts, the amount of spacing (i.e., clearance and creepage distances), the results of dielectric strength tests, and the rigidity of enclosures and barriers.

Accessible Parts. Measuring the voltage between an accessible part and the ground can determine whether the part is hazardous live.¹ If the measured values exceed specified safety levels, the accessible leakage current and capacitance should also be measured.

In general, the leakage (or touch) current refers to the current that flows from an accessible part through the protective ground conductor to the ground. In the absence of a grounding connection, the leakage current could flow from any conductive part, or the surface of the nonconductive parts, to the ground if a conductive path were available.

Figure 1. Measuring a circuit for any leakage current for dc and ac with frequencies of up to 1 MHz. (click to enlarge)

Ac leakage current is caused by a parallel combination of capacitance and dc resistance between a voltage source (the ac line) and the grounded conductive parts of the equipment. The leakage caused by the dc resistance usually is insignificant compared with the ac impedance of various parallel capacitances. The capacitance may be intentional (e.g., in radio-frequency interference filter capacitors) or unintentional (e.g., spacing in printed circuit boards, insulation between semiconductors and grounded heat sinks, primary to secondary capacitance in transformers). For dc and ac with frequencies up to 1 MHz, a measuring circuit for leakage current is used (see Figure 1).

For an electrical device to be permitted for normal use, the product of the capacitance in farads and the potential in volts should not exceed 45 microcoulombs for a maximum 15 kV peak-to-peak (p-p) or dc voltage measured between the accessible part and the ground. In addition, one-half of the product of the capacitance in farads and the square of the potential in volts should not exceed 350 millijoules for a voltage measured between the accessible part and the ground above 15 kV p-p or dc.

Protective Spacing. Protective spacing separates hazardous live parts from other such parts, grounded parts, and nonhazardous parts to prevent electric shock in normal and single-fault conditions.^1,2 The spacings, or clearance and creepage distances, are specified in the standards so as to withstand the voltages that appear on the equipment. The spacings also account for rated environmental conditions and any protective devices fitted within the equipment or required by the manufacturer.

Clearances are specified to withstand the maximum transient overvoltages that can be present on the power-supply circuit, as a result of either an external event (such as a lightning or switching transient) or the operation of the equipment. If transient overvoltages cannot occur, clearances are based on the maximum working voltage.

The clearance distance necessary depends on the type of insulation (basic or double) and the pollution degree, a numeral characterizing the expected pollution of the microenvironment. According to the working microenvironment for most laboratories, electrical equipment can be categorized into either pollution degree 2 (nonconductive pollution occurs) or pollution degree 3 (conductive pollution occurs). In all cases, the minimum clearance for pollution degree 2 is 0.2 mm, and 0.8 mm for pollution degree 3.

For creepage distances between two circuits, the working voltage that stresses the insulation needs to be considered. Creepage will always be at least as large as the distance specified for clearance. If the calculated creepage distance is less than the clearance, the creepage shall be increased to equal the clearance.

The comparative tracking index (CTI) characterizes the progressive formation of conductive paths on the surface of solid insulation materials due to the combined effects of electric stress and electrolytic contamination on the surface. For calculating creepage distances, the insulation materials are separated into one of the following groups: I with CTI > 600; II with 600> CTI >400; III with 400 > CTI > 100). Laboratory electrical equipment generally uses material from group III. In addition, the creepage distance for double insulation is twice the value specified for basic insulation.

Dielectric Strength Tests. The purpose of dielectric strength tests is to evaluate the insulation under electrical stress. These tests involve applying for 1 minute a high voltage from the primary circuit to the grounding circuit and the low-voltage secondary circuits. The test voltage depends on the working voltage and the grade of insulation used.

During the test, no breakdowns or flashovers should occur. Insulation breakdown occurs when the current flowing as a result of applying the test voltage increases rapidly in an uncontrolled manner, and the insulation does not restrict the flow of the current. A corona or single momentary flashover is not considered an insulation breakdown. The common waveforms used in dielectric strength tests include ac root-mean-square (rms) voltage, Ac peak-to-peak voltage, dc voltage, and the 1.2/50-nanosecond peak impulse voltage. Ac may be chosen for simplicity, dc to avoid capacitive current, and the impulse test to reduce power dissipation in the components.³

For the dielectric tests, the protective impedance that is parallel with the insulation to be tested should be disconnected. In addition, the dielectric and leakage current tests are conducted after humidity tests are done at a temperature of 38–42°C and a relative humidity of 90–95% for 48 hours. Those parts that are normally used in electrical equipment and are sensitive to humidity but do not influence safety do not need to be subjected to the humidity test (e.g., disk drives).

Enclosure Rigidity. The rigidity of the electrical equipment's enclosure is tested by applying a force of 30 N with the hemispherical end of a hard rod that is 12 mm in diameter on any accessible part of the enclosure. Any access to hazardous live parts is not allowed.

Single-Fault Conditions

The following should be investigated to prevent accessible parts from becoming hazardous live in single-fault conditions: the correctness of the grounding, the proper separation of parts between a double insulation, the use of protective impedances, and, if it is used, the automatic disconnection of the electrical supply in case of a fault.^1,2

Grounding. The ground bond test uses a measurement of resistance to determine if the electrical equipment would remain safe in the event that the internal live conductors touched the casing. This test stresses the ground connection with a high current and causes a failure on a weak connection. All accessible conductive parts of the product must be grounded or separated from hazardous live circuitry with double or reinforced insulation, since they could become hazardous live in case of a fault. Alternatively, a conductive protective screen or barriers bonded to protective ground terminals can be used.

The grounding lead from the main supply cord should be terminated by a closed-loop connector that is secured to a threaded stud or screw on the chassis with a nut and washer. The threaded stud or screw should be a suitable size for the bond wire. The grounding terminal should not be used for other purposes such as fixing mechanical parts. Where accessible plugs and sockets are used, the plug-socket combination must be such that the ground connection is made first and broken last.

The ground conductor for each electrical component must be at least as large as the conductor carrying the largest current in that component. The ground conductor may be uninsulated; but if it is insulated, it must have green or yellow colored insulation. Soldered ground connections must be mechanically secured prior to soldering. Crimping connections should also crimp both the conductor and the insulator with the tools recommended by the connector manufacturer.

The grounding system is tested by passing a current that is 25 amperes (A) or twice the rated current of the electrical equipment, whichever is greater, from a maximum 10-V ac rms or dc source for 1 minute between the ground pin and each grounded accessible conductive part. This test verifies that the resistance between the accessible parts is reliably grounded and that the protective grounding terminal or grounding contact is not greater than 0.1 (omega) for equipment with detachable power-supply cords. The main cord impedance does not form part of the specified bonding impedance.³

If the equipment contains overcurrent protection devices for all poles of the main electrical supply, the current for the bonding impedance measurement should not exceed twice the rated current of the internal overcurrent-protection devices. This is also the case if the wiring between the electrical supply and the overcurrent-protection devices cannot become connected to accessible conductive parts in a single-fault condition.

It is important not to confuse the ground-bonding test with the ground continuity test. This latter test only verifies that the protective ground connection exists. Since it is a low-current test, it does not verify that this connection can withstand fault current.

Separation. In laboratory electrical equipment, there should be basic insulation between hazardous live parts in the main circuitry and the following: accessible grounded parts, low-voltage grounded circuitry, and low-voltage parts that are connected to accessible parts not grounded through a protective impedance or are supplied through a protective impedance.^1,2 In addition, there should be double or reinforced insulation between hazardous live parts and low-voltage circuitry that is nongrounded, and accessible nongrounded parts.

Figure 2. Insulation requirements according to EN 61010-1. (click to enlarge)

EN 61010-1 explains the insulation required between different parts in laboratory electrical equipment (see Figure 2).

Protective Impedance. A protective impedance ensures that accessible conductive parts cannot become hazardous live as a result of a single-fault condition. This impedance can be a high-integrity single component that is not liable to become defective as to cause a risk of hazard, a combination of components, or a combination of basic insulation and a current- or voltage-limiting device.

Mechanical Hazards

To prevent mechanically related injuries when using laboratory electrical equipment, the edges and corners should be smooth and rounded, and the moving parts that could injure a patient or operator must not be accessible.¹

The equipment should not cause a hazard when subjected to shock and impact. After completing the tests (i.e., drop, impact, enclosure rigidity), the equipment should also pass a dielectric test and be inspected to check that: hazardous live parts are not accessible; enclosures show no cracks which could cause a hazard; barriers have not been damaged or loosened; spacings have not been reduced; nonaccessible moving parts in normal use are not exposed; and no damage has been done which could cause spread of fire.

The equipment also should not overbalance when it is tipped to a 10° angle, with all the doors, covers, and drawers put in their most unfavorable positions. The equipment should also not overbalance when a certain amount of force is applied to it. For equipment that is more than 1 m high and weighs 25 kg or more, a force of 250 N or 20% of the mass of the unit, whichever is less, is applied on top of the unit; for floor-standing equipment less than 1 m high, a force of 800 N is applied downwards.

Risk of Fire

A risk of fire exists when a combustion source and fuel are present. The fuel may be polymeric materials such as plastic enclosures, wires, connectors, and printed circuit boards. Fire hazards may result from excessive temperatures either under normal operating conditions or due to overload, component failure, insulation breakdown, or loose connections. Fires originating within the electrical equipment should not spread beyond the immediate vicinity of the source of the fire, nor cause damage to the surroundings of the equipment.

To reduce the number of combustion sources, limits are imposed on temperature increases, and minimum spacings are required. To limit the propagation of fire, minimum flammability ratings are also imposed for both internal and enclosure plastics. Other measures to reduce the risk of fire include: providing overcurrent and overheating protection; limiting the quantity of combustible materials used; shielding or separating combustible materials from those likely to be ignition sources; and using enclosures or barriers to limit the spread of fire within the equipment.

There are a number of ways to meet the requirements for protecting against the spread of fire, such as by using the following: limited-energy circuits in which the current under any condition of load, including short circuit, does not exceed 8 A after 1 minute of operation and where the open-circuit potential does not exceed 30 V rms or 60 V dc; components with suitable flammability ratings; adequate openings on enclosures containing unlimited circuits. In those situations in which protecting against the risk of fire depends on separating circuits, they should be separated at least by basic insulation.³

Laboratory electrical equipment should operate safely within a temperature range of 5– 40°C. In most cases, the maximum temperature of a part is determined by measuring the temperature increase of the part and adding it to the maximum ambient temperature for safe use (40ºC). The temperature of the windings should be determined by the resistance method or by using temperature sensors with a negligible effect on the temperature of the winding. Temperatures are measured when a steady state has been reached.

Enclosures should be such that combustible materials outside the enclosure are prevented from being ignited by molten metal, burning insulation, or flaming particles from within the equipment. Enclosures, barriers, and guards must be made of materials that are not highly combustible. These materials include steel; aluminum that is corrosion protected; and heat-resistant, tempered, wired, or laminated glass. Plastics can also be used, but flame ratings must be observed. Openings in bottom enclosures should also be avoided, but in some cases, they are allowed.

Thermoplastic enclosures should withstand a mold stress treatment for seven hours at a temperature of 70°C. Parts made of insulating materials that are used to support parts connected to the main electrical supply are subjected to ball pressure tests for one hour at 125°C, or the Vicat softening test according to ISO 306 method A at 130°C. Thermoplastic materials used in laboratory electrical equipment should be minimum flame rated as 94V-1 or FV-1 for all printed circuit boards, enclosures, and any flame barriers; 94V-2 or FV-2 for all polymeric materials used within the unit; and 94HB or F-HB for external decorative parts.

Fluids and Chemicals

EN 61010-1 specifies construction requirements for protecting against hazards from fluids and chemical substances.¹ The electrical equipment needs to be designed so that no hazard will occur as a result of insulation getting wet or uninsulated internal parts becoming hazardous live. Loss of cooling or other liquids used in the equipment also should not cause electric shock or the spread of fire.

Electrical equipment that uses liquids under normal conditions should withstand the spillage test without any adverse effects. In this test, 200 ml of water is poured steadily onto each point where liquid might gain access to electrical parts for 15 seconds from a height of 10 cm. Certain equipment should also pass the overflow test. In this test, the liquid containers in the equipment that has them are completely filled, and a further quantity of liquid equal to 15% of the capacity of the container, or 250 ml, is then poured in for 60 seconds. In addition, equipment that is intended to be moved while a container is full of liquid is tilted to a 15° angle in the most unfavorable direction.

After conducting these tests, the equipment should function normally and comply with the dielectric test requirements. Where appropriate, conformity is also checked by examining the compatibility between the potentially aggressive substances (i.e., corrosive, toxic, or flammable liquids) and the contacted parts of the equipment.

If potentially hazardous substances are used in the equipment, the operator should not be in danger of getting wet or being able to inhale quantities likely to be hazardous. The areas of the equipment containing such substances should be equipped with protective covers or other similar means of protection.

A part subjected to pressure having a pressure x volume content greater than 200 kPa and a pressure greater than 50 kPa should not cause a hazard through rupture or leakage and should withstand the hydraulic test pressure. No leakage is allowed for pressure vessels that contain toxic, flammable, or otherwise hazardous substances. The maximum pressure a part can be subjected to in all conditions should not exceed the maximum permissible working pressure for the part. The equipment should incorporate an overpressure protection device where excessive pressure could occur.

Additional Construction Requirements

Low-voltage leads from the secondary circuitry should be additionally fixed by a tie-wrap or similar fixing to prevent the wires from contacting hazardous live primary or secondary circuits, if they were to become free. Similarly, wires at hazardous live voltages must be secured so that they cannot touch secondary circuitry if they were to break free from a connector. Wiring must also be protected from coming into contact with sharp edges.

Electrical equipment containing an ultraviolet light source that is not designed for external ultraviolet illumination should not permit unintentional escape of UV radiation on the operator. In addition, overpressure protection devices should satisfy the following requirements: connected as close as possible to the part intended to protect; an adequate discharge capacity; permitting easy access for inspection, maintenance, and repair; not capable of being adjusted without the use of a tool; a discharge opening not directed toward any person; and no shutoff valve between an overpressure safety device and the parts it is intended to protect.

Conclusion

Electrical equipment is a potentially dangerous source of numerous hazards, such as fire and electric shock. Since electrical equipment has become a standard part of most IVD devices, IVD manufacturers should take the proper precautions and run the necessary tests to ensure that their electrical products and components can be used safely and will not cause harm to users or patients.

References

1. Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use: Part 1: General requirements; Second edition, IEC 61010-1 (Geneva: International Electrotechnical Commission, 2001).

2. Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use: Part 2-101: Particular requirements for in vitro diagnostic (IVD) medical equipment; First edition, IEC 61010-2-101 (Geneva: International Electrotechnical Commission, 2002).

3. S Loznen, "Product-Safety Requirements for Medical Electrical Equipment," Compliance Engineering 7, no. 3 (1995) 17–30.

Steli P. Loznen, MSc, is the head of medical devices evaluation services at the Standards Institution of Israel (Tel Aviv). He can be reached via sloznen@sii.org.il.

Photo Courtesy Beckman Coulter Inc.

Copyright ©2003 IVD Technology