Originally Published IVD Technology March 2005
Processing Technologies
Critical components in
IVD electrical equipment
Ensuring that IVD components meet safety standards can save IVD manufacturers money and headaches down the road.
Steli P. Loznen
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| Figure 1. Component life cycle (click to enlarge). |
Like construction requirements and testing, the selection of components represents an important aspect of compliance with any safety standard. To achieve safety in IVD electrical equipment, construction materials and components should be selected and arranged to perform in a reliable manner, without creating a hazard, for the anticipated life of the equipment (see Figure 1). When components are not adequately inspected, the probability of failure is much higher and unacceptable risks can be generated.
Critical components (e.g., power supplies, transformers, supply cords, fuses, power entry modules, mains capacitors, EMI filters, circuit breakers, power switches, wiring materials, motors, fans, thermoplastic materials, EMC coatings, optoisolators, batteries, and interlock switches) should be evaluated against national or international safety standards (see Figure 2). They must be provided with a nationally recognized testing laboratory (NRTL) or European approval, such as a mark from Underwriters Laboratories (UL; Northbrook, IL) or another agency such as the Canadian Standards Association (CSA; Toronto), Verband Deutscher Electrotechniker (VDE; Offenbach, Germany), or the TÜV Rheinland Group (Cologne, Germany). A CE marking is not legally required for components. The approval must be documented by a copy of the approval certificate or by the license for the component (the use of catalogs data sheets is not a proper way to prove compliance). Particular attention should be paid to conditions of acceptability in the end product to ensure correct application and that specified electrical ratings are not exceeded.
Although it is possible to buy nonapproved components for less money than approved components, the buyer will need to pay for the additional cost of testing these components. The best option is to use components that have already been approved by the same agency that will approve the product. For example, to get a UL listing, use UL-recognized components and VDE-approved components for European Union–approved end products.
Ratings of IVD Equipment
IVD electrical equipment should include ratings for voltage, frequency (for ac supply sources), power (in VA or W), or current input.11 For example, a rating of 230 V, 50 Hz, 500 VA means that the equipment should be connected to an ac mains supply with a voltage of 230 V and a frequency of 50 Hz, and will use a maximum input power of 500 VA.
Power Supply
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| Table I. Examples of regional color coding of conductors on power supply cords (click to enlarge). |
When selecting components, particularly power supplies, it is important to ensure that the approval of the component is up to date. One difficulty sometimes encountered is that new editions of and amendments to components standards often introduce changes to the requirements. If the end product is to comply with the latest requirements, the components selected must also comply.
For European approvals, the product, as well as all of its components, must comply with the latest standard published in the Official Journal. For instance: when IVD equipment needs approval using the 2001 edition of EN 61010-1, use power supplies that are identified as having been evaluated for EN 60950 and followed by a year (2000 for the 3rd edition). Anything older is not likely to be usable without additional testing and inspection.
For the United States it is necessary to choose a power supply that is UL recognized (category QQGQ2 with no deviations) or that bears a mark from another NRTL. For Canada, power supplies need to bear a CSA or C-UL approval for standard C22.2 No. 950 (for SELV output, an output level of 3 or higher is requested). Power supplies meeting the requirements of IEC 60601 (for medical equipment) can generally be used in EN 60950 (information technology equipment) and EN 61010-1– based products (laboratory products including IVD equipment), but the reverse is not necessarily true.2
A direct plug-in adaptor intended for use with IVD equipment should be UL listed (EPBU) to the standard for class 2 power units (UL 1310) if supplied with class 2 outputs, or be listed (QQFU) to the standard for power units other than class 2 (UL 1012) if supplied with non–class 2 output. An approval from another NRTL may also be acceptable.
Power Supply Cord Set
IVD electrical equipment supplied in the EU must have a European-harmonized cord set suitable for the rating of the unit, according to EN 60245 designation 53 (rubber sheathed) or EN 60227 designation 53 (PVC sheathed).3 Alternatively, the equipment may be provided with an appliance inlet, and each country will supply a European-harmonized detachable cord set (see Table I).
A nondetachable cord must be provided with a bushing and a strain relief. It must enter the equipment through an inlet bushing having a smooth, well-rounded surface and withstand the cord anchorage test. If the cord set is supplied without a plug, appropriate installation instructions must be included.
For a unit rated up to and including 6 A, the cross-sectional area of the conductors in the power supply cord must be a minimum 0.75 mm2. For products rated between 6 A and 10 A, the minimum cross-sectional area required is 1.0 mm2.1 Any clamping screws used as part of anchorage should not bear directly on the cord, and at least one part of the cord anchorage should be securely fixed to the equipment. The terminals used to make the ac connection between the equipment and its cord can be of stud, pillar, or screw types. All these have to meet the dimensional requirements.
When preparing power supply cords for permanent attachment to equipment, the protective-grounding conductor should be made slightly longer than the circuit conductors so that, in the event the strain-relief mechanism fails, the protective-grounding connection will be the last to break.
Disconnection Means from Supply Source
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| Figure 2. Summary of component compliance options in IVD equipment (click to enlarge). |
IVD equipment must have a means for disconnecting all current-carrying conductors. This may be a switch or circuit breaker, an appliance coupler that can be disconnected without the use of a tool, or a mains plug without a locking device that can be removed from the wall socket outlet without the use of a tool. For permanently connected equipment and multiphase equipment, only a switch or circuit breaker is allowed. If the plug is considered to be the disconnection device, it must be easily accessible. Safety instructions should specify both plug function and location. For single-phase portable equipment, a plug on a cord not longer than 3 m is considered to be easily accessible. The mains plug must be rated to at least 125% of the instrument current rating.
Switches must be rated equal to or exceeding the load they will control, and must disconnect all ungrounded (hot) connectors (double-pole, single-throw– type switches). The switches and circuit breakers should comply with IEC 60947-1 and IEC 60947-3 requirements. Switches need to be mounted securely so they do not rotate in normal use. “O” and “I” symbols for ON and OFF positions should be marked adjacent to the mains switch, circuit breakers, and push-button switch. Spacing for switches should comply with IEC 60328 requirements (a minimum 0.3-mm contact gap per pole).
Indicator lights, if provided, must comply with IEC 60073 with regard to coloring. Since the power inlet terminals become live after switching the unit on, they must be protected with shrink-tubing or insulated quick-disconnect– type connectors. If either is used, it must be rated at a minimum of 300 V and 105°C. If a product includes a one-third or greater horsepower motor, a separate switch is required.
Internal Powered Devices
When selecting a battery, it is important to analyze factors such as backup run time, recharge time, power level, memory effect, charge retention, service life, and low-temperature use. The selection of a battery should be based on the highest discharge rate and the lowest operating temperature expected. At these extreme conditions, the battery must meet the IVD equipment’s discharge load requirements.
In general, battery voltage is selected according to the power level needs of the equipment: 12 V for up to 100–150 W, 24 V for up to 300–500 W, and 48 V for greater than 500 W. Additionally, protection means such as fuses, diodes, and resistors should be included to prevent the risk of accidentally shorting the battery.
Nonrechargeable lithium batteries require two blocking components: a diode to prevent reverse charging and a current-limiting component in its circuit. A rechargeable lithium battery only requires a current-limiting component (see Figure 3).
Important safety precautions should be followed for batteries. Do not mix batteries of different types or ages. Do not expose batteries to strong impacts or to temperatures above 60°C. Housing compartments should be made of acid- and alkali-resistant materials and should allow gases to escape during charging and discharging. For wet-cell batteries, use a separate compartment to avoid electrolyte spillage should cells rupture.3 In addition, make sure there are means to prevent battery leakage when the system is turned off. Use a power-factor-corrected circuit in systems where battery backup power exceeds 200 W.
On equipment instructions, indicate proper battery insertion in order to prevent incorrect polarity. Also, be sure to provide adequate information about recharging (e.g., audible or visual depletion warnings that remain activated until the battery is replaced, information on appropriate charger, a means to note the charging state of the battery, and instructions for removing nonrechargeable batteries that are unlikely to be used).
On backup battery applications, it is important to use blocking diodes to prevent current from the main power source from flowing into the battery circuit. For best results, use a low-current-leakage type of diode so that the charge capacity from a reverse-current leak does not exceed 1–2% of the nominal capacity. Make sure a battery is not connected in series after a power source as this will increase current flowing into the battery.
Components for Overtemperature Protection
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| Figure 3. Example of a battery protection circuit (click to enlarge). |
An overtemperature protection device operates in single-fault condition and should be rated to interrupt the maximum voltage and current of the circuit in which it is employed. Although overtemperature protection devices can be designed to be self-resetting, in self-test IVD medical equipment, such devices are not allowed. Negative temperature coefficient devices, used as temperature controllers, should comply with either IEC 60730-1 or UL 1434.
Components for Overcurrent Protection
Devices that provide overcurrent protection prevent excessive current from causing any interruption to the current flow in a circuit.4 Overcurrent protection devices include fuses, positive temperature coefficient devices (PTCs), power line feed resistors, and flameproof resistors. If a modem is included in IVD electrical equipment, the overcurrent protector should be a series component placed in front of the overvoltage protector on either the tip or ring for closed-loop applications, and on both the tip and ring for grounded applications. IVD electrical equipment should be protected by fuses, circuit breakers, thermal cutout impedance limiting circuits, or similar means.
Standards requirements for fuses include IEC 60127, UL 198G, and CSA 22.2 No. 59-1972. IEC 60127 requires fuses to withstand currents of 120% of related current. UL and CSA guidelines specify that the fuse must open with load conditions of less than 110% of the rated current. The time–current characteristics of fuses are known by their abbreviations and color codes (see Table II). Low breaking capacity is in the order of magnitude of 30 to 200 A, while high breaking capacity is typically measured from 1500 to 15,000 A. (In the event of a fault, an inadequate breaking capacity can result in a persistent arc, causing serious damage.) The fuse current rating must be selected on the basis of fault-mode current and should be higher by 1.25 to 1.5 than the full-load current (rated input current for the equipment). The fuse voltage rating should be at least equal to the line voltage.
Fuse holders with fuses intended to be operator-replaceable must not permit access to live parts during replacement. Fuses should preferably be fitted in all supply conductors. In some equipment, the operation of the overcurrent protection device may need to be detected and indicated.
Fuses in accordance with IEC 60127 should be marked with the rated current, voltage, and type of fuse. For example, a T1AL250V fuse is slow blow rated 1A/250V with low breaking capacity or F1AH250V (fast blow rated 1A/250V with high breaking capacity). The type and rating of any operator-replaceable fuses should be marked adjacent to each fuse holder.
It is recommended that fast-acting fuses (type F) in equipment have surge currents that are 10 times the full load current during the first 10 milliseconds of operation.2 PTC devices should comply with IEC 60730-1 and should be considered as a fixed (limiting) impedance.
Overvoltage Protection
Components that provide overvoltage protection include gas discharge tubes (GDTs), metal oxide varistors (MOVs), and transient voltage surge suppression (TVSS; avalanche breakdown) diodes. GDTs possess high-surge-current and low-capacitance ratings but their performance degrades with use. MOVs, which are clamping devices, present a high-surge current but have slow response times. TVSS diodes are suitable for low-voltage applications that do not require large amounts of energy to be shunted. These components should limit the surge voltage to a safe level for the circuit or component being protected and should be able to withstand several thousand volts of a fast dv/dt impulse. For electrostatic discharge protection, Schottky diodes, zener diodes, and ceramic capacitors can also be used.
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| Table II. Abbreviations and color codes for fuse time–current characteristics. (click to enlarge). |
For surge protection, a 500-W transient voltage surge suppression diode is typically adequate. A TVSS used in line-to-earth applications should comply with UL 1449. If not properly selected, a TVSS device used in the primary circuit could explode or create some other hazard.
Selecting an MOV depends upon its suitability for the operating voltage. Surge current, energy absorption, average power dissipation, as well as the maximum possible voltage rise must be considered (in the latter, voltage rise needs to be higher than the electric strength of the component or circuit that is to be protected).
Mains Capacitors
Capacitors (X) bridging operational insulation (connected between phase-phase or phase-neutral) should comply with X-capacitors requirements in IEC 60384-14.5.5 For capacitors with working voltages of up to 250 V rms, a supplementary protection device (i.e., fuse) can be used to protect the capacitor in the event of a short circuit. Capacitors (Y) bridging basic insulation (connected between primary and ground) should comply with the electric strength test for basic insulation (for a working voltage of 250 V rms, the test voltage should be 1500 V ac or 2121 V dc).
For electrolytic capacitors, an additional and separate layer of insulation (i.e., extruded tubing) should be provided. The thickness of this layer needs to be in accordance with the standard requirements for spacing.
Capacitors should not be connected between live parts and nonearthed accessible parts. Any capacitors rated higher than 100 nF must be marked with the voltage rating and capacitance. Capacitors should not be connected across thermal cutouts.
Motors
Motors must be tested to ensure that they do not create a fire hazard. IVD manufacturers should avoid motors approved for construction only since these motors have not been inspected for locked-rotor and running-overload protection. Only motors that are recognized as having impedance or thermal protections should be used.3 To evaluate the airflow rate (Q; volume flow) necessary for a fan, first determine the amount of heat generated inside the equipment (P). After deciding the permissible temperature rise inside the equipment (³T), calculate the airflow rate necessary for transfer of the heat with the following equation: Q (cfm) = (1.76 ¥ P) / ³T, where P is the internal heat dissipation in W, ³T is the allowable temperature in degrees Celsius, and airflow rate is in cubic feet per minute (cfm). The airflow rate can also be expressed as m3/hour = 1.6 cfm, m3/min = 0.028 cfm, m3/sec = 0.00045 cfm, or liter/sec = 0.47 cfm.2 Critical systems that incorporate fan cooling should be provided with a thermal warning or shutdown when loss of airflow is detected.
Emergency Stop
Types of emergency-stop devices include push-button, pull-cord, and pedal-operated switches.6 The devices must be of a self-latching type and should be positioned so as to be readily accessible by the operator and others who may need to operate them. They must be available and operational at all times, regardless of the operating mode, and must override all other functions and operations. The emergency-stop function must not impair the effectiveness of safety devices or devices with safety-related functions. The operation of an emergency stop should not depend on electronic logic (i.e., hardware or software) or the transmission of commands over a communications network or link.
Emergency-stop actuators may be mushroom-type push buttons, wires, ropes, bars, handles, or foot pedals without protective covers. They should be colored red with a yellow background, if possible. In certain circumstances, it may also be useful to provide labels.
Conclusion
It is important to select the proper safety components in IVD electrical equipment. Using nonapproved components in such equipment places the IVD manufacturer in a precarious position, especially when it comes to testing, documentation, and the ongoing conformity of the components. In other words, do not assume that the burden of safety testing rests on others. The equipment manufacturer must take complete responsibility for the end product and the components that go into it.
References
1. S Loznen, “Product-Safety Requirements for Medical Electrical Equipment,” Compliance Engineering 12, no.3 (1995): 17–30.
2. “Product Safety and Third-Party Certification,” in The Electronic Packaging Handbook, ed. Glenn R Blackwell (CRC Press + IEEE Press, 2000).
3. Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use, Part 1: General requirements, 2nd ed., IEC 61010-1 (Geneva: International Electrotechnical Commission, 2001).
4. Miniature Fuses, Part 1: Definitions for Miniature Fuses and General Requirements for Miniature Fuse-Links, 1st ed., IEC 60127-1 (Geneva: International Electrotechnical Commission, 1999).
5. Fixed Capacitors for Use in Electronic Equipment, Part 14: Fixed capacitors for electromagnetic interference suppression and connection to the supply mains, 2nd ed., IEC 60384-14 (Geneva: International Electrotechnical Commission, 1993).
6. Safety of Machinery. Emergency stop equipment, functional aspects—Principles for design, EN 418 (Brussels, European Committee for Standardization, 1992).
Steli P. Loznen, MSc, is the head of medical devices evaluation services at the Standards Institution of Israel (Tel Aviv). He can be reached at sloznen@sii.org.il.
Copyright ©2005 IVD Technology
