Component Selection

This article examines what designers should look for when selecting components for ITE. It also discusses the necessary safety approvals for components and the documentation that designers should provide to the test lab. When selecting components, designers should watch for certain red flags. For example, when checking a manufacturer's literature or Web site, remember that the statement "designed to comply with X standard" does not necessarily mean that the component complies with that standard. A component must be properly approved to a standard that is harmonized with the product standard. Acceptable component standards are generally listed in each safety standard.

With regard to components, most safety standards state: "Where safety is involved, components shall be used in accordance with their specified ratings . . . and they shall comply with the applicable safety requirements of a relevant IEC (International Electrotechnical Commission) or ANSI (UL) standard as appropriate." The flowchart in Figure 1 provides a guideline for selecting suitable components.

Designers typically select components that have several agency approvals. Unfortunately, they often do not investigate further to determine which standard the components were approved to. For IEC-based safety standards, which are becoming universal, components incorporated in the design must be investigated and approved to IEC-based component standards or to the actual product standard that is being used for the end product.

Both the Canadian Standards Association (CSA) and Underwriters Laboratories (UL) publish standards for North America. To satisfy the requirements of standards that originate in North America, CSA- and UL-approved components are generally suitable without further investigation. However, the current trend is to harmonize North American safety standards with worldwide (IEC) standards, which means that many older North American standards are being replaced by IEC-based standards. For example, CSA and UL are replacing their standards for ITE (UL 478 and CSA C22.2 No. 220) and for telephone equipment (UL 1459 and CSA C22.2 No. 225), with IEC 60950 to produce what is now known as CSA C22.2 No. 60950 and UL 60950, 3rd edition.^1–7 These new harmonized safety standards require that components comply with IEC standards. Components may also need to comply with UL and CSA standards as noted in the individual country deviations listed in each standard.

When these two standards were harmonized with IEC standards, many components that were previously acceptable became no longer compliant. The most common reason is that the minimum spacings (creepage and clearance) between hazardous voltages and safety extra-low voltage (SELV) in the IEC-based standards are greater than the spacings required in the older North American standards. Components such as circuit breakers, line filters, relays, capacitors, and fuses are generally acceptable in harmonized standards if they have been approved to the applicable IEC-based standard. Other components such as optoisolators, motors, transformers, and resistors, which do not always have a specific IEC-based standard, may be required to be approved to another IEC standard such as 60950, 60601, or 60335 to be suitable.^8,9

Not all components are required to be certified to an IEC-based standard. As components become less critical in terms of safety to the user, fewer (if any) requirements apply. The term safety-critical components refers to components that are relied on to satisfy the requirements of the standard, including protection for the user and the surroundings; components that are purely functional are not considered critical in this sense. North American approval is acceptable for some components such as UL-recognized printed wiring boards (PWBs) and UL-recognized plastics. IEC approval of these components is not required.

The following section discusses some common components and their approval and construction requirements.

Main Disconnect Switch. A main disconnect switch is the switch that users depend on to turn off the equipment when there is a potentially unsafe condition. The disconnect switch marking can only be a bar and circle. The disconnect switch should not have a red actuator (red is reserved for emergency stops). However, when the switch is in the off position, it must have a minimum 3-mm air gap between the contacts. Most switches lack this 3-mm air gap. Specifications must be carefully examined in order to use a given switch for the main disconnect. Note that the disconnect device does not have to be a switch. Other disconnect devices may include the power cord or switch or circuit breaker installed near the equipment. All switches carrying hazardous voltages must be approved or tested to the appropriate standard.

Capacitors (Line-to-Line or Between One Line and Neutral). Such capacitors must be X1 or X2, and approved by one of the major agencies such as UL or VDE using IEC 60384-14:1993 requirements or the equivalent.¹⁰ An X2-approved capacitor is smaller than an X1 capacitor and can be used up to 250 V ac (see Figure 2).

Figure 2. Capacitor requirements between line and neutral.

Capacitors (Line-to-Ground). Line-to-ground capacitors must comply with IEC 60384-14:1993, subclass Y1, Y2, or Y4. Y2 and Y4 capacitors have a bodies coating that can be classed as basic insulation at the voltage rating of the capacitor. This insulation means that the body of Y2 capacitors used in hazardous voltage areas can touch grounded metal, but they can't make contact with SELV circuits. Y1 capacitors have a body coating that can be classed as reinforced insulation. The bodies of Y1 capacitors used in the primary can make contact with SELV circuits. Y-type capacitors must be approved by an agency such as UL or VDE.

Varistors. Varistors should be approved and should be used within a fire enclosure. Line-to-line varistors are acceptable provided that additional protective sleeving (see Figure 3) is used to protect against scattering of potentially hazardous debris in the case of a catastrophic failure or explosion. When used between line and neutral conductors, varistors must be placed after the mains fuse unless they are specifically approved for use without the protection provided by the mains fuse. If used between the line and neutral conductors, but before the mains fuse, varistors are only acceptable if the component is approved to an international standard such as IEC 61051 or CECC 42200.^11,12 Varistors are often certified to be protected by a maximum fuse value; varistor manufacturers can provide the required value of the fuse.

Figure 3. Varistor.

Varistors are semiconductor devices and, therefore, cannot be treated as high-integrity devices. They must always have additional protection. Varistors used in equipment or an installation that are coupled with safe construction and approvals meet these protection requirements unless the disconnection of earth is possible as a single fault. If a single fault is possible, the slow deterioration of the device with time, which allows an increase in earth leakage current, is not acceptable. Varistors, therefore, are not allowed in this type of construction.

Optoisolators. When serving as isolation, optoisolators are treated like transformers in that they must satisfy internal and external creepage and clearance distances, and dielectric strength requirements. The optical barrier has no specified thickness requirement, but the internal spacings must meet the minimum thickness specified for the insulation in question. If reinforced insulation is required, it usually means internal spacings must be a minimum thickness of 0.4 mm. Most agencies require that optoisolators be approved to VDE 0884 to ensure that the 0.4-mm thickness is met.¹³ The inside of an integrated circuit (IC) package can be considered Pollution Degree 1.¹⁴ Currently, there is no IEC standard for optoisolators, but the proposed standard is IEC 60747-5.¹⁵

Semiconductor Devices. Mounting ICs to earthed heat sinks may be suitable as basic insulation, but not as reinforced or supplementary insulation. The material thickness must be measured and fully described in the agency report for the end-use equipment. Approval is usually not required.

Fuses. Fuses are certified to various standards. The acceptability of a fuse in an end-use product depends on the standard used to certify the fuse. The characteristics of fuses certified to IEC 60127 are different from those for fuses certified to North American standards.¹⁶ If a fuse is employed to provide protection under single-fault (open or short) abnormal operating conditions, a fuse certified to IEC 60127 is acceptable, but safety instructions must specify that only that specific fuse be used. North American standards do not allow alternate fuses that have not been tested in the end-use product.

Fuses provided for fire-hazard and overload protection must be certified for use in the country of use. Conformity should be verified for the fire-hazard and overload protection requirements. The requirements are based on each calibration point of the IEC fuse according to the applicable IEC 60127 standard sheet. Similarly, it is critical to ensure that North American calibrated fuses provide protection across the range of interrupting (breaking) conditions. In North America, IEC fuses are not acceptable for protection of primary or secondary wiring for applications in which a fuse is required to provide protection equivalent to branch circuit protection. They are also not acceptable when a fuse is used to provide a Class 2 output.

IEC fuses with low interrupting capacity are only acceptable in primary or secondary circuits if the circuit's short-circuit current does not exceed the interrupting capacity of the IEC fuse. This condition must be verified. IEC fuses are identified with an L for low or an H for high to denote their breaking capacity. Hazardous live parts must not be accessible to operators who must replace fuses. All interrupting devices must have an adequate interrupt capacity for the supply circuit to which the equipment is intended to be connected. Interrupting devices must be able to handle the available fault current of the supply circuit. The same condition applies to circuit breakers.

Wires. The following information should be helpful for selecting certified wires. When the dielectric strength required is greater than 3000 V and the insulation thickness greater than 0.4 mm, use wire types TEW, REW (XLPVC, XLCPE), or TR-32 (600 V). When the dielectric strength required is greater than 1500 V and the insulation thickness greater than 0.4 mm, use wire types TXF, TXFW, TLW, TBS, SEW-2, or SEWF-2. Remember that all wires must be suitable for the actual temperature where the wire is located in the equipment. All of the wires listed above have different temperature ratings, so this factor should be considered when selecting wire.

All wire (including wire that is passed through a PCB) and other connections must be mechanically secure before soldering (see Figure 4). To meet the requirement of "no hazard after a single failure," all wire must be prevented from coming loose.

Figure 4. Wire restraint.

Earthing (or Ground) Wire. Ground wire must be green with a yellow tracer or yellow with a green tracer. Wire of this color scheme generally cannot be used elsewhere in the equipment. For Europe, the standard is a minimum of 30% green and 30% yellow. The typical ground wire used in North America has a very thin yellow stripe and therefore will be rejected by European test houses. The protective earthing connection point is identified by the symbol shown in Figure 5.

Figure 5. Symbol identifying protective earthing connection point.

A typical buildup for a grounding stud is shown in Figure 6. The protective earthing connection must be secured under its own nut and star washer. Other bonding connections can be added separately. The protective earthing conductor cannot be mounted on a removable component or on a fastener used for a component.

Figure 6. Ground stud construction.

Power Supplies and Dc-Dc Convertors. The selection of an appropriately approved power supply is critical. For IEC 61010-1, a power supply approved to IEC 60950 is generally acceptable.¹⁷ Power supplies should be approved to at least the second edition of IEC 60950 with amendments 1, 2, and 3. For power supplies approved by European test houses, look for the following standard and typical year information on the license copy for the power supply being considered: EN 60950:2000 or EN 60950:1992, including A1 (1993), A2 (1993), A3 (1996), and A4 (1996).^18,19 The third edition of IEC 60950 is the current edition. It is now being used by CSA, UL, and many European agencies.

In all cases, the conditions of acceptability for each agency approval as well as the proof of agency approval (i.e., license copies) must be obtained. These can be requested from the power supply manufacturer. CE marking by itself is not recognized or accepted as an indication of approval. CE self-declaration, which is done in most cases without third-party verification, is also not an indication of approval. Power supplies with only a self-declaration are considered not approved and will require a complete evaluation.

Motors. The declared pollution degree for the overall product will also apply to the internal environment of the motor for the purpose of spacings evaluation. Motors must be approved. A locked-rotor test should have been conducted; otherwise, this test must be done during the end-product evaluation. Although conducting this test later is not a big issue for small dc motors, it is more critical for larger ac motors that require an 18-day test. Be careful when looking at the approvals for a motor; often, the approval covers only the motor's construction and does not include the locked-rotor test.

Transformers. Transformers must meet the creepage and clearance requirements for the pollution degree and insulation type required by the product standard. Figure 7 illustrates a typical method of obtaining adequate creepage distance by using what is often referred to as a VDE bobbin. This term came into use because VDE has required this type of construction for years. More recently, a variety of sizes of triple-insulated magnet wire (listed in Annex U in IEC 60950) have become available. These triple-insulated wires can be used to meet the requirements of reinforced insulation and can be used without the need for additional spacings.

Figure 7. Transformer construction.

Overtemperature Protection Devices. Overtemperature devices include those operating in single-fault condition. These devices must be constructed, tested, and approved so that reliable function is ensured. They must be rated to interrupt the maximum voltage and current of the circuit in which they are employed.

Mains Voltage-Selecting Devices. Mains voltage-selecting devices must be constructed and located so that a change from one voltage to another or from one type of supply to another cannot occur accidentally. All such switches must be approved for the voltage, current, and spacings required by the application.

Lights and Indicators. As mentioned earlier, the color red is reserved for emergency switches and must not be used for the mains disconnect switch. Panel lights and other indicators can be any color without restriction as long as they do not cause confusion. For Europe, light-emitting diodes are subject to testing for harmful radiation according to the requirements of IEC 60825-1. Testing of LEDs is not required in North America.

Relays. An inadequate creepage or clearance distance within a relay is probably the most common problem encountered when evaluating components in a product design. It is important to remember that relays are usually providing isolation, so they are subject to the same examination as any other isolation component within a product. Approval is necessary, and internal spacings usually need to be evaluated. Ensure that the relay has been evaluated to an IEC standard to verify that the internal spacings comply with those required by the harmonized standard.

Printed Wiring Boards. It is generally understood that spacings on a PWB need to be evaluated, but a common mistake is that designers attempt to use solder masks to reduce the spacings requirements when the masks have not been evaluated for that purpose. Spacing is measured from the edge of the metal solder point, not the edge of the component pin (see Figure 8). Components must be physically mounted in such a way that no hazard can arise in the event of the device failing catastrophically.

Figure 8. Component restraint.

Be sure to check every component for adequate approvals to cover the ratings of the product and make sure the specific product standard is suitable. Do not assume it complies just because it has an approval mark. Use the following resources to help verify approval information:

Following the advice provided in this article can help ensure a much more satisfying product approval experience.

References