Safety Design and Certification for Test and Measurement Products Part 2

1203safeprod2

Part 1 of this article appeared in the November 2003 issue of EE.

It is essential that engineers understand the underlying principles of safety so that they can design safe products. Safety does not cover the performance or functional characteristics of the product. Instead, the evaluation only has to prove that the product meets the standards and doesn’t pose a hazard to the user.

1203safeprod2Designers should take into account the normal operating conditions of the equipment and likely fault conditions, consequential faults, foreseeable misuse, and external influence such as temperature, altitude, pollution, moisture, and overvoltages. Products must meet the standards and be inherently safe when possible, even after a single fault. Caution statements should not take the place of safe design.

Steps to a Safe Design

These steps to producing a safe design are presented in order of priority:

  1. Components—Identify safety-critical components with safety marks to confirm they meet U.S. and EU standards: UL/CSA for North America and VDE/TUV for Europe.

  2. Construction and Design—Meet all construction and design requirements such as insulation, PCB spacings, enclosures, labels and markings, wiring, materials, and user documentation.

  3. Testing—Products shall pass all relevant safety tests after components and construction are in order. Tests include dielectric withstand, ground continuity, temperature, and abnormals. Complete product safety evaluation and testing should be performed by qualified people to ensure conformity.

Safety Principles

Users must be protected from electrical hazards during normal product operation and after a single fault. User-accessible parts are protected through the use of various forms of insulation, enclosures, and other means. A part is accessible if it can be touched with a finger or pin.

Hazardous live voltage is >42.4 Vpk/60 VDC, which may cause shock. Working voltage is the highest rms value of AC or DC voltage that can occur across any particular insulation.

Safe voltage limits in normal conditions are 30 Vrms/42.4 Vpk/60 VDC in IEC 61010-1:1990 and 33 Vrms/46.7 Vpk/70 VDC per IEC 61010-1:2001. If hazardous live voltage is bridged to safety extra-low voltage (SELV), damage to the product could result, and the user may risk electric shock or burn.

Nonhazardous voltage circuits are everywhere and may be accessible to the user. These SELV circuits are <42.4 Vpk/60 VDC. Examples include accessible connector pins for printers, keyboards, and PCs that typically are SELV and considered safe to touch. For that reason, SELV circuits must be adequately insulated from hazardous live voltages to protect the user.

Basic Insulation2 Pollution Degree 2 Measurement Category II Double or Reinforced Insulation2 Pollution Degree 2 Measurement Category II
Working Voltage (rms or DC) up to Clearance Creepage on PCB (CTI >175) Creepage In Equipment (CTI >100) rms Test Voltage Working Voltage (rms or DC) up to Clearance Creepage on PCB (CTI >175) Creepage In Equipment (CTI >100) rms Test Voltage
50 0.2 0.2 1.2 350 50 0.2 0.4 2.4 510
100 0.2 0.2 1.4 490 100 0.4 0.4 2.8 740
150 0.5 0.5 1.6 820 150 1.6 1.6 3.2 1,400
300 1.5 1.5

3

1,350

300

3.3

3.3

6

2,300

600

3.4

3

6

2,200

600

6.5

6.5

12

3,700

1,000

5.5

5.5

10

3,250

1,000

11.5

11.5

20

5,550

Table 1.  Creepage and Clearance Per IEC 61010-1:1990

1. Table 1 is for illustration only.  Refer to IEC61010-1 for tables and actual values.

2.  Distances in millimeters;  PCB is not coated.  Conformal coating does not reduce distance.

Insulation is achieved through various forms such as barriers, grounding, or distance. Basic insulation is a single layer or distance and the first protection level. Supplemental insulation is several layers of insulation or a distance equal to two times basic insulation.

With double insulation, if there is a fault on one layer, the basic insulation still remains to protect the user. Reinforced insulation is a single body of insulation. It equals several layers of insulation or a distance equal to two times the basic insulation.

Figure 1 illustrates an insulation system where hazardous live voltage is separated from nonhazardous voltages by double insulation, and the enclosure is separated from hazardous live by basic insulation. If the enclosure were not safety grounded, double insulation would be required.

Safety-Critical Components

Safety-critical components may affect the safety of the product or user. Safety-critical components should comply with the relevant component standards and be rated for use per the end-product standard.

Safety-critical components typically are located in hazardous live circuits (>42.4 Vpk/60 VDC) with special focus on components that bridge double/reinforced insulation (SELV to 250/300 V). Examples are optical isolators, transformers, relays, fuses, and AC input components such as inlets, switches, terminal blocks, and power supplies.

UL and IEC component standards are not yet harmonized for most components. As a result, two safety marks may be required to verify compliance for the United States and Europe. Certification body marks are evidence that the component meets the standards.

With respect to safety, the CE Marking is for products, not components. Ignore the CE Marking when found on components.

Insulation

1203safeprodProducts designed with insulation between conductive parts adequately protect the user from hazardous live voltages. The minimum insulation (spacing) values depend on five factors:

  1. Measurement category.

  2. Pollution degree.

  3. Working voltage.

  4. Insulation: basic, supplementary, double/reinforced.

  5. Comparative tracking index (CTI).

Clearance is the shortest distance between two conductive parts measured through air. Creepage is the shortest distance between two conductive parts measured along a surface. Creepage always should be at least as large as clearance. Interpolation of creepage in Table 1 is permissible. Here are some examples of required insulation between circuits:

  • Double/reinforced insulation between hazardous live voltages (>42.4 Vpk/60V DC) and SELV (<42.4 Vpk/60 VDC).

  • Basic insulation between hazardous voltages and safety ground circuits/enclosures.

  • Double/reinforced insulation between hazardous voltages and an ungrounded metal enclosure.

  • Basic insulation within hazardous voltage circuits preferred.

Measurements on PCBs and parts are noted between the two closest conductive parts such as edges of pads around soldered connections on PCBs. In-equipment distances are spacings between parts not on PCBs, such as connector pins or pins on optical isolators.

The CTI is used to determine spacing distances (creepage) on PCBs, connectors, and other parts. CTI expresses the voltage that causes tracking across insulating materials. CTI values typically are specified in vendor specifications with the lowest CTI used when unspecified such as CTI >175 on PCBs and CTI >100 in equipment (not on PCB).

Using Figure 2 and Table 1, one evaluation method first determines the pollution degree (typically 2) and measurement category (CAT II) for the operating environment. A block diagram sections circuits as hazardous, SELV, and safety grounds:

  • Block 1 = Hazardous live voltage circuit section >42.4 Vpk/60 VDC.

  • Block 2 = SELV secondary circuit section <42.4 Vpk/60 VDC.

  • Block 3 = Grounds such as safety-ground traces and/or metal enclosure.

Measure the distances on PCBs and parts to see if the spacings meet the requirements. The closest distance between two parts should be at least as large as the value in the appropriate table in IEC 61010-1. Measurement examples are PCB trace-to-trace, connector pin-to-pin, optical isolator input-to-output pins, and ground trace/metal enclosure to live circuits.

Conclusion

With international standards becoming the de facto rules worldwide and consumer awareness of safety increasing, it is incumbent on manufacturers to understand and apply a sound safety design policy. By reading the standards and understanding safety concepts, product manufacturers will be better equipped to design products that comply with established safety norms.

To get started, use the information contained in this article along with the standards and the Safety Design Checklist (Table 2). It is important to remember the steps to a safe design are components, construction and design, and testing.

About the Author

David Lohbeck is a senior safety engineer at National Instruments. Previously, he worked for Motorola, Memorex, Dell, and TUV in the field of international product safety and EMC. Mr. Lohbeck is the author of the CE Marking Handbook: A Practical Approach to Global Safety Certification. He received a B.S. from Arizona State University and an M.A. from the University of Phoenix. National Instruments, 11500 N. Mopac Expressway, Bldg. C, Austin, TX 78759, 512-683-8474, e-mail: dave.lohbeck@ni.com

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December 2003

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