It's not safe unless it's tested

Electronic products should never expose users to shock hazards. Fortunately for your product's users, regulations specify that your products pass numerous electrical-safety tests before they can receive an official compliance marking and be released to the market.

Most companies hire a certified test lab to perform the necessary compliance tests, but some choose to achieve certifications to perform their own compliance tests (see "Bringing safety in house "). Before sending a product to a lab, you may want to perform precompliance tests that raise your confidence level and help you uncover and fix problems before incurring the costs of formal testing. Many companies also test all production units prior to shipping.

Tests for leakage current, for example, let you verify that users won't feel a shock. Ground tests give you confidence that your product won't harm anyone in case of misuse. Insulation-resistance measurements reveal potential problems in insulating materials, and hipot tests ensure that every unit your company produces can withstand high voltages without compromising its safety integrity. (Some products also must withstand harsh environmental conditions while maintaining their electrical safety integrity; see "Beyond electrical tests ".)

Figure 1. The tester simulates a person's body resistance and measures the EUT's leakage current.

"Leakage current is fundamental to UL and IEC standards," says Ken Boyce, assistant global chief engineer at UL (Northbrook, IL). "Current shouldn't flow from a product through a user's body."

Because a person can feel as little as 0.5 mA of current, you must measure the current that leaks from the AC mains, through your product, and to the user. Leakage testers measure a product's leakage (or "touch") current by simulating the resistance of a human body (typically 1000 Ù or 1500 Ù shunted by 0.15 µF). They measure the current that flows from a product's case, through the meter's body impedance-modeling network, to ground.

Products that use a grounded (three-wire) AC mains plug are called Class I products. Those without grounds (two-wire AC cords) are called Class II products. Class II products require two forms of insulation between the power circuits and operator-accessible parts. Because they lack a safety ground, Class II products typically must not leak more that 0.25 mA. Class I products may leak up to 0.5 mA and still comply with regulations, because the ground wire will short higher levels of current away from the user.

To successfully complete a line-voltage leakage test, you must test your product under the conditions specified in the standards that apply to it. (Body impedance models, test procedures, and limits vary among standards.) Some standards require tests at 110% of rated AC mains voltage (132 VAC for a product rated at 120 VAC). They require you to reverse the polarity of the hot and neutral AC mains lines and test the product with the hot and ground connections only. Figure 1 shows a typical test circuit, in which switch S2 reverses the hot and neutral polarity, switch S1 opens the neutral line, and switch S3 opens the ground wire.

The ground wire of a product's AC mains line must protect users from faulty currents. Therefore, a ground lead must provide a low-impedance path, and it must withstand high current. Ground continuity tests tell you if a product's ground network is connected to Earth ground. To measure ground continuity, you can use either a safety analyzer or a DMM set to measure resistance. A ground connection's impedance between the farthest point from ground to the AC mains ground pin should be less than 0.1 Ù.

A ground-continuity test indicates whether a product is connected to ground, but it doesn't indicate the current-carrying capacity of that ground connection. A ground-bond test verifies a product's ability to divert fault currents to ground. Ground-fault currents can occur from a miswired or misused product.

Figure 2. A ground-bond tester forces high current through an EUT and measures the impedance between the product and Earth ground.

Makers of Class II products such as appliances often measure insulation resistance. Insulation-resistance tests performed during a product's design phase and on samples over time can tell you if operating your product causes its safety to deteriorate. To measure insulation resistance, you subject a product to a voltage, typically 500 VDC, and measure the current to calculate resistance.

Dwayne Davis, technical service manager at Associated Research (Lake Forest, IL) says "EN 60204-1, the standard for machinery safety, states that for test voltages at 500 VDC, the minimum insulation resistance should be 1 MÙ. EN 60335-1, the standard for safety of household and similar electrical appliances, states that for test voltages at 500 VDC, the minimum insulation resistance should be 5 MÙ."

You can measure insulation resistance with a megohmmeter and an insulation-resistance tester. Some hipot testers also measure insulation resistance.

Dielectric withstand tests, often called high-potential ("hipot") tests, verify that a product's insulation can withstand high voltages. All electrical safety standards require a hipot test on design prototypes and, in many cases, on production units. Figure 3 shows test circuits for both Class I and Class II products.

Figure 3. Hipot tests subject two-wire devices (top) and three-wire devices (bottom) to voltages far in excess of those in normal operation.

You can also perform hipot tests with DC voltages. If you use DC, then multiply the AC voltage by 1.414 to find the voltage setting needed to test your product. Most standards also require a stepped test voltage, which allows time for a product's internal capacitance to charge. Only after the internal capacitance charges can you measure the leakage current. If you apply the test voltage too soon, the capacitance will cause the EUT's current draw to exceed the test limit, causing a false-negative result. The measured current represents the normally drawn current plus any leakage currents in the EUT.

Once you are satisfied that your product has passed its precompliance tests, you can send it for formal testing. If a certified testing lab confirms that your product passes all required electrical safety tests, then you can affix the appropriate compliance label to your product.