Developments in Electrical Safety Testing

The traditional electrical safety testers, with limited front-panel control and screwdriver adjustments, cannot compete with today's instruments, which can provide much greater functionality for the same or even less cost. These new devices offer easier, faster, and more-thorough testing, and can be especially beneficial for meeting the stringent requirements of medical device testing.

Electrical Safety Tests

The typical electrical safety tests performed on medical devices are hipot, leakage current, and ground continuity and bond tests.

Hipot Test. In the hipot test, often called the voltage breakdown or dielectric withstand test, a product's insulation is stressed beyond normal use. Hipot testing has been in use in various forms for decades. Many years ago, Underwriters Laboratories and other international agencies took a strong position in requiring that virtually any appliance or electronic product must undergo hipot testing before exiting the production line. A standardization of hipot testers gradually evolved, so two different testers will yield the same go or no-go results.

The standard hipot test to meet many agency requirements is to apply a test voltage that is two times the normal operating voltage plus 1000 V. This voltage is usually sinusoidal ac, but in some cases a dc voltage test can be used. For finished appliances with hard-wired power cords, the test voltage is applied between the high, or hot, and neutral conductors shorted together, and the power-line ground or exposed metal parts.

Leakage Current Test. The leakage test measures current flow from user-accessible parts to ground when a product is operating at normal voltage (see Figure 1). If this current is excessive, the user could receive an electrical shock. Because of the shock hazard, safety agencies have set standards for the maximum amount of current that may leak. Because hipot tests, which are more stringent, are usually required for all electrical products in a production line, leakage tests of most electrical products are specified as design rather than production tests. However, because of the sensitive nature of medical equipment, leakage tests are required on all medical products as production tests.

Figure 1. Setup for a leakage current test. An equivalent circuit of the human body consists of a 1-kW resistor with a 0.015-µF capacitor and 10-kW resistor. The leakage current is measured under various fault conditions, such as no ground, or with line and neutral connections reversed. Voltage is applied first with normal line and neutral connections, followed by a test with the connections reversed (S1), and then with no ground (S2).

Ground Continuity and Bond Tests. The ground continuity test verifies that a fault path exists between any exposed conductive metal surface and the power-line ground. Should a fault occur in the product where the power-line voltage is connected to a user-accessible surface, high current will flow through the connection to the power-line ground rather than through the user, tripping a fuse or breaker. The ground bond test identifies the current-carrying capacity of the path, which usually is 25 or 30 A. The continuity and bond tests are often prerequisites for proceeding to a hipot test. It is wise for a test operator to verify the ground integrity of the product before applying high voltages to it.

New Testing Features

Combining a host of new features, electrical testers are better equipped than ever before to meet the needs of medical device testing.

Programmable Test Parameters with Setup Storage. With the age of microprocessors, the programming of test conditions, such as current limit, voltage level, and ramp time, is easily placed in the hands of the test engineer. The ability to store these conditions in tester memory for later recall ensures that products can be tested the same way, time after time.

Digital Readout. With the change from analog meters to digital readouts, interpretation of results is no longer subjective. Clear, concise display of test conditions, such as programmed test voltage and current limit, and test results, such as actual measured leakage current, is routine.

Current Detection. New hipot testers impose a voltage on the device under test, sense the current, and compare this measured current to a user-programmed maximum limit. Should this limit be exceeded, the tester shuts down its high voltage and alerts the user to a device failure.

Most agency standards specify that there should be no excessive breakdown current. Since these standards do not refer to any maximum limit, limits of 5 or 10 mA are commonly used. Also, maximum leakage current alone is no longer the only criterion for pass or fail results. Although not specified in any safety standard, a minimum current detection can be just as important. After all, any device will pass the maximum limit of the hipot test if the tester never makes contact with the device. A small amount of current typically flows through the device and associated cables when properly connected to the tester. Using minimum current detection, it is possible to recognize the difference between the presence of this small current and no current at all, which indicates that the device is not properly connected.

Arc Detection. Although arc detection is not presently addressed in any safety standards, it is likely to be considered in the future because it can further minimize hazards or product failures. Some new testers have the ability to monitor current transients of a few microseconds in duration to identify an impeding fault that might appear later in a product's life.

Figure 2. Resistive and capacitive current.

Real and Total Current Distinction. When testing highly capacitive devices, it is often desirable to make a distinction between real and total current. Total current is the vector sum of resistive and capacitive leakage current (see Figure 2). If the tester monitors only the total current, a substantial change in real current can often go undetected. The ability to separate the real and capacitive currents is becoming an important requirement for ac hipot testing. In fact, some test requirements clearly specify the measurement of real rather than total current.

Sequential Testing. During electrical safety testing, it is often useful to be able to run a series of tests automatically. For example, a test operator may want to run the ground continuity and bond tests prior to any other tests or wish to run an insulation test, a hipot test, and then a second
insulation test. In the second case, the first insulation test provides a baseline resistance measurement. The hipot test stresses the device, and the second insulation resistance test determines the effect of the hipot test on the insulation. Sequential testing can be a very valuable tool in detecting latent product failures before the customer is exposed to them.

Operator Safety Features. Significant operator safeguards have been built into new hipot testers, such as voltage shutdowns, alarms, and indicators. Many testers have interlock terminals, in which the high voltages cannot be activated until the operator closes a cover around the test device or uses both hands to start the test. Current monitoring and fast voltage shutdown are also being employed in new testers to enhance operator safety. One feature involves the simultaneous monitoring of the current exiting a hipot tester and the current running through the device under test. If the voltages are different, the tester assumes that the operator has come in contact with the output voltage and shuts down.

Multiple Functions. Many testers today can perform multiple tests. In a high-volume environment in which only one measurement, such as ac hipot, needs to be taken, less-expensive, single-function units are the best choices. However, for environments in which several tests must be performed, testers that offer a range of tests, such as ac or dc hipot, ground bond, insulation resistance, and current leakage, are better solutions. Such multifunction units are also good choices for companies that, though they need only one test now, anticipate expanding test requirements in the future.

Safety analyzer with five test functions.

Automation of Results. When choosing a tester, part of the decision should be based on how the test results will be analyzed. In some applications, the only required analysis is the observation of the pass or fail light or buzzer at the conclusion of the test, but for others, test results may need to be transferred to a computer for traceability, further analysis, or archiving. Today's testers commonly have IEEE-488 or RS-232 interfaces to automate such data transfer.

Conclusion

James Richards is senior marketing engineer for QuadTech Inc. (Maynard, MA). He can be e-mailed at jrichards@quadtech.com.