Electrical Safety Testing: Types of Tests
Product safety standards contain three primary sets of safety compliance test requirements: (1) constructional specifications related to parts and the methods of assembling, securing, and enclosing the device and its associated components, (2) performance specifications or “type tests” – the actual electrical and mechanical tests to which the test device sample is subjected, and (3) production line tests which are required on all products.
The following tests are generally a subset of the performance test. The test methods and the pass/fail limits were established as a basis of providing a margin of safety in cases of misuse and expected single fault component failures.
A dielectric strength or hipot test determines the suitability of the dielectric or insulation barrier between hazardous and non-hazardous parts. A dielectric barrier is commonly required by all established safety standards between hazardous circuits and user accessible circuits or surfaces. The dielectric strength test is a fundamental method of ensuring that a product is safe before it is placed on the market.
The dielectric barrier protects the user from exposure to dangerous electrical potentials. The most common points of application for a dielectric withstand test are between AC primary circuits and low voltage secondary circuits, as well as between AC primary circuits and user-accessible conductive parts/ground. Confirming that the proper dielectric barrier exists between these areas verifies the existence of a level of protection from electric shock hazards under normal and single fault conditions. A dielectric withstand test (dielectric strength test) appears in nearly every product safety standard and is a fundamental test employed to check a fully assembled product as it exits the production line.
Insulation resistance measurements are generally conducted to determine the actual resistance between the two points of test. This test is similar to a DC hipot test except that it displays resistance rather than leakage current. It serves as a practical and effective method of verifying suitability of the product for use by the public. More details on insulation resistance testing.
All products that use an AC line source as power have some associated leakage current when the device is turned on and operating. This leakage current normally flows from the AC line source through the ground path in the product and back to earth ground through the ground blade on the power cord. On products without a ground blade or those in which the ground is malfunctioning, a potential can develop on metal surfaces of the product. If an individual then comes in contact with the exposed metal surface, this individual then becomes the ground path for the product.
Under this condition, a certain amount of leakage current flows through the person exposed to the metal surface. If the leakage current is extremely low, typically less than 0.5mA, the person should not notice he/she is in the path of the current flow. At levels higher than this, the person can experience a startle reaction or worse.
For this reason, products that do not use a ground on the power cord generally are limited to a maximum leakage current of 0.5mA or less. Products that exceed this level normally have a ground on the power cord to conduct the leakage current back to ground, thereby protecting a person who comes in contact with any exposed metal on the product.
Limits on leakage current are significantly less on medical products. The leakage current discussed here is different from the measurement of leakage current during a dielectric withstand or hipot test. During a dielectric withstand test, a high voltage generally greater than 1000V is applied between the hot and neutral lines and the ground of the DUT. The leakage current is then measured. In a leakage current test, the product is on and operating via standard line voltage, such as 120VAC. The leakage current is then measured using a special circuit that simulates the impedance of the human body.
A ground continuity test checks that a path exists between all exposed conductive metal surfaces and the power system ground. This ground circuit provides the most fundamental means of electrical shock protection for a user. If a fault occurs in the product that causes power line voltage to be connected to a surface a user might touch, a high current will flow through the connection to the power system ground, causing a circuit breaker to trip or a fuse to blow, thus protecting the user from shock. The ground continuity test is normally performed using a low current DC source (<1 Amp) to determine that there is a low resistance between the ground blade on the power cord and any exposed metal on the product.
A polarization test is a simple test that verifies that a product supplied with a polarized line cord (either a 3-prong plug or a 2-prong plug with the neutral prong larger than the other) is properly connected.
A ground bond test verifies integrity of the ground path by applying a high current, low voltage source to the ground path circuit, typically a 25 or 30A current. This test is similar to the ground continuity test with the additional benefit of verifying how a product will perform under actual fault conditions. When a ground fault occurs, current starts to flow through the ground circuit. If the current-carrying capacity is high enough and the circuit resistance low enough, the system operates properly and the user is protected from shock.
Production Line Testing
Most manufacturers perform production line tests as a means of ensuring overall product quality. If, however, a product carries an approval mark of an independent testing laboratory, that laboratory usually requires mandatory production line testing to make sure that the product continues to meet its requirements over a long period of time.
In the US, test laboratories typically require production line tests for dielectric withstand (hipot) and ground continuity. European agencies usually require a ground bond test in addition to the dielectric withstand and ground continuity tests. Approval agencies also require regular periodic calibration of the production line test equipment to make sure it meets their standards. They also conduct follow-up inspections on a regular schedule to verify construction of the product and the procedures used to test it. The manufacturer is normally required to keep calibration certificates and inspection documentation on file at all times.