Leakage Current Test

To provide a margin of safety for the consumer, regulatory agencies usually require that a product exhibit a line voltage leakage current of less than 0.5mA. With some products equipped with 3-prong plugs and warning stickers, the permissible leakage current may be as high as 0.75mA, but the typical limit is 0.5mA. Since hipot tests are usually required for 100% of the units in a production line, and since hipot tests are more stringent, line voltage leakage tests are normally specified as design or type tests and not as production line tests. Line voltage leakage tests are typically required on all medical products as a production test.

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Line voltage leakage test are conducted with a circuit similar to that shown in Figure 17, measuring the leakage current 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, and then with no ground.

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The measurement of leakage current is a requirement for type testing of any mains powered product. A compliance laboratory or National Recognized Test Lab (NRTL) normally performs the type testing on a sample of products during the design phase. Once the type testing is complete generally no further leakage testing is required on a production basis with the exception of medial products. Leakage current measurements are routinely performed on the production line for medical products for safety reasons.

The frequency response in figure 12 b) relates to the low-pass filter in 12 a), i.e. the MD only. This is not the required frequency response of the voltage measuring instrument! The "b)" at the voltage measuring instrument does not point to figure b) but to the note "b)" below figure 12! It says: "Resistance ≥ 1 MΩ and capacitance ≤ 150 pF".

You have to be careful when choosing the voltmeter. The standard says, AC limits apply to currents having a frequency not less than 0.1Hz, cl. 8.7.3 b). This means that everything above 0.1Hz shall be considered AC, everything below shall be considered DC. When you perform leakage current measurements you have to distinguish between AC and DC components for some leakage current types, e.g. patien leakage current. You could just set the multimeter to AC to measure the AC component and DC for the DC component. But the multimeter has a certain internal cut-off frequency that you can look up in the multimeter specifications.
For the Fluke 87V I cannot really find this value, but the accuracy specifications start only at a frequency of 45Hz. This indicates, that in AC mode only frequencies higher than 45Hz are really measured. Lower frequencies are damped.

This might lead to different results, when different voltmeters are used.

Using a scope is s good idea but grounding might be a killer. Best idea is to use a battery driven scope, thas is isolated from the supply circuit of the measurement setup. But a scope has the same issue with the cut-off frequency when switching to AC coupling (we're using a scope with cut-off frequency of appr. 8Hz). Most scopes have a DC coupling mode, that includes the AC component. So you really see the whole signal. In order to do the standard compliant distinction of AC and DC component, you can either do post-processing with the required cut-off frequency of 0.1Hz. Or the scope has internal signal analysis functions (FFT...) to analyze the frequency components.

Some more point to consider regarding the supply network:

It important to isolate the measurement supply network from the actual mains supply by an isolating transformer. And you need a variable AC-source, because the standard requires to use 110% of the rated maximum supply voltage and the maximum rated supply frequency. IF the isolating transformer is after the AC-source (i.e. the AC source feeds the isolating transformer), you also have to measure the actual output of the isolating transformer separately, because of diffences between the input and the putput voltage of transformers, whoch usually are load dependent.

And another thing is important. You might need to have a second voltage availabe (for touch current with SIP/SOPs, F-type applied parts and if there are metal accessible parts that are not protectively earthed), where you can at least change the polarity independently from the supply voltage of the device under test! This voltage can be derived from the supply voltage, so you don't need a second variable source, but polarity must be changed independently.

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