PRODUCT SAFETY SOLUTIONS
© 2017 Educated Design & Development, Inc. All rights reserved. Rev C
Page 4 of 19
B. How Is Leakage Current Measured?
To measure leakage current, the scenario described above must be simulated. The worst case exists when
the human body is the sole path for the leakage current. Therefore, when conducting this test the product is
energized with the grounding conductor disconnected, and all other possible ground paths isolated.
A meter representing the human body (Body Impedance Model) is then connected between the grounding
conductor of the product (or the enclosure) and the grounded conductor of the power system (neutral). The
grounded conductor is used rather than earth ground for several reasons including reliability/repeatability of
the test. (Distance from the ground point and other ground currents could affect the test). Some Standards
also require referencing both hot and neutral to simulate polarity reversal.
NOTE: In some field testing situations, where the product is reliably connected to ground through the
building structure (i.e. mounted to a grounded pad, or connected to grounded water pipes, etc.), it may be
appropriate to measure the leakage current in the grounding conductor or from any exposed surfaces to
ground. (Refer to “Field Testing Grounded Products”, Chapter 2, for additional details.)
During this test, the unit is energized and any switches within the product controlling primary power (and likely
to be operated in normal use) are opened and closed in all possible combinations. Since the amount of
leakage current is proportional to the supply voltage, this test is typically done at the product's maximum rated
input voltage.
C. The Human Body Model
As previously indicated, a meter with precision impedance correlating to the impedance of the human body
is used to represent the human body. Extensive tests have been done to approximate the impedance of the
human body. These tests have shown that in different environments, with the user subjected to different
physical situations, the body impedance differs. On average, the human body impedance is equivalent to
1500 ohms shunted by a 0.15 µF capacitor.
It should be noted that most of the body impedance is a result of skin resistance. Therefore, in cases where
a person’s skin is likely to be cut (such as in a hospital), the human body resistance model may be different.
Similarly, moisture lowers the skin resistance. Most Standards deal with these issues by lowering the leakage
current limit for hand held products and products that may be exposed to moisture. Furthermore, it has been
proven that the perception to shock (threshold of perception) is directly related to frequency and therefore,
different body impedance models may be used when testing products whose leakage current contains
significant high frequency leakage.
D. Leakage Current Limits
The limits for leakage current vary among Standards. Depending upon the environment the product is being
used in and assumptions concerning the user, different limits may be imposed. These limits are not only
imposed to reduce the risk of shock, but also to minimize the possibility of reaction by the intended user. This
is important to emphasize because many assume the limits are only imposed to minimize injury from electric
shock. Although currents in the low milliampere range can cause injury or death in children and some adults,
they can also cause involuntary reactions that cause injury. In other words, a person may touch a product
and receive an unexpected shock which, in itself, does not cause injury, but instead causes the individual to
react in a manner that results in injury.
