CHAPTER
10 Testing for Residual Current and Isolating Its Sources
I
n a perfect world, alternating current does just that: it alternates back and forth, and it
does so evenly, between the hot and neutral conductors (or in the case of 240 VAC,
between the L1 and L2 conductors). In effect, the current coming down the hot wire
should be exactly equal to the current flowing back down the neutral conductor. The cur-
rents cancel each other out because they are perfectly balanced.
On many AC appliances, however, there is always a small amount of current leakage,
also called residual current, which finds its way into the grounding conductor. For example:- appliances with motors, such as refrigerators and air-conditioning units
- appliances equipped with line noise filters, such as battery chargers and stereo
amplifiers - any device that uses power factor correction circuitry, such as, again, refrigerators and
air-conditioning units (see Chapter 5)
Additionally, as appliances age, materials such as heating elements and the winding
insulation in motors and generators gradually degrade. These conditions also cause resid-
ual current.
In this chapter we will discuss both sources of AC leakage and how to test for it using
two instruments: a clamp-on AC leak tester and a megohmmeter.AC Leakage from Appliances
Residual current from appliances is a normal situation and not dangerous as long as the
level of leakage is maintained at an extremely low level. The International Electrotechnical
Commission (IEC) limits acceptable leakage to 0.01 mA to 0.75 mA, depending on the
specific appliance. In the United States, Underwriters Laboratories specifications emulate
the IEC standard. Suffice it to say that acceptable leakage is limited to extremely low levels.
However, with multiple appliances on board, the normal leakage from each appliance
adds up, and this cumulative leakage ends up traveling along the grounding conductor.
Low-level AC current leakage begins to occur, allowing yet more AC current into the
grounding system. This is where the danger begins. This leakage is often well below theCopyright © 2007 by Edwin R. Sherman. Click here for terms of use.