Ground leakage is the current that reaches the ground through an unintended path. There are two categories: unintentional ground leakage caused by insulation or equipment failure and intentional ground leakage caused by the way the equipment is designed. “Design” leakage may seem strange, but sometimes it is inevitable-for example, IT equipment often generates some leakage even if it is working properly.
Regardless of the source of the leakage, it must be prevented from causing electric shock. This is usually done by using RCD (leakage protection device) or RCBO (leakage circuit breaker with overcurrent protection). They measure the current in the line conductor and compare it with the current in the neutral conductor. If the difference exceeds the mA rating of the RCD or RCBO, it will trip.
In most cases, the leakage will work as expected, but sometimes the RCD or RCBO will continue to trip for no reason-this is “annoying trip”. The best way to solve this problem is to use a leakage clamp meter, such as Megger DCM305E. This is clamped around the wire and the neutral conductor (but not the protective conductor!), and it measures the ground leakage current.
To determine which circuit caused a false trip, turn off all MCBs in the power consuming unit and place the ground leakage clamp around the power cable. Turn on each circuit in turn. If it causes a significant increase in leakage, this is likely to be a problematic circuit. The next step is to determine whether the leak was intentional. If so, some form of load spreading or circuit separation is required. If it is an unintentional leak—the result of a failure—the failure must be found and repaired.
Don’t forget that the problem may be a faulty RCD or RCBO. To check, perform an RCD ramp test. In the case of a 30 mA device-the most common rating-it should trip between 24 and 28 mA. If it trips with a lower current, it may need to be replaced.