ABSTRACT
We are frequently in touch with electrical equipment of all kinds, ranging from domestic appliances and hand-held tools to industrial motors. We shall illustrate the need of eective equipment grounding by considering a single-phase circuit composed of a 230-V source connected to a motor M (see Figure 17.1). Note that neutral is solidly grounded at the service entrance. In the interest of easy understanding, we shall divide the discussion into three heads:
1. Ungrounded enclosure. Figure 17.1 shows the case of ungrounded neutral enclosure. If a person touches the metal enclosure, nothing will happen if the equipment is functioning correctly. But if the winding insulation becomes faulty, the resistance Re between the motor and the enclosure drops to a low voltage (a few hundred ohms or less). A person having a body resistance Rb would complete the current path as shown in Figure 17.1. If Re is small, the leakage current IL through the person’s body could be dangerously high. As a result the person would get electric shock which may be fatal. erefore, this system is unsafe.
