<P> A functional earth connection serves more than protecting against electrical shock, as such a connection may carry current during the normal operation of a device . Such devices include surge suppression, electromagnetic - compatibility filters, some types of antennas, and various measurement instruments . Generally the protective earth system is also used as a functional earth, though this requires care . </P> <P> Distribution power systems may be solidly grounded, with one circuit conductor directly connected to an earth grounding electrode system . Alternatively, some amount of electrical impedance may be connected between the distribution system and ground, to limit the current that can flow to earth . The impedance may be a resistor, or an inductor (coil). In a high - impedance grounded system, the fault current is limited to a few amperes (exact values depend on the voltage class of the system); a low - impedance grounded system will permit several hundred amperes to flow on a fault . A large solidly grounded distribution system may have thousands of amperes of ground fault current . </P> <P> In a polyphase AC system, an artificial neutral grounding system may be used . Although no phase conductor is directly connected to ground, a specially constructed transformer (a "zig zag" transformer) blocks the power frequency current from flowing to earth, but allows any leakage or transient current to flow to ground . </P> <P> Low - resistance grounding systems use a neutral grounding resistor (NGR) to limit the fault current to 25 A or greater . Low resistance grounding systems will have a time rating (say, 10 seconds) that indicates how long the resistor can carry the fault current before overheating . A ground fault protection relay must trip the breaker to protect the circuit before overheating of the resistor occurs . </P>

Earth conductor provides a path to ground for