Lightning and Facility Grounding The basic reason for grounding is to keep lightning from entering a facility via the power conductors. This is accomplished by earthing one of the power conductors that enters a facility at the service entrance. This conductor is the neutral in three-phase power, or the grounded conductor in single-phase power distribution. The idea is to provide an earth path for lightning that is essentially outside of the facility. If the lightning path is inside a facility, there can be a real threat to life and property. The rules are the same whether the power conductors enter overhead or enter in underground conduit. Separate grounding (earthing) of one of the power conductors creates several safety issues that must be addressed. There are many conductors in a facility that are earthed, including plumbing, gas lines and building steel. Suppose there is an electrical fault to any one of these earthed items. If a breaker or fuse does not disconnect the power, there is a real chance of electrical shock. Unfortunately earth connections are rarely below 10 ohms and a fault to an earthed conductor might only draw 5 amperes. This current is not sufficient to trip a breaker. If some piece of plumbing is electrically “hot”, there is danger of electrical shock. The NEC thus requires that all facility conductors that could come in contact with power wiring must be electrically connected together. The fault path resistance must be milliohms, not ohms. This set of conductors is known as the grounding electrode system in a facility. When there is a fault to this grounding electrode system, a high current will flow, thus tripping a breaker or blowing a fuse. The NEC requires that there can only be one grounding electrode system in a facility. Splitting the grounding electrode system into two sections is against the law. A building that rests on earth must have all of its exposed conductors connected together and connected to earth at the service entrance. The neutral power conductor must also be earthed once and only once at the service entrance. It is illegal to earth the neutral a second time. The grounding electrode system may be earthed at many points. These rules must be followed or the fault protection system may be compromised.

Lightning and Facility Grounding

The basic reason for grounding is to keep lightning from entering a facility via the power conductors. This is accomplished by earthing one of the power conductors that enters a facility at the service entrance. This conductor is the neutral in three-phase power, or the grounded conductor in single-phase power distribution. The idea is to provide an earth path for lightning that is essentially outside of the facility. If the lightning path is inside a facility, there can be a real threat to life and property. The rules are the same whether the power conductors enter overhead or enter in underground conduit.

Separate grounding (earthing) of one of the power conductors creates several safety issues that must be addressed. There are many conductors in a facility that are earthed, including plumbing, gas lines and building steel. Suppose there is an electrical fault to any one of these earthed items. If a breaker or fuse does not disconnect the power, there is a real chance of electrical shock. Unfortunately earth connections are rarely below 10 ohms and a fault to an earthed conductor might only draw 5 amperes. This current is not sufficient to trip a breaker. If some piece of plumbing is electrically “hot”, there is danger of electrical shock. The NEC thus requires that all facility conductors that could come in contact with power wiring must be electrically connected together. The fault path resistance must be milliohms, not ohms. This set of conductors is known as the grounding electrode system in a facility. When there is a fault to this grounding electrode system, a high current will flow, thus tripping a breaker or blowing a fuse.

The NEC requires that there can only be one grounding electrode system in a facility. Splitting the grounding electrode system into two sections is against the law. A building that rests on earth must have all of its exposed conductors connected together and connected to earth at the service entrance. The neutral power conductor must also be earthed once and only once at the service entrance. It is illegal to earth the neutral a second time. The grounding electrode system may be earthed at many points. These rules must be followed or the fault protection system may be compromised.