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The neutral line from the pole is connected to a ground near the service panel; often a conductive rod driven into the earth. The service drop provides the building with two 120 V lines of opposite phase, so 240 V can be obtained by connecting a load between the two 120 V conductors, while 120 V loads are connected between either of the two 120 ...
The three conductors are usually coloured red, black, and white. The white serves as a common neutral, while the red and black each feed, separately, the top and bottom hot sides of the receptacle. Typically such receptacles are supplied from two circuit breakers in which the handles of two poles are tied together for a common trip.
Long-distance electromagnetic telegraph systems from 1820 onwards [a] used two or more wires to carry the signal and return currents. It was discovered by German scientist C.A. von Steinheil in 1836–1837, that the ground could be used as the return path to complete the circuit, making the return wire unnecessary. [2]
These are typically composed of a central rod made of fibre reinforced plastic and an outer weathershed made of silicone rubber or ethylene propylene diene monomer rubber . Composite insulators are less costly, lighter in weight, and have excellent hydrophobic properties. This combination makes them ideal for service in polluted areas. [10]
There should be a break in copper between this strip, and the main ground plane of the circuit. The two should be connected at only one point. This way, if there is a large current between connector shields, it will not pass through the ground plane of the circuit. A star topology should be used for ground distribution, avoiding loops.
A receptacle with a green dot is a so-called "hospital grade" device; The primary reason to use a hospital-grade receptacle at a patient bed location is to ensure that a receptacle with a greater contact tension is provided to minimize possibilities that an attachment plug supplying medical or life support equipment may be disconnected because ...
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This is called a "false" or "bootleg" ground and is a serious safety hazard [10]: 49 often undetected by common receptacle testers. [1]: 48–49 Replacing the obsolete receptacle with a GFCI receptacle is the safest alternative, other than installing a new cable from the main circuit breaker panel.