Why Three Prongs? (1996)
For those outside the US, or generally confused how US domestic power distribution works, let me explain in the context of the 3 prong plug and NFPA 70 NEC (National Electrical Code) for the lay reader. We&#x27;ll start outside the house and go all the way to the plug:<p>0. Before the transformer secondary on the pole outside the house, the wiring is dictated by the NESC, which is like the NEC for utilities. We&#x27;ll leave that there.<p>1. Most US homes have a single phase, &quot;center tapped&quot; transformer secondary. This gives three voltage potentials to feed the home: (Nominal) 0V - from the center tap; +120Vrms - from one &quot;end&quot; of the transformer; -120Vrms from the other end of the transformer. And, yes, I am glossing over the phase relationship here. All three potentials are connected through the chunky copper of the transformer, which provides a current path to complete the circuit.<p>2. The three potentials feed the house meter and disconnect. The 0V potential may, or may not, be insulated on the path back to the transformer secondary, and often doubles as the mechanical support for the incoming lines (the &quot;service&quot;). The other two potentials will be insulated wiring.<p>3. Inside the main service panel the +120V and -120V inputs are split between the breakers. Every other breaker is connected to +120V or -120V. Typically a breaker that spans multiple slots is connected from -120V to +120V. That is the &quot;240Vrms&quot; for ovens, hot water heaters, dryers, air conditioners, etc.<p>4. Adjacent to the metal bits that fan out the +120V and -120V is a strip of metal that allows multiple connections to 0V potential. Colloquially this the &quot;neutral bus,&quot; but in the NEC this is termed a &quot;grounded service conductor.&quot; All of the white wires from the circuits in the house (&quot;branch circuits&quot;) are tied to the spaces in this metal bus bar. This metal bar is also electrically connected to the 0V potential entering the house from the transformer center tap.<p>5. Also inside the main service panel there is strip of metal that allows connection for a number of &quot;grounds.&quot; It looks just like the &quot;neutral&quot; connection bar, but instead has a number of green wires attached to it. Colloquially these wires a called &quot;ground,&quot; but the NEC calls them &quot;Equiment Grounding Conductor&quot; or EGC.<p>6. Between the grounded conductor (&quot;neutral&quot;) bar, and the EGC (&quot;ground&quot;) bar there is a removable conductive link. In the main panel this link remains installed, but in secondary panels it is removed. The link is removed in &quot;subpanels&quot; to insure correct operation in the event of a fault. Fault conditions are discussed below.<p>7. Also connected to the EGC and grounded conductor is a third wire. The NEC terms this the &quot;grounding electrode conductor&quot; or GEC. The GEC then goes to a water pipe or &quot;grounding&quot; rod(s). Thus the GEC is the connection between physical Earth voltage potential and the power panel. It is unfortunate that the EGC and GEC are such similar abbreviations.<p>In the event I&#x27;m out of characters, I&#x27;ll pause and reply to myself now.