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In contrast to a simple light switch, which is a single pole, single throw (SPST) switch, multiway switching uses switches with one or more additional contacts and two or more wires are run between the switches. When the load is controlled from only two points, single pole, double throw (SPDT) switches are used.
Electrical equipment may be designed with a floating ground for one of several reasons. One is safety. For example, a low-voltage DC power supply, such as a mobile phone charger, is connected to the mains through a transformer of one type or another, and there is no direct electrical connection between the current return path on the low-voltage side and physical ground (earth).
An outlet is defined by the NEC as "a point in the wiring system at which current is taken to utilization equipment". [1] This definition includes receptacles, lighting, motors, etc. Ordinary switches control but do not consume electricity, and therefore are not defined as outlets in this sense.
A key switch 3-way socket has the switch incorporated in the lamp socket and requires no external wiring between switch and socket. This would be typical in a 3-way floor-standing floor lamp. A 3-way socket that is to be wired to a separate 3-way 2-circuit switch, is called a keyless 3-way socket. [4]
On the other hand double switching is inherently less prone to wrong side failures; it helps overcome short-circuit faults that are hard to test for. Partial double switching might double switch the lever controls, and the track circuits between one signal and the next, while single switching the track circuits in the less critical overlap ...
The reason for using two wires rather than four is simple economics—half the materials cost half as much to purchase and install. Note the use of the past-tense "cost," as installation of two-wire copper local loops for telephony was done primarily during the mid 20th century.
If circuit 2 is an audio system and circuit 1 has large AC currents flowing in it, the interference may be heard as a 50 or 60 Hz hum in the speakers. Also, both circuits have voltage on their grounded parts that may be exposed to contact, possibly presenting a shock hazard. This is true even if circuit 2 is turned off.
IEC 60364 Electrical Installations for Buildings is the International Electrotechnical Commission (IEC)'s international standard on electrical installations of buildings.This standard is an attempt to harmonize national wiring standards in an IEC standard and is published in the European Union by CENELEC as "HD 60364".