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Telephone system central offices contain large battery systems to provide power for customer telephones, telephone switches, and related apparatus. [2] Terrestrial microwave links, cellular telephone sites, fibre optic apparatus and satellite communications facilities also have standby battery systems, which may be large enough to occupy a separate room in the building.
hazard lights switch 85c hazard sound on Switches 81 opener 81a 1 out 81b 2 out 82 lock in 82a 1st out 82b 2nd out 82z 1st in 82y 2nd in 83 multi position switch, in 83a out position 1 83b out position 2 Relay; 85 relay coil - Important if relay coil has flyback diode in parallel 86 relay coil + Important if relay coil has diode in parallel
First-generation wiring systems became susceptible to abuse by homeowners who would replace blown fuses with fuses rated for higher current. This overfusing of the circuits subjects wiring to higher levels of current and risks heat damage or fire. Knob-and-tube wiring may also be damaged by building renovations. [11]
A somewhat similar system called "concentric wiring" was introduced in the United States around 1905. In this system, an insulated electrical wire was wrapped with copper tape which was then soldered, forming the grounded (return) conductor of the wiring system. The bare metal sheath, at earth potential, was considered safe to touch.
Blue, previously used as a phase colour, is now the colour for neutral. Black, which was previously used for neutral, now indicates a phase. Household wiring does not usually use three-phase supplies and the clash only occurs in three-phase systems. Wiring to the old standard can be detected by use of a red wire.
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The Carter system, also known as the Chicago system, was a method of wiring three-way switches in the era of early knob-and-tube wiring. This now-obsolete wiring method has been prohibited by the USA National Electrical Code since 1923, [ 2 ] even in new knob-and-tube installations which are still permitted under certain circumstances.