Friday, May 30, 2008

Utilities (Electrical Systems): Wiring System

- The term includes all devices that are normally installed in wall outlet boxes, including receptacles, switches, dimmers, and pilot lights.

- Identified by number of poles and wires.
- Special types such as explosion proof and specific usage types such as range receptacles are available.

Switch devices:
- Switches up to 30A that can be outlet-box mounted fall into this category.
- Normal constructions are single-pole, 2-pole, 3-way and 4-way.
- Operating handles are toggle type, key, push, touch, rotary and tap-plate types.
- Programmable switch is available which can readily program to switch the controlled circuit or device at preset times.

Outlet and Device Boxes:
- Generally of galvanized stamped sheet metal. PVC now available.
- Most common sizes are the 4” square and 4” octagonal boxes for fixtures and junctions.
- 4” x 2-1/8” box used for single devices where no splicing is required.

- emergency systems are used to supply electric power to equipment essential for safety to human life, upon interruption of normal supply
- included here are illumination of areas of assembly to permit safe exit.

Standby systems are divided into two categories: required and optional.
- The former are intended to power systems whose stoppage might create hazards or hamper fire-fighting operations.
- Optional systems are at the discretion of the owner intended to protect property and prevent financial loss.

An engine-generator set comprises three components: the fuel system, the set itself, and the space housing the unit.

Battery equipment used only to supply limited amounts of emergency power, primarily for lighting.

Grounding Electrode System:

The grounding electrode system is a method by which the neutral and grounding conductors are connected to the common "earth" reference. The connection from the electrical system to the grounding system is made in only one place to avoid ground loops.

The grounding electrode system is _not_ intended to carry much current. Ground faults (Ie: hot to grounded case short) are conducted down the ground wire to where it is interconnected with the neutral and hopefully the breaker/fuse trips. The grounding electrode does not participate in such a situation. While the conductors involved in this are relatively large, they're sized for lightning strikes and other extremely short duration events. The grounding electrode system is specifically _not_ expected to have enough conductivity to trip a 15A breaker.

The grounding electrode often has a moderately high resistance. For example, according to the NEC, an acceptable ground electrode system may have 25 ohms of resistance – only 5A at 120V, not enough to trip a 15A breaker.

A grounding electrode system usually consists of a primary grounding electrode, plus possibly a secondary electrode. A primary electrode can be (if in direct contact with the earth): 10' of ground rod. 10' of well casing or metallic water pipe (must be connected within 5' of pipe entrance to house). 20' of copper wire buried in the bottom of the footings. A secondary electrode will be required if the primary is a water pipe or (NEC) if the primary electrode is >25 ohms to the dirt.

Surges, spikes, zaps, grounding and your electronics:

Theoretically, the power coming into your house is a perfect AC sine wave. It is usually quite close. But occasionally, it won't be. Lightning strikes and other events will affect the power. These usually fall into two general categories: very high voltage spikes (often into 1000s of volts, but usually only a few microseconds in length) or surges (longer duration, but usually much lower voltage).

Most of your electrical equipment, motors, transformer-operated electronics, lights, etc., won't even notice these one-shot events. However, certain types of solid-state electronics, particularly computers with switching power supplies and MOS semiconductors, can be damaged by these occurances. For example, a spike can "punch a hole" through an insulating layer in a MOS device (such as that several hundred dollar 386 CPU), thereby destroying it.

The traditional approach to protecting your electronics is to use "surge suppressors" or "line filters". These are usually devices that you plug in between the outlet and your electronics.

Roughly speaking, surge suppressors work by detecting overvoltages, and shorting them out. Think of them as voltage limiters. Line filters usually use frequency-dependent circuits (inductors, capacitors etc.) to "tune out" undesirable spikes - preventing them from reaching your electronics.


Junction Box
A junction box is a box used only for connecting wires together. Junction boxes must be located in such a way that they're accessible later. Ie: not buried under plaster.

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