ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Pobierz pdf (Strona 37)

This section introduces some of the physical properties of electrical wiring in

order to understand its capabilities (both advantages and limitations) for the trans-

mission of data.

Impedance

Electrical wiring is characterized by an impedance Z (the absolute value of the resis-

tive, inductive, and capacitive components of the elements in the electrical network).

It is not a fixed value. Devices are constantly being connected or disconnected from

the electrical wiring. This modifies the wiring’s impedance, making it difficult to

model the communication medium, and therefore the transmission channel.

Additionally, the impedance of a device can vary as a function of its operating

mode, speed, age, design, and so forth.

Studies have shown that the impedance of electrical devices powered by house-

hold electricity typically falls between 10Ω and 1 kΩ.

Capacitance and Inductance

The various devices connected to the electrical network all have a certain capaci-

tance and inductance with regard to the electric current (110V, for example) that is

present on the circuit, alternating at a line frequency of 50 or 60 Hz.

The inductance (L) of a circuit or electrical dipole, also called self-inductance, is

a value that expresses the inductive flux created by the electrical current passing

through it. The displacement of electric charges in a material having nonzero mag-

netic susceptibility (μ) creates a magnetic field (H) and a magnetic induction (B).

In the case of a material with a delimited surface, typically an electrical cable,

the magnetic field produced by the current passing through the circuit creates an

inductive flux. The inductance may be confined to the circuit or may interact with

another electrical circuit.

The inductance can be expressed as a function of the magnetic field (φ) and elec-

trical current (I) using the formula:

In the case of a sinusoidal voltage (as is the case for household electricity), this

equation is expressed efficiently using Ohm’s law as a function of the voltage (U),

current (I), and frequency (f):

2π

(expressed in henries)

The capacitance (C), also called capacity, of an electrical circuit is a value

expressing the potential energy stored in an electrical field created between two

adjacent conductive surfaces of opposite electrical charge.

This potential energy, or capacitance, is proportional to the electrical charge

stored by the electrical dipole formed by the two surfaces. This electrical charge can

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ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Strona 37