ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 188)

are therefore relatively insensitive to electromagnetic disturbances, or aerial cables,

in which case they are more sensitive to electromagnetic disturbances but much less

so than inside cables that are subject to disturbances close to those of various

domestic devices.

Table 8.8 summarizes the results obtained for various PLC technologies.

Interference

The interference notion is essential in PLC networks. The PLC signal that propa-

gates over electrical wirings causes electromagnetic emissions in the 1 to 30 MHz

frequency band in the cables’ immediate environment and is itself disturbed by the

electrical devices connected to the electrical network.

In addition, a link between two PLC stations does not necessarily have the same

characteristics in both communication directions. The physical characteristics of

the communication medium (impedance, charge, capacity) can therefore change

according to the signal propagation direction.

The various national, European, and international standardization bodies have

set up regulations intended to determine the electromagnetic emissions limits for

PLC devices operating over an electrical network. As we saw in Chapter 1, the elec-

tromagnetic emissions of these devices must remain less than a set maximum

quasipeak value. This PSD (power spectral density) boundary value has been

defined by the IEC CISPR 22 amendment as being –50 dBm/Hz.

Effects of Interference on the Electrical Network

The PLC network is subject to interference and electromagnetic disturbances origi-

nating from the electrical devices connected to the network outlets.

Figure 8.14 illustrates the disturbance sources that a PLC local area network

can receive.

The use of electrical devices and their actuation generate various noises (broad-

band, impulse, Gaussian, and so forth) that can be evaluated to an average noise of

amplitude 30 dBìV/m over the entire 1- to 30-MHz frequency band.

It is difficult to make an exhaustive list of devices generating these noises, but

many devices have been identified as potential sources: plasma displays, halogen

lamps, vacuum cleaners, light regulators, microwave ovens, television sets, com-

puter screens, air conditioning, heating appliances, and so forth.

Figure 8.15 illustrates the various disturbances of electrical devices as a mean

value of various measurements performed on many domestic installations accord-

ing to the day hours. The end of the day is obviously loaded with disturbances since

many devices simultaneously operate on the electrical network. In the figure, we see

that the disturbance amplitude varies according to the frequency, with two higher

amplitude peaks being around 10 MHz and 20 MHz.

The technologies have greatly improved to ensure the robustness of data com-

munications over electrical wirings, but it may be necessary to take some precau-

tions with some electrical devices like halogen lamps or vacuum cleaners connected

to the same outlet as a PLC device.

Interference 169

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 188