Disturbances in the common mode are more difficult to identify since the circuit closes through the exposed conductive part. The earth itself can also be a source of disturbance as shown in figure 9, where the disturbance is generated by voltage Ut due a fault current that flows through the exposed conductive part.
The higher the voltage Up of the source and/or the parassitic capacitance Cp, the higher will be the value of the stray current.
Figure 11: capacitive coupling circuit Figure 9: common mode disturbance circuit
1.3 Inductive coupling Inductive coupling occurs when the receiver pairs with the magnetic induction flow associated with the source. In this case, any coil formed by an electric conductor with surface area S will generarte voltage U at its ends. This happens frequently in switchgear for apparatus installed near power lines with high current running through them. Using the diagram in figure 9, this situation is illustrated in figure 10:
Figure 10: inductive coupling circuit
1.4 Capacitive coupling Capacitive coupling occurs owing to the effects of interaction between the receiver and the electric field of the source due to parassitic capacitances (see figure 11). This situation is also frequent in switchgear owing to the vicinity of the energized power circuit. It is important to note that the parassitic capacitance value is proportional to the surface area and inversely proportional to the distance between the two circuits.
1.5 Radiated coupling To assess the presence of this type of coupling, wavelength λ of the disturbance must be calculated using the following formula: c λ=— f where c is the propagation speed and f the frequency of the disturbance. The wavelengths of certain types of signals are given below.
Type of signal
Frequency
Wavelength λ
Grid voltage
50 Hz
6000 km
Telephone
4 kHz
75 km
HF musical
20 kHz
15 km
Lightning strikes
1 MHz
300 m
Television
5.5 MHz
54 m
Data (Ethernet)
100 MHz
3m
Modulated for GSM
900 MHz
33 cm
Data (optical fiber)
2.5 GHz
12 cm
Microprocessors
20 GHz
10.5 mm
Since this type of coupling occurs in relation to lengths comparable to the wavelength, it is not present at network frequency but affects overhead lines if lightning strikes in their vicinity and can be a serious problems for electronic boards.
