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Electrical Noise
Effects of electrical noise on the electronic equipment are greatly reduce when the techniques
outlined below are closely followed.
1.
Do not run low power control signals and high power wiring in the same raceway.
NOTE: If mixing wire cannot be avoided, then the low voltage control input and output
wiring must be shielded. The shield for these wires should only be connected to ground only
at the source end of the signals.
2.
Never connect both ends of a shielded cable to ground. This may cause a ground loop
condition which could be very difficult to locate.
3.
It is suggested that all the wires in the system be kept as short as possible.
Magnetic Coil Noise
In the case of DC relays, a diode is installed across the coil in a direction that will cause the
voltage transient to be dissipated through the diode.
Figure 15
Magnetic Coil Noise (DC Line)
In the case of AC coils a capacitor and resistor are installed across the coil to supress the
unwanted transient.
Figure 16
Magnetic Coil Noise (AC Line)
The specific values of resistance and capacitance may vary depending on the inductance of the
coil. Consult the relay manufacturer for the proper values to use. These suppressor networks
greatly extend the life of the contacts controlling the coil because the transient energy, which can
easily reach 1000 volts, shunts through the suppressor’s rather than arcing across the controlling
contacts as they open.
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