Cutler-Hammer D320 PLC, User Manual

Page: 195 / 258

Chapter 8: Troubleshooting Noise Problems
183
Sources of Noise
There are three main sources of noise. Some of these sources generate large noise amplitudes. The
occurrence time can be very short (impulse type) or continuous (power line induced). Some noise
levels can damage the D320 PLC components and peripheral devices.
1. Noise Generated by Electronic Equipment
All electronic devices radiate noise in the form of a magnetic field. The magnetic field is
created around the printed circuit board or the wiring of electronic devices due to the flow of
electrical current. The amplitude of the magnetic field changes over time due to changes in
the flow of the electrical current. The magnetic field strength increases as the amount of the
electrical current flow increases.
As a device crosses the magnetic field, electrical currents will be induced. The induced
current could be summed vectorially with the normal electrical currents. In some cases this
could cause cancellation of electrical current flow (essentially shutting down the circuit). In
other cases this could create large surge currents that cause severe damage to the circuit. In
most cases the summation of the currents cause errors in readout and control values. Some
sources of this kind of noise are relays, magnetic contactors, inverters, computer monitors,
and motors.
2. Noise from Power Cables
When various loads are connected to a single power source the current draw conditions and
impedance imbalance can cause unwanted noise. The noise created by these sources can
affect other devices connected to the power source, via spikes, sags, reflected high speed
switching noise, and ground pulse. This is the most frequent cause of noise in a PLC’s
environment.
3. Noise from Natural Causes and Work Practices
Lightning, welding, shared cable trays, “grandfather’d plant wiring,” and static electricity can
also be sources of noise.
In the first case, the noise is caused within the equipment and is called internal noise. In the second
case, the noise is caused by external factors and referred to as external noise. These two types of noise
may also be referred to as artificial system noise.
The noise caused by natural occurrences can not be prevented, but can be controlled. Precautions such
as good grounding techniques, surge suppressors, and burying cables underground can help minimize
the affect. This type of noise may be referred to as natural noise.