Wiring For Trouble
Free
Signal Conditioning
Article written by Joe Wilkerson, President of
Wilkerson Instrument Co., Inc.
Previously published in European Process Engineer magazine
and In-tech magazine
Signal conditioning equipment for process signals has kept pace with
modern technology, but many users never realize the full potential of the
equipment because of poor installation and wiring practices. Such
practices can degrade equipment performance from a small percentage of
error to the point where the equipment is unusable.
The most common complaints heard by this company are invariably
associated with wiring problems. The most common problem encountered is an
installation where all wiring is pulled in a common conduit or
cable trough. A relay coil being switched or a relay contact switching an
inductive load can easily generate a transient in excess of 1000 volts.
This transient can easily couple into a signal conditioner and cause
severe measurement problems.
Many products have input signal levels as low as 5 millivolts full
scale and an output of 10 volts. With a gain of 2000, it is easy to
understand the necessity of using good installation and wiring practices.
Accurate signal measurement and conditioning is vital if one is to
maintain quality in their process control. Understanding the correct way
to install and wire this important equipment is the proper responsibility
of anyone who specifies control panels, wiring installations, or who
manages factory personnel who install such equipment.
DEFINITIONS OF SOME COMMON
PROBLEMS
Ground Loop - A ground loop exists whenever an extraneous
current flows in a conductor carrying the desired signal. If the
extraneous current is related to the power mains, it generally manifests
itself as "noise". If the extraneous current is DC, it can create an
offset that can be difficult to recognize. The measured signal becomes the
sum of the signal and ground loop current.
Magnetic Coupling - AC currents can be induced into the
conductors carrying the desired signal. The AC can be power mains,
magnetic transients from switching inductive loads, or magnetic fields
from coils, transformers, or motors.
Capacitive Coupling - AC voltages can be capacitively coupled to
the signal carrying conductors. This is accomplished by locating the
signal leads near an AC voltage source such as power mains, SCR drive
inputs or outputs, or any other source of AC potential.
Radio Interference - Radiation from radio transmitters can
create measurement problems by having some portion of the signal
conditioning circuit rectify the radio energy and adding the resultant DC
to the process signal. The radio energy source can be a portable two-way
radio or a fixed installation radio or television transmitter. The
interference can be momentary with portable radio equipment or a permanent
offset with fixed installations. AM radio transmitters can often modulate
the process signal with the voice or music modulation on the radio
carrier.
Lightning - Lightning interference is sporadic and the primary
concern is for survival of the equipment. Central Florida in the United
States has the most frequent and intense lightning storms in the country.
It is nicknamed the "lightning belt". Survival is the desire in this
environment.
INSTALLATION CONSIDERATIONS
The location of signal conditioning equipment is important for the
prevention of the common problems defined above. Avoid mounting equipment
next to high voltage sources such as breakers, fuse blocks, or terminal
strips. Also avoid magnetic field sources such as large transformers,
motor control relays, or motors. Wiring paths should also be considered
when mounting equipment. Insure paths exist to route signal wires without
having to run them in proximity to noise sources.
Ground Loop Avoidance - Avoiding a ground loop is simple. Never
let an extraneous current flow in the signal leads. If a device must drive
two or more loads with a voltage signal, use individual leads from the
output to the input of each driven device.
When the driven device and the driver must both connect to earth
ground, use an isolator to break the ground path for extraneous currents.
Magnetic Coupling Avoidance
Every effort should be made to keep signal leads away from AC magnetic
field sources such as motors, transformers, or large relays. If it is
necessary to route signal wiring near these devices, use a twisted pair of
conductors for each signal. A pair of conductors with ten to twelve twists
per foot offers an effective method of reducing magnetic pickup.
A major source of magnetic interference is created by running untwisted
signal leads parallel to and in proximity to conductors carrying AC
currents.
Twisting the signal leads and the power leads are an effective way to
reduce this form of magnetic interference.
Mounting signal conditioning equipment in powerful magnetic fields can
create interference inside the circuit boards and internally in the
integrated circuits used in the equipment. The most reasonable and
effective cure for this condition is to move the equipment away from the
magnetic field.
Capacity Coupling Avoidance
AC potentials near signal carrying conductors can capacitively couple
the AC to the signal conductors. The higher the frequency of the AC source
the more extreme the potential interference problem. Common sources of
interference are SCR drives, relay coil circuits, welders, and fluorescent
lamp conductors.
If the interference can not be avoided by equipment location and wire
paths, shielded wire should be used between products.
Twisted shielded wire is the best choice because the twist provides
magnetic interference protection and the shield prevents capacitive
coupled interference.
The shield should be connected to the circuit common of the driven
device. It should never be connected to both devices. Current could flow
in the shield and magnetically couple to the signal leads. If the signal
conditioner is mounted near a high potential source, capacitively coupled
interference can occur by direct coupling to components inside the
product. Metal housed products can be effective in preventing direct
coupling, but the preferred practice is to locate the equipment in a
better environment.
Radio Interference
Well designed products will include internal filters to desensitize the
product to radio frequency signals. There is no cure for direct radiation
pickup by components in the product except extensive shielding. Metal
housings with RFI gaskets can be used to shield the product. If the RFI
source is powerful enough, individual RFI filters may be required on each
conductor entering the metal housing.
Lightning
Lightning is basically radio interference except when a direct hit
occurs on a power line or signal cable. Standard RFI shielding techniques
work on radiation from lightning -to a point. The energy levels associated
with lightning radiation can induce currents in conductors that will
destroy products. Good lightning protection requires the following
techniques as a minimum:
- Mount all equipment in a metal housing.
- All signal leads should have a gas discharge transient protector to
circuit common.
- Circuit common should have a gas discharge transient protector to a
good earth connection (very short heavy lead).
- Power connections should have a transient protector from each line
to earth.
- Use twisted shielded pair for all signal leads.
- Run all wire underground where feasible - otherwise use metal
conduit which is well grounded.
Fundamental Requirements
