Partlow MRC 7000, Installation & Operation Manual

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2. If possible, eliminate mechanical contact relay(s) and replace with solid state relays. If a
mechanical relay being powered by an instrument output device cannot be replaced, a
solid state relay can be used to isolate the instrument.
3. A separate isolation transformer to feed only instrumentation should be considered. The
transformer can isolate the instrument from noise found on the AC power input.
4. If the instrument is being installed on existing equipment, the wiring in the area should be
checked to insure that good wiring practices have been followed.
2.5.1.2 AC POWER WIRING
Earth Ground
The instrument includes noise suppression components that require an earth ground connec-
tion to function. To verify that a good earth ground is being attached, make a resistance check
from the instrument chassis to the nearest metal water pipe or proven earth ground. This
reading should not exceed 100 ohms. Each instrument should have a dedicated earth
ground. Do not chain link multiple instrument ground wires.
Neutral (For 115VAC)
It is good practice to assure that the AC neutral is at or near ground potential. To verify this, a
voltmeter check between neutral and ground should be done. On the AC range, the reading
should not be more than 50 millivolts. If it is greater than this amount, the secondary of this
AC transformer supplying the instrument should be checked by an electrician. A proper
neutral will help ensure maximum performance from the instrument.
2.5.1.3 WIRE ISOLATION/SEGREGATION
The instrument is designed to promote proper separation of the wiring groups that connect to
the instrument. The AC power wire terminals are located near the top of the instrument
boards. The analog signal terminals are located near the bottom of the instrument boards.
Maintain this separation of the wires to insure the best protection from electrical noise. If the
wires need to be run parallel with any other wiring type(s), maintain a minimum 6 inch space
between the wires. If wires must cross each other, do so at 90 degrees to minimize the
contact with each other and reduce cross talk. Cross talk is due to the Electro Magnetic Field
emitted by a wire as current passes through it.
2.5.1.4 USE OF SHIELDED CABLE
Shielded cable helps eliminate electrical noise being induced on the wires. All analog signals
should be run with shielded cable. Connection lead length should be kept as short as pos-
sible, keeping the wires protected by the shielding. The shield should be grounded at one end
only. The preferred grounding location is at the sensor, transmitter or transducer.
2.5.1.5 NOISE SUPPRESSION AT THE SOURCE
Usually, when good wiring practices are followed, no further noise protection is necessary.
Sometimes in severe electrical environments, the amount of noise is so great that it has to be
suppressed at the source. Many manufacturers of relays, contactors, etc. supply "surge
suppressors" which mount on the noise source.
For those devices that do not have surge suppressors supplied, RC (resistance-capacitance)
networks and/or MOV (metal oxide varistors) may be added.
Inductive Coils - MOV's are recommended for transient suppression in inductive coils con-
nected in parallel and as close as possible to the coil. See Figure 2-2, page 10. Additional
protection may be provided by adding an RC network across the MOV.
Contacts - Arcing may occur across contacts when the contact opens and closes. This results
in electrical noise as well as damage to the contacts. Connecting a RC network properly
sized can eliminate this arc.
For circuits up to 3 amps, a combination of a 47 ohm resistor and 0.1 microfarad capcitor
(1000 volts) is recommended. For circuits from 3 to 5 amps, connect 2 of these in parallel.
See Figure 2-3, page 10.