P
AGE
9
In applications where a High Voltage Transformer is used, (i.e. ignition systems) the second-
ary of the transformer should be isolated from all other cables.
This instrument has been designed to operate in noisy environments, however, in some cases
even with proper wiring it may be necessary to suppress the noise at its source.
2.2.1.4 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
possible, keeping the wires protected by the shielding. The shield should be grounded at one
end only. The preferred grounding location is the sensor, transmitter, or transducer.
2.2.1.5 NOISE SUPPRESSION AT THE SOURCE
Usually when good wiring practices are followed, no further noise protection necessary.
sometimes in severe electrical environments, the amount of noise is so great tht it has to be
suppressed at the source. Many manufacturers of relays, contactors, etc., supply "surge
suppressors" which mount on the noise source.
For these devices that do not have surge suppressors supplied, RC (resistance-capacitance)
networks and/or MOC (,etal 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. Aditional protection
may be provided by adding an RC network across the MOV.
FIGURE 2-2
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 capacitor
(1000 volts) is recommended. For circuits from 3 to 5 amps, connect 2 of these in parallel.
See Figure 2-3, page 10.
Coil
0.5
mfd
1000V
220
ohms
115V 1/4W
230V 1W
