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4 Electrical Installation
ELECTRIC SHOCK/FIRE HAZARD. Installation
should be only performed by technically com-
petent personnel. It is the responsibility of the
installing engineer to ensure that the configu-
ration is safe. Local Regulations regarding elec-
trical installation & safety must be observed
(e.g. US National Electrical Code (NEC) or Ca-
nadian Electrical Code). Failure to follow these
instructions could result in personal injury or
death and/or equipment / property damage.
Avoiding EMC Problems
This controller has passed EMC compliance tests to
EN61326. There should be no difficulty achieving this
level of compliance in use, but it should be borne in
mind that the wiring of the installation can significantly
reduce the efficiency of instrumentation immunity due
to the ease with which high frequency RF can enter via
unprotected cables.
The following general recommendations can reduce
the possibility of EMC problems.
1. If the instrument is being installed in existing equip-
ment, wiring in the area should be checked to en-
sure that good wiring practices have been followed.
2. The controller should be mounted in a properly
earthed metal cabinet. All round metal shielding is
important, so the cabinet door may require a con-
ductive sealing strip.
3. It is good practice to ensure that the AC neutral is at
or near ground (earth) potential. A proper neutral will
help ensure maximum performance from the instru-
ment.
4. Consider using a separate isolation transformer to
feed only the instrumentation. A transformer can
protect instruments from noise found on the AC
power supply.
Cable Isolation & Protection
Four voltage levels of input and output wiring may be
used with the unit:
1. Analog inputs or outputs (for example thermocou-
ple, RTD, VDC, mVDC or mADC)
2. Relays & Triac outputs
3. Digital Inputs & SSR Driver outputs
4. AC power
The only wires that should run together are
those of the same category.
If any wires need to run parallel with any from another
category, maintain a minimum space of 150mm be-
tween them. If wires MUST cross each other, ensure
they do so at 90 degrees to minimize interference.
Keep signal cables as short as possible. If an earthed
thermocouple is used or if the sensor has a screened
cable, it should be earthed at one point only, preferably
at the sensor location or cabinet entry point, by means
of a metal gland. Ideally all analogue and digital signals
should be shielded like this, but for unscreened cables,
large diameter ferrite sleeves at the cabinet entry point
are an effective method of reducing RF interference.
Looping cables through the ferrite sleeves a number of
times improves the efficiency of the filtering. For mains
input cables the fitting a suitable mains filter can pro-
vide good results.
Noise Suppression at Source
If possible, eliminate mechanical contact relays and
replace with solid-state relays. Noise-generating de-
vices such as Ignition transformers, arc welders, mo-
tor drives, relays and solenoids should be mounted in
a separate enclosure. If this is not possible, separate
them from the instrumentation, by the largest distance
possible.
Many manufacturers of relays, contactors etc supply
‘surge suppressors’ to reduce noise at its source. For
those devices that do not have surge suppressors sup-
plied, Resistance-Capacitance (RC) networks and/or
Metal Oxide Varistors (MOV) may be added.
Inductive coils: MOVs are recommended for transient
suppression in inductive coils. Connect as close as
possible, in parallel to the coil. Additional protection
may be provided by adding an RC network across the
MOV.
Figure 5. Transient Suppression
with Inductive Coils
Contacts: Arcing may occur across contacts when
they open and close. This results in electrical noise as
well as damage to the contacts. Connecting a properly
sized RC network 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
two of these in parallel.
Summary of Contents for Chromalox 4081
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