57
tion. The controller’s accuracy and resolution will result in an indicated temperature change of a
couple of degrees if the measured resistance temperature falls between two discrete values. This
is sometimes referred to as quantization error.
If the bounce or instability is excessive, check:
• Wire used for extension of the
RTD should be three-wire, twisted and shielded with the shield
grounded at the controller only. Each of the three lead wires must be of the same gauge.
• The ideal installation has a separate conduit for the
RTD leads (if they have been extended). It
is not usually a problem to run low signal levels in the same conduit as the power leads even
in high-power applications, as long as the
RTD wire is a twisted, shielded type with an insula-
tion rating equal to or greater than the highest voltage in the conduit. Follow the proper Elec-
trical Code for your particular installation.
• Terminal connections that are not tight can add resistance to an
RTD circuit. Check the tight-
ness of all screw terminal connections at time of installation and during subsequent mainte-
nance checks.
• Check the specifications for the particular cable being used to ensure that it does not have
excessive capacitance when used in long lengths. This can cause a temperature offset
between what the controller reads and what the
RTD actually measures. This again is nor-
mally not a problem since the controller compensates for all but the worst cases of this.
• Lastly, it is possible for the
RTD itself to fail on an intermittent basis but this failure mode
should be considered unusual. This kind of failure is probably the most difficult to find but
fortunately it is also the least likely as a failure mechanism.
6.2.3 GROUND FAULT
Ground-fault alarms can be due to incorrect installation as well as leakage resulting from wet
system components or faulted cables.
The 920 series switch interface detects ground faults by summing the outgoing and return trace
currents through an internal current transformer. Under normal operating conditions (no ground
fault condition) this current will be zero. When there is a flow of current from one of the trace
supply wires to ground, a ground-fault condition occurs.
If a ground-fault alarm is present on start-up of a new installation, it is likely due to a wiring error
or damaged cable. To verify this condition:
• Check that the heating circuit neutrals return to the controller and are not connected directly
to the distribution panel. This can be a common problem if the installation is a retrofit situa-
tion.
• On paralleled circuits, be certain that
ALL neutrals return. The late addition of a circuit may
not be obvious.
Use the monitoring feature available at the 920 Operator Console or the Model 780/GCC-9000
Group Communications Controller to view the measured ground-fault current at the heat-trace
controller. If this value is at the maximum that the controller can measure, it is usually an indication
that the wiring is incorrect. If the value is less than 250 mAmps, an actual ground-fault condition
may exist in the cable.
Note: The controller monitors the integrity of the ground-fault (
GF) detection transformer and
associated wiring. If a fault is detected, the controller will report a
GF value of 300 mAmps.
To Next Page
Loading...
