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monitored, the HIGH and LOW RESISTANCE ALARMS offer cable monitoring that is relatively
immune to voltage fluctuations.
Range: 2.00 to 2000.00 Ω
Procedure: The
NOMINAL RESISTANCE value can only be set if either the LOW RESISTANCE and/or
the
HIGH RESISTANCE ALARMS are enabled. Once the controller and the heating cable have been
installed, the following procedure should be used to determine the
NOMINAL RESISTANCE setting:
• Adjust the
CONTROL SETPOINT temperature to turn on the output switch.
• Allow the load to come up to design temperature and its power consumption to stabilize.
• Using the 920 Operator Console, access the
RESISTANCE reading and record its value. Return
the
CONTROL SETPOINT temperature to its proper setting.
• Enter the recorded resistance value as the
NOMINAL RESISTANCE setting.
Note: The setup procedure outlined above may have to be repeated a number of times to arrive
at a correct nominal resistance setting. This value will be affected by the heating cable tempera-
ture, which in turn is affected by ambient temperature, insulation level, a full or empty pipe or
vessel, etc.
3.8.17 OVERCURRENT TRIP ALARM (SSR ONLY)
Purpose: The overcurrent trip feature is always enabled when using an
SSR output switch and is
used to provide protection for the output switch. Enabling this alarm will only inform the user of
an excessively high current condition and that the output switch has been latched off. During a
high current condition, the controller attempts to soft start a heating cable by a technique involv-
ing measured in-rush current and the
SWITCH CURRENT RATING. If the controller is unable to start
the cable, it will eventually trip its output switch off and will not retry or pulse its output switch
again. At this point the
OVERCURRENT TRIP ALARM is latched on.
Notes:
• The controller is not a safety cutout or an overcurrent protective device as defined by the
National and Canadian Electrical Codes (
NEC and CEC). A protective device such as a circuit
breaker or fuse must be included as part of a proper design and be selected in accordance
with the requirements defined in the National Electrical Code (
NEC) and/or the Canadian Elec-
trical Code (
CEC).
• The controller cannot protect the SSR from short circuits or excessive overcurrent condi-
tions. Always ensure that the power is off prior to performing any maintenance or trouble-
shooting of the heating circuit. Verify that no damage has occurred to the cable or the
controller prior to re-energizing the circuit.
Alarm Mask:
ENABLE or DISABLE
Procedure: Adjust the SWITCH CURRENT RATING setting to the actual current rating of the SSR.
Enable or disable the alarm as required. Note that the
OVERCURRENT TRIP ALARM does not have
to be enabled in order to adjust the
SWITCH CURRENT RATING setting.
Note: This alarm should be left enabled since an overcurrent trip condition would normally rep-
resent a serious problem. This is a factory-set alarm value and disabling the alarm does not dis-
able the overcurrent trip function. In some applications the use of self-regulating cables will
produce very high in-rush currents during cold startup. These currents may exceed the overcur-
rent trip limit and the controller will not be able to soft start the trace circuit. If this condition per-
sists, please contact your nearest sales office for recommendations and solutions to this
problem.
3.8.18 SWITCH FAILURE ALARM
Purpose: The purpose of the
SWITCH FAILURE ALARM is to indicate that an output switch failure
has occurred. The controller determines that if the output switch is turned off and there is load
current present, then the output switch has failed closed and the alarm is latched on.
Alarm Mask:
ENABLE or DISABLE
Procedure: Enable or disable the alarming of an output switch that has failed in the closed position.
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