nvent 910 Series Installation, Operating And Maintenance Instructions page 41

or a lost phase. The problem with such a close setting is that it inevitably leads
to nuisance alarms, particularly when voltage fluctuations are present. By using the HIGH
RESISTANCE ALARM, nuisance alarms due to voltage dips may be minimized.
• Use of the HIGH RESISTANCE ALARM assumes that the controller power is derived from the
same circuit as the tracing power.
• No HIGH RESISTANCE ALARMS will be generated if the measured voltage is below the LOW
VOLTAGE ALARM setpoint, regardless of whether the LOW VOLTAGE ALARM is enabled. This
stops an alarm from being generated when the circuit power is turned off.
If the LOW VOLTAGE ALARM is disabled ensure that the LOW VOLTAGE setpoint is set to a
relevant level otherwise no HIGH RESISTANCE ALARMS will occur.
• HIGH RESISTANCE ALARMS will only be generated if the output switch is on.
5.5.15 HIGH RESISTANCE ALARM FILTER TIME SETTING
Purpose: The HIGH RESISTANCE ALARM FILTER will prevent HIGH RESISTANCE ALARMS
from being indicated until a high resistance condition has existed for the duration of the HIGH
RESISTANCE ALARM FILTER time.
Range: 0 to 12 seconds
Procedure: Adjust the HIGH RESISTANCE ALARM FILTER time to the desired value. Note that
the HIGH RESISTANCE ALARM must be enabled in order to adjust the HIGH RESISTANCE
ALARM FILTER time.
IMPORTANT:
• If an alarm condition appears and then disappears before the alarm filter time has expired,
the filter timer is reset and the alarm condition must exist again for the entire alarm filter
time before the corresponding alarm will be indicated.
• If the user resets an alarm while the alarm condition is still exists, then the alarm will not be
indicated again until the entire alarm filter time has expired.
5.5.16 NOMINAL RESISTANCE SETTING
Purpose: This parameter defines the nominal expected heater resistance. A value must be
entered by the user to allow the HIGH and LOW RESISTANCE ALARMS to be used. In instal-
lations where the power source may experience periodic fluctuations (surges and/or brownout
conditions), alarming on resistance deviation offers an improved method of monitoring tracer
integrity than simple LOW and HIGH CURRENT ALARMS. Since the ratio of voltage to current is
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.
IMPORTANT: 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 temperature, which in turn is affected by ambient temperature, insulation level, a full or
empty pipe or vessel, etc.
5.5.17 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 using a technique involving
measured in-rush current and the SWITCH CURRENT RATING. If the controller is unable to start
nVent.com | 41