T-372
7–18
The heaters are wired directly back to the contactor and if a heater failure occurs during a trip, the heater set containing
that heater may be disconnected at the contactor.
The next pre-trip (P1) will detect that a heater set has been disconnected and indicate that the failed heater should be
replaced.
7.16.1
Megger Testing the Heaters
WARNING
!
Always turn OFF the unit circuit breakers (CB-1 & CB-2) and disconnect main power supply
before working on moving parts.
All of the checks performed during this procedure should be carried out using a 500v Meg-ohm tester.
1. Connect the ground wire from the insulation tester to a fixed ground point, preferably the ground plate in the
control box.
2. At the load side of the heater contactor, check the insulation resistance to ground.
If readings are > 2 Mohm, then the heaters are operating properly and no action is needed.
If readings are < 1 Mohm, then the faulty heater needs to be identified. Proceed to step 3 for units
with
a
heater access panel or step 4 for units
without
a heater access panel.
If readings are between 1 and 2 Mohm, then the heaters need to be re-tested with the following steps:
a. Reconnect the unit to power and power the unit on.
b. Set the unit set point to a minimum of 10°C higher than the current temperature of the container. Allow
the unit to go into heat mode, reach the temperature set point and maintain for 10-15 minutes.
c. Power the unit off. Allow the unit to cool to ambient temperature.
d. Connect the ground wire from the insulation tester to a fixed ground point, preferably the ground plate in
the control box.
e. At the load side of the heater contactor, check the insulation resistance to ground.
If readings are > 1 Mohm, then the heaters are operating properly and no action is needed.
If readings are < 1 Mohm, then the faulty heater needs to be identified. Proceed to step 3 for units
with a heater access panel or step 4 for units without a heater access panel.
3. Identify the faulty heater(s) for units with a heater access panel:
a. Open the access panel and cut out all wire splices to isolate all heaters inside of the unit.
b. Repeat the Megger test on each individual heater. Connect the ground clip to the outer metal sheath of
the heater and the test clip to one of the wires from the same heater.
c. Replace any heater where the readings are < 1 Mohm.
4. Identify the faulty heater(s) for units without a heater access panel:
a. Remove all six connections from the Heater (HR) contactor load side, which splits the six heaters into
three separate pairs.
b. Identify the following three wires: DHTL, DHML, DHBL. There is one from each load connection.
c. Repeat the Megger test on each pair of heaters to identify the faulty heater pair. Connect the ground clip
from the insulation tester to a fixed ground point on the unit, preferably the ground plate in the control
box. Connect the test clip to one of the wires stated above.
d. Test all three wires and replace any heater pair that has readings < 1 Mohm.
5. If the unit is loaded, and the heater can not be immediately replaced, perform the following steps:
a. Identify the wire at the opposite end of the faulty heater pair: DHTL - DHTR, DHML - DHMR, DHBL -
DHBR.
b. Isolate the two wires.
