Levelcom 100 - Page 56

Page 56
©2013 Technical Marine Service, Inc. LC-100 V2.97
The control option was designed with the intent of controlling a pump or a valve to maintain
the level of a tank. If such control is to be implemented it is important to consider what will
happen in case there is a failure in the system. The LevelCom 100 will de-energize the relay
when any system failure condition is indicated. These include supply air failure, plugged
sense lines, or onboard hardware failures. When any of these conditions occurs, the
LevelCom 100 is no longer able to measure the level in the tank and it enters a fault mode
and the relays are de-energized. This way the relays go to the same state they would go to if
the LevelCom 100 lost power.
It is very important to consider what will happen to the controlled pump or valve in this case.
Should the pump run (fail on) or should it shut down (fail off) in this case? If the pump is
filling the tank it should probably shut off to avoid overflowing the tank. However if the pump
is emptying the tank, and the pump can handle continuous operation it might be better for the
pump to fail running, again to avoid overfilling the tank. Since the LevelCom 100 relays have
form C contacts it is possible to handle all possible combinations.
First it is important to identify the required failure action. This will determine whether to use
the normally open contact, or the normally closed contact of the relay. Then the proper
setting of the setpoints will result in the desired action. Remember that the relay ON
SETPOINT controls where the relay coil is energized, the OFF SETPOINT where the relay
coil de-energizes. The function of the required relay contact can make this seem backward.
In the following a pump or valve will be referred to as Device. The word Deactivated will refer
to the condition where no electric power is flowing to the Device, Activated will refer to the
condition where electric power is flowing to the device.
Case 1:
Device must fail to a Deactivated condition. Device is to be Activated at a high liquid level,
Deactivated at a low liquid level.
Wire Device to the normally open contacts of the relay. Set ON SETPOINT to the high level,
OFF SETPOINT to the low level.
Case 2:
Device must fail to an Activated condition. Device is to be activated at a high liquid level,
deactivated at a low liquid level.
Wire Device to the normally closed contacts of the relay. Set OFF SETPOINT to the high
level, ON SETPOINT to the low level.
Case 3: