APG KA Cable Suspended user manual - Manual (Page 10)

KA Series Rev. A2, 12/08
10
Automation Products Group, Inc.
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• Understanding the Switch Action/Terminology
In order to understand the operation of KA liquid level controls, imagine
them operating like pushbutton switches. A normally open (NO) switch is one
that is open when there is no liquid present and closes as the liquid rises. Such
a switch is symbolized as in Diagram 1. The opposite action normally closed
(NC) switch operates as in Diagram 2. Note that the term “normally” means the
switch state when there is no liquid present — it does not mean the “usual”
operating state of the switch.
• Single Versus Dual Point Control
The simplest single level KA fl oat switches (types 1L or 1H) have no deadband
(= diff erential or hysteresis). They switch ON and OFF at one level point. Such
switches work well as alarm point indicators but are unsuitable for automatic
maintenance of the level of liquids in containers.
Usually, tank levels are maintained by pumps or electrically controlled
solenoid valves in the supply or drain lines. Holding the level exactly at one
point is diffi
cult if there is fl ow in and out of the container, because it would
involve the control switch going on and off constantly. This leads to excessive
wear of the components. Thus, the normal practice is to have two control
levels involved. At one level the pump or valve is turned ON and at the other
level it is turned OFF.
These situations can take advantage of the unique capabilities of the KA fl oat
switches. Rather than using two separate single level fl oat switches, one fl oat
switch (type 2L or 2H) can provide ON and OFF level controls. The absolute
height and separation (deadband) of these points can be easily adjusted.
The ON control is a “make” in the circuit and the OFF is a “break” in the circuit.
The deadband is supplied by the fl oat switch. Other KA fl oat switches provide
additional facilities for high and low alarms as well as control of duplex pump