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Switches
In the world of HVAC control, there is basically one type of
device used to complete a digital input (DI) circuit. A switch, employed in
various forms, is this device.
A switch is an electrical device used to enable or disable flow of
electrical current in an electrical circuit. Switches may be actuated in a
variety of ways, including movement of two conducting materials into direct
contact (mechanical), or changing the properties of a semi-conducting material
by the application of voltage (electronic).
Switches are typically rated in terms of voltage, voltage type (AC
or DC), current carrying capacity, current interrupting capacity,
configuration, and load characteristic (inductive or resistive). Also specified
are applicable ranges of ambient conditions over which the ratings are valid.
Current carrying capacity (or current rating) is the maximum current that may
continuously flow through the closed switch contacts without exceeding the
maximum permissible temperature.
Process medium property sensing switches are also rated by
parameters such as adjustment range, accuracy or repeatability, and deadband or
differential. The range of a control switch is specified by upper and lower
process values between which the switch has been designed to operate. The
accuracy or repeatability of a control switch is a value typically measured in
process units or percent of range that represents the expected maximum
deviation from setpoint at which the switch will operate under test conditions.
The switch differential or deadband is the change in process value required to
cause the state of the switch to change. For example, a pressure switch that
makes at 10 psig and breaks at 8 psig has a 2 psig differential.
Switch contacts are characterized in much the same way as relay
contacts.
Figure 2.1 describes the most common contact configurations using industry
standard terminology and symbols. Many other configurations are available.
Continue on to Chapter 2: Part 1: Types of
Switches
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