Application Note

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Application Note

Process switches

A process switch is a device that

can sense a process variable —

such as temperature or pressure

— and change the state of one or

more sets of switch contacts

when that variable reaches a

predetermined value. This value

is called a setpoint. A switch can

have multiple setpoints. Let’s look

at some important c

onc

epts of

how proc

ess switches work.

Contacts. Contacts come in

pairs, and a pair is either nor

mally open or normally closed.

“Normally” means without ener-

ization — just the way the

contacts would be on the shelf or

if you disconnected the power

ires from the switch.

Many process switches have

four sets of contacts — two

normally open and two normally

closed. But, there are many

variations. A single switch may

operate just one set of contacts,

or it may operate multiple sets

Process and temperature

switch applications with

the 740 Series DPCs

What about actuation? You

might want the switch to failsafe

upon loss of level in a cooling

tank. So, normal level would acti-

vate the switch (compared to its

shelf position). Upon loss of level,

the switch deactivate — that is, it

will assume the same state that

it would be in if it were on the

shelf. For an example of this

control logic, look at a typical

toggle-style light switch. You will

notice the word “ON” under the

toggle handle and the word OFF

above it. To reveal the word “ON,”

you must flip the switch up. If the

toggle mechanism were to fail

mechanically — which could

happen if, for example, it were to

melt due to arcing — the toggle

handle would drop into the “OFF”

position due to gravity. That is the

failsafe position of these switches.

It’s common to implement process

switches the same way.

Setpoint. A switch may have

multiple setpoints

. For example,

many level switches come with

low-low, low, high, and high-

high sets of contacts — each with

its own setpoint.

But, it can get more c

omplex

than that, depending on the

required control scheme and the

type of sw

itch used. There are

many ways to accommodate

complex switching schemes —

including the use of an analog

transmitter serves as the input to

a virtual switch (implemented in

software).

From the Fluke Digital Library @ www.fluke.com/library

of normally open and normally

closed contacts. You select which

contacts to use based on the

desired output for a given condi-

tion and a given failsafe condition.

Control logic. You must think

of switch actuation and contact

state separately. Actuating the

typical process switch means

opening one set of contacts and

closing another at the same time.

Whether actuation opens or

closes a set of contacts depends

on whether you are using the

normally open or normally closed

contacts and whether the switch

is in an activated or deactivated

state during normal operation.

Failsafe operation is the first

criterion to assess when deciding

which set of contacts to use. For

example, you should use normally

closed contacts if breaking the

circuit will result in a failsafe

condition. Because loss of power

and an open circuit (via a broken

circuit wire, broken connection,

or intentional operation) have the

same effect on circuit operation,

the normally open contacts would

e the c

orrect ones to use. Upon

loss of power, these contacts

will open. So, you would want

them to b

e closed for normal

operations and to open when

operations go into alarm or

ontrol change c

onditions

It is not true that, for example,

a high level sw

itch w

ill nec

essar

ily close c

ontacts when you reach

a high level condition. Good con-

trol practic

es usually dictate the

opposite.

This note discusses appli-

cations for process

switches and calibrating

temperature switches

using the Fluke 740 Series

Documenting Process

Calibrators (DPCs). Let’s

begin by looking at what a

process switch is and

what it does.

Summary of content (5 pages)

PAGE 1
Process and temperature switch applications with the 740 Series DPCs of normally open and normally closed contacts. You select which This note discusses applicontacts to use based on the cations for process desired output for a given condition and a given failsafe condition. switches and calibrating Control logic. You must think temperature switches of switch actuation and contact using the Fluke 740 Series state separately. Actuating the Documenting Process typical process switch means Calibrators (DPCs).
PAGE 2
Closed 50 °C Setpoint Deadband Reset Open High Limit Process Variable Open Reset Deadband 20 °C Setpoint Closed Low Limit Figure 1. 2-point switch with settings for low and high setpoints. Here’s an example of a complex application. A level switch may allow a “normal” indication (such as a light) to display at any level up to 82 %. At 82 %, the switch causes normal indication to go off — placing the indication between a normal state and an alarm state.
PAGE 3
Deadband. This is closely related to reset. Deadband prevents a switch from cycling around a setpoint. Hysteresis provides some deadband, automatically. But for some processes, hysteresis is not enough to prevent undesirable on/off cycling. So, many switches have additional deadband intentionally designed into them. That deadband may be fixed, fixed selectable, or variable. For example, an electronic thermostat used for a heat pump may have a fixed selectable deadband of 1.5 degrees or 3 degrees. Range.
PAGE 4
8. 9. Figure 5. MEASURE/SOURCE split screen, contacts reset. 7. You should now see the split screen (Figure 5). Select the “Auto Test” softkey and the “Continue” softkey. The DPC will now ramp the simulated thermocouple potential into the limit switch back and forth past the nominal setpoint and record the sourced temperature values for the actual setpoint, and then show that value in the upper left-hand corner of the DPC display.
PAGE 5
Our examples in this application note have been for In the preceding step by step temperature switches. The 740 description, the switch has been series DPCs allow you to test removed from its operational cir- pressure switches, too — in fact, cuit and the switch contact in 11 different engineering units. closure is monitored to determine Pressure switch tests are similar state change.