Part 4 - Digital Inputs - Switching Circuits Página 1 de 4

TECHNICAL RESOURCES
BACK | PRINT

Intrinsic Safety Circuit Design

Part 4 - Digital Inputs - Switching Circuits

? Abstract
? Highlights
? Introduction
? Switch Amplifiers
? Switching Through Safety Barriers
? Conclusion

? Return to the Intrinsically Safe Main Page or visit www.isbarriers.com

Abstract:

Digital inputs are simple switching operations where a switch or contact is operated in a hazardous area and
the on/off signal is sent back to the distributed control system (DCS). Switches are simple devices and do not
need to be approved. The most common method of protecting switches is with an isolated, intrinsically safe
relay, commonly referred to as switch amplifier. These can drive inputs back to a DCS or motor starters and
pumps which require heavier contact ratings. Switching through safety barriers is also possible when the input
is taken back to a DCS. The same barriers used for analog inputs can also be used for digital inputs.

Highlights:

? Digital inputs include: contacts, mechanical, reed, limit, float and proximity switches.
? All digital inputs can be made safe with an isolated intrinsically safe relay, commonly referred to as a
switch amplifier, which can drive DCS inputs or other electrical devices requiring more power.
? NAMUR style, 2-wire proximity switches are most commonly used for intrinsically safe applications.
? Digital inputs can be switched through barriers back to a DCS.
? The barriers used for digital inputs are the same as used in analog inputs and digital outputs.

Introduction

Digital inputs constitute almost one-third of all process signals. They also are known as binary, on-off, 0/1, or
simple switching signals where a switch is either opened or closed. The most common examples of these are
mechanical or reed contacts, transistors, limit, float, on-off, and pushbutton switches. As defined in paragraph
3.12 of the ANSI/ISA-RP12.6-1987, switches are simple devices that neither generate nor store more than 1.2
V, 0.1 A, 25 mW, or 20 J. Since switches are simple devices, they do not have to be approved as intrinsically
safe. If they are connected to an approved intrinsically safe associated apparatus (barrier), the circuit is
deemed to be intrinsically safe.

To make a switch intrinsically safe, the user may select a switch amplifier or a safety barrier. A switch amplifier
is an intrinsically safe relay that solves virtually all switching applications. However, if power is not available in
the control panel or if panel space is an important consideration, a grounded safety barrier may be a better
choice. There is not a significant cost savings of one alternative over the other. Each has its own advantages
and disadvantages, as shown in Table 1.

http://www.crouse-hinds.com/CrouseHinds/resources/intrinsically_safe/techref/article4.cfm 01/04/2003
Part 4 - Digital Inputs - Switching Circuits Página 2 de 4
Switch Amplifiers
Advantages Disadvantages
- Simple application - Needs power supply
- No ground required - Larger in size
- No internal resistance
- LEDs to indicate power and
monitor operations
- Sensistive to detect closed
contacts in corrosive areas

Safety Barriers
Advantages Disadvantages
- Smaller in size - Requires grounding
- Does not require power supply - Has internal resistance
Table 1. Advantages and disadvantages of switch amplifiers and safety barriers.

Switch Amplifiers

The most common application is switching through an intrinsically safe relay (Fig. 1). Relays, which normally
are powered by 110 VAC or 24 VDC, have a low voltage and current which are safe at the contact in the
hazardous area. When this contact is closed, the relay transfers the signal from the hazardous location to the
non-hazardous side. A closed switch on the hazardous side operates a relay or optocoupler output on the
non-hazardous side. The signals are electrically isolated so that grounding is not required.

Figure 1. Switch amplifier - 2 channels.

When proximity switches became a popular means of sensing the presence of objects and materials, the
NAMUR-style sensor was developed. Contrary to popular opinion, NAMUR is not an approval standard. It was
organized by the German chemical industry to develop operating standards for proximity switches. A NAMUR-
style proximity switch is a 2-wire DC sensor that operates at 8.2 V with switch points operating between 1.2 to
2.1 mA. This NAMUR standard later was superseded by the German Standard DIN 199234, Measurement
and Control: Electrical Sensors Used For Intrinsically Safe 2-Wire DC Systems. Because these switches
required a remote amplifier for operation, most switch amplifiers standardized on an intrinsically safe voltage
of 8.2 V and current of 8 mA at the contacts in hazardous areas. This provided enough power to operate
NAMUR-style proximity switches safely.

The amplifiers are sensitive enough to detect closed contacts in corrosive or abusive areas. Despite the fact
that the intrinsically safe voltage and current at the contacts are very low, most modern switch amplifiers will
detect a closed contact when the resistance of the circuit is less than 3000 ohms. Intrinsically safe switches
can be located a long distance from the switch amplifiers and still function properly.

Switch amplifiers are available with two different output contacts to the safe side, relays and optocouplers.

http://www.crouse-hinds.com/CrouseHinds/resources/intrinsically_safe/techref/article4.cfm 01/04/2003
Part 4 - Digital Inputs - Switching Circuits Página 3 de 4
The more commonly used relay versions are applied in slow speed switching to operate smaller pumps,
motors, or other electrical devices. Optocouplers are transistors operated by photo diodes to close the output
contacts. These outputs have lower contact ratings but an almost infinite switching capability. Optocouplers
are used for switching back to a DCS or for high-speed counting operations up to thousands of times per
second (KHz).

Switching Through Safety Barriers

When a 110 V supply is not available in the control panel, safety barriers frequently are used for digital inputs
back to a DCS. There are two methods of switching: current sourcing or current sinking. Both of these
methods can use the same types of barriers that were used for transmitters (see Part 3 of this series, March
INTECH).

The current sourcing method of switching in Fig. 2 could use the same signal and return barrier that was used
for 4-20 mA transmitters. The voltage to the switch is supplied through the supply channel. The second
channel is used for signal return. A closed switch will close the contact in the DCS. Most digital input signals
operate with 24 V and 10 mA. If the same barrier is used for switching as 4-20 mA transmitters, there will be
about a 3 to 4 V drop across the barrier.

Figure 2. Current sourcing switching.

The barrier used for current sinking switching can be a single-channel DC barrier as seen in Fig. 3. When the
switch is open, the DCS input will sense 24 V. When the switch is closed, the DCS will recognize a lower
voltage. This lower voltage is calculated as a voltage divider circuit.

Figure 3. Current sinking switching.

Make sure the rated voltage of the barrier, Vn, is equal to or greater than the voltage supply. Since most
switching uses 24 VDC, select a barrier rated at 24 V. The internal resistance of the barrier is not as critical
since the current in digital inputs usually is very small. However, it always is good practice to select a barrier
with low resistance. Check the approvals of the barriers to make sure that they are rated for the proper
hazardous area group location.

Intrinsically safe relays, also referred to as switch amplifiers, can be applied universally for all digital inputs.
However, if safety barriers are used, the same barriers used to make analog inputs intrinsically safe can be
used for either current sourcing or current sinking switching.

Conclusion

http://www.crouse-hinds.com/CrouseHinds/resources/intrinsically_safe/techref/article4.cfm 01/04/2003
Part 4 - Digital Inputs - Switching Circuits Página 4 de 4
The next article in this series will explain how to make digital outputs intrinsically safe.

Return to the Intrinsically Safe Main Page

Copyright © 2000 Cooper Crouse-Hinds

http://www.crouse-hinds.com/CrouseHinds/resources/intrinsically_safe/techref/article4.cfm 01/04/2003