Part 5 - Intrinsically Safe Outputs Made Easy Página 1 de 5

TECHNICAL RESOURCES

Intrinsic Safety Circuit Design

Part 5 - Intrinsically Safe Outputs Made Easy

Table of Contents

? Abstract
? Highlights
? Introduction
? LEDs
? Analog Outputs
? Conclusion

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

Abstract:

The most common intrinsically safe digital outputs are solenoid valves and LEDs. Analog outputs are I/P transducers. Except
the LEDs, these devices must be approved to be used in an intrinsically safe circuit. The same barriers that are used for analo
and digital inputs can be used to make all solenoid valves intrinsically safe. An isolated solenoid driver can also be used. Ana
outputs for I/P transducers always use the same barrier. LEDs require a simple voltage drop calculation to select the proper
barrier. A summary of barrier usage shows that all temperature-sensing, analog and digital inputs and outputs can be made
intrinsically safe with only 2 to 4 barriers.

Highlights:

? Digital outputs refer to solenoid valves and LEDs; analog outputs refer to I/P transducers.
? All solenoid valves use one barrier which is the same one used for analog and digital inputs.
? All I/P transducers use 12 volt, 150 ohm barriers.
? A simple voltage drop calculation is required to select the proper barrier for LEDs.
? A simple 5-step procedure and table is provided to select the barriers for all circuits.

Introduction

Digital outputs refer to closed contacts in a distributed control system (DCS). They transfer a voltage to a process area to
operate a field device. The two most commonly used digital output field devices, solenoid valves and LED displays, can easily
be made intrinsically safe. For solenoid valves, the same types of barriers are used that make analog and digital inputs
(transmitters and switch contacts) intrinsically safe. LEDs may require a different barrier.

There is good news and bad news for making circuits (or loops) containing solenoid valves intrinsically safe. The bad news is
that unlike transmitters which have minimum operating voltages, valve manufacturers often describe their valves with a nomin
operating current or voltage. To select the proper barrier one needs to know the minimum operating characteristics under the
most extreme conditions. Without these characteristics it can be quite difficult to select a barrier that will allow the circuit to
function properly and still meet the entity parameters of the valves. Conditions that may affect the operating characteristics ar
high ambient temperatures, position of the actuator, and length of cable runs.

The good news is that there are only a handful of approved intrinsically safe solenoid valves to choose from. For this article,
manufacturers tested their intrinsically safe valves with the most common barrier used in analog and digital input circuits 24
VDC barrier with a resistance equal to or less than 350 ohms (Fig. 1).

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Figure 1. Solenoid valve.

To determine the correct barrier, start with the basics. Since most digital output circuits operate with 24 VDC switched on the
positive side, use a positive DC barrier rated at 24 VDC. Knowing the minimum operating current of the valve and the internal
impedance of the coil, you can calculate the maximum allowable impedance for the barrier and the cable.

For example, assume a valve has a minimum operating current of 28 mA and a coil impedance of 400 ohms. The maximum
allowable impedance of the circuit is 857 ohms (24/.028 = 857). If the internal impedance of the solenoid coil is 400 ohms, the
allowable impedance of the barrier and cable would be 457 ohms (857 - 400 = 457). The resistance of one mile of #18 AWG
wire at 60 C is about 40 ohms (resistance of #18 AWG wire at 60 C is 0.00737 ohms/ft.). This makes the maximum resistance
of the barrier 457 - 40 = 417 ohms.

Selecting the barrier now is simple:

1. Select a simple DC positive barrier. (The rated voltage should be 24 V.)

2. Calculate the maximum allowable resistance of the barrier as in the example.
3. Confirm that the entity parameters of the solenoid valve match those of the barrier (refer to Part 1 of this series, Octob
1992 INTECH).

Associated Apparatus (barrier) Apparatus (field device)

Open circuit voltage Voc <= Vmax
Short circuit current Isc <= Imax
Allowed capacitance Ca >= Ci
Allowed inductance La >= Li

LEDs

LEDs (light emitting diodes) are simple devices since they do not store energy (capacitance or inductance); therefore, they do
not need to be approved. However, they still must be used with safety barriers to make circuits intrinsically safe. Typical LEDs
are rated at 24, 18, 12 or 6 V and operate at about 25 mA. Since there will always be a voltage drop across the barrier, the be
application is to choose an LED rated at less than 24 VDC. Use a barrier rated at 24 V, then subtract the rated voltage of the
LED. This difference is the allowable voltage drop on the barrier at the rated current. Use Ohm's Law (V = IR) to calculate the
internal impedance of the barrier.

Example:

? LED rated at 12 V at 25 mA
? Allowable voltage drop 12 V (24 - 12 = 12)
? Internal impedance of the barrier = 480 ohms (12/.025 = 480)

Choose a 24 V positive DC barrier with an internal impedance of about 480 ohms (Fig. 2).

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Figure 2. LED pilot light.

Analog Outputs

Analog outputs refer to I/P transducers, also known as I/Ps (pronounced "Ida Pease"). An I/P transducer produces a pneuma
output proportional to the electrical current input that it receives. The more current that is applied to the transducer, the more
pressure is allowed into the system to drive a device. As opposed to a solenoid valve which is either in an opened or closed
position, a transducer is a proportional valve. I/P transducers are referred to as analog outputs because a variable output, the
current signal, is sent from the DCS to the transducer.

I/P transducers need entity approval. They act like resistors in the circuit, so three facts must be known to select the correct
barrier: transducer impedance; maximum burden of the driver that sends the current signal; and transducer entity values.
Burden, rated in ohms, measures the maximum load the DCS can drive.

To select the barrier, use the following characteristics:

? Transducer impedance is 150 ohms

? Burden of the drive is 1000 ohms

The barrier must have an internal resistance less than 850 ohms (1000 - 150 = 850). Verify the rated voltage of the barrier by
calculating the voltage drop of the circuit. For example, use the same barrier and cable values as in the solenoid valve examp
The total impedance (impedance of barrier + transducer + cable) of the circuit would be 540 ohms (350 + 150 + 40). At the
maximum current of 20 mA, the voltage drop would be 10.8 V (540 x 0.20 = 10.8). Select a barrier rated equal to or higher tha
10.8 V. A barrier rated at 12 V or higher with an internal resistance of 150 ohms also would be a good choice (Fig. 3). Confirm
that the entity parameters of the barrier correspond with those of the transducer.

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Figure 3. 4-20 mA I/P transducer.

This series of articles has shown how the most common applications of temperature measurements and analog or digital
inputs/outputs can be made intrinsically safe with a few intrinsic safety barriers.

Selection is simple:

1. Determine if the field device is a simple or nonsimple (energy storing) device that needs approval and has entity
parameters.
2. Select the type of barrier needed to protect the individual ungrounded lines of the circuit. Normally, temperature sensor
use an AC barrier. For analog inputs and current sourcing switching, use the supply and return barrier. The remainder
(analog and digital outputs and some switching circuits) require DC barriers.
3. Select a barrier with a rated voltage equal to or greater than the voltage of the circuit.
4. Confirm that the internal resistance of the barrier will allow enough voltage for the field device to operate properly.
5. Confirm that the entity parameters of the barrier match those of the field device.

Use Table 1 as a guide in selecting grounded safety barriers. There will always be exceptions to these guidelines, so verify yo
selection with the manufacturer of the barriers or field devices.

Device Barrier Rated Internal

Type Voltage Resistance Notes (IT=INTECH)

Thermocouples AC >1 V < 1000* Thermocouples are simple

devices; do not need approval.
(Dec. '92 IT)

RTDs AC >1 V < 1000* RTDs are simple devices;

do not need approval.
(Dec. '92 IT)

Digital Inputs AC Dry contacts are simple

devices; do not need approval.
(Dec. '92 IT)

D/I - current Switch 24 350** Dry contacts are simple

sourcing amplifiers devices; do not need approval.
(Apr. '93 IT)

D/I - current Supply & 24 350** Dry contacts are simple

sinking return devices; do not need approval.

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(Apr. '93 IT)

A/I Supply & 24 350 Transmitters need approval.

transmitters return Check entity parameters.
Conversion resistor of 250 ohms
is on negative side.
Minimum lift-off voltage of
is 12 or less.
(Mar. '93 IT)

A/I DC 24 350 Same, except conversion resistor

transmitters is on + side.
(Mar. '93 IT)

D/O solenoid DC 24 350 Solenoid valves need approval.

valves Check entity parameters.
(Jul. '93 IT)

A/O transducers DC >12 >150 Transducers need approval.

Check entity parameters and
DCS burden.
(Jul. '93 IT)

* Select a barrier with a low resistance

** Other barriers with a different resistance can be used.
However, these barriers match those of the analog inputs.
Table 1. Guide to selecting grounded safety barriers

Conclusion

The last installment in this series will discuss the general rules of grounding, installation, and maintenance of intrinsically saf
systems.

Return to the Intrinsically Safe Main Page

Copyright © 2000 Cooper Crouse-Hinds

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