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Intrinsic Safety Circuit Design

Part 2 -Thermocouples & RTDs

Table of Contents

? Abstract
? Highlights
? Introduction
? Examine The Barrier Parameters
? Thermocouple System Design Pointers
? RTD System Design Pointers
? Conclusion

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

Abstract:

Temperature-sensing devices, thermocouples and RTDs are simple devices and do not need to be approved
as intrinsically safe. One intrinsically safe barrier can be used to protect all thermocouples and 2, 3 or 4-wire
RTDs. The barrier designed so that connections can be made without regard to polarity of the wires. The low
impedance of the barrier will not influence the temperature readings. Consistent wiring must be used in the
thermocouple circuit to avoid thermocouple effects caused by dissimilar metals on the barrier connection.

Highlights:

? All thermocouples and RTDs are simple devices and do not need approvals.
? One intrinsically safe barrier can be used to make all thermocouples and RTDs intrinsically safe.
? Isolated temperature converters accept signals from temperature sensors and convert them to a mA
signal which is intrinsically safe.
? 3-wire RTDs provide better signals than 2-wire RTDs.
? Use of consistent wiring on thermocouples will provide more accurate signals.

Introduction

When thermocouples and RTDs (resistance temperature devices) are installed in hazardous areas, barriers
are required to make their circuits intrinsically safe. These intrinsic safety barriers prevent excess energy from
possible faults on the safe side from reaching the hazardous area. Without the barriers, excessive heat or
sparks produced by the fault condition could ignite volatile gases or combustible dusts.

Hundreds of different barriers are available from North American suppliers. The multitude of products can give
control engineers nightmares as they try to select the proper barrier for common instrumentation loops. The
search can be simplified, however. One type of barrier can be selected to make all thermocouples and RTDs
intrinsically safe so that polarity problems are avoided and calculations are not necessary.

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Figure 1. Typical values of barrier in thermocouple circuit.

Normally, the design of all intrinsically safe circuits centers around one of two approaches: the universal
approach, which uses a universal device that often is isolated so that a ground for safety is not required; or the
engineered approach, which uses grounded safety barriers.

? Isolated temperature converters. These universal devices measure temperature in hazardous areas, but
at a higher cost. (Dispensing with the need for a ground is a convenience that may cost two to three times as
much as grounded safety barriers.) Isolated temperature converters accept a low-level DC signal from a
thermocouple or 3-wire RTD and convert it into a proportional 4-20 mA signal in the safe area. They also are
available with set points that trip an on-off signal to the safe side when the temperature reaches a designated
level. These units must be approved as intrinsically safe.

Advantages of isolated temperature converters as compared to grounded safety barriers include:

? Good signal response

? No ground required for safety
? More versatile application
? One product for all applications

Disadvantages include:

? Larger in size
? Requires calibration
? More expensive
? May not work with all thermocouples and RTDs

? Grounded safety barriers. These are passive devices that prevent all excess energy from a fault occurring
on the safe side from reaching the hazardous area. Under normal conditions the barriers allow the circuit to
function properly by allowing signals to pass between the field device and the control room. In a fault condition,
the barriers limit voltage and current to levels that are not sufficient enough to ignite gases. For a more
detailed explanation, refer to Part 1 of this series (October 1992 INTECH).

Advantages of grounded safety barriers as compared to isolated temperature converters include:

? Less expensive

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? Precise signal response
? Very small (less than 1/2 in. wide)
? Simple application
? One barrier for all types of thermocouples and RTDs

Disadvantages include:

? Requires ground
? Requires some engineering

Examine The Barrier Parameters

Articles in this series will focus on methods to select the proper grounded safety barriers. Before we analyze
thermocouple and RTD circuits, we should examine the functional parameters necessary to select the proper
barrier. These parameters are: polarity of circuit; rated voltage of barrier; and resistance of barrier.

? Polarity. The circuit's polarity must be known in order to choose the correct type of barrier. DC barriers are
rated either as positive or negative. AC barriers can be connected to circuits with either a positive or negative
supply. SIGNAL & RETURN barriers are used for transmitter and switching applications. All of these barriers
are available in single- or double-channel versions. However, because double-channel barriers save space
and money by being connected to two legs of a loop, they are becoming the standard.

? Rated voltage. Like any electrical device, safety barriers have a rated nominal voltage, Vn, referred to as
working voltage. The barrier's Vn should be greater than or equal to the supply to the barrier, much like the
rated voltage of a lamp must be equal to or greater than the supply to it. If the voltage supply to the barrier is
much greater than its Vn, the barrier will sense a fault. The protective zener diodes will conduct, causing
leakage currents and inaccurate signals on the loop. Most barriers have a rated working voltage that
guarantees a minimal leakage current from 1 to 10 micro amps if it is not exceeded. If the supply voltage to the
barrier becomes too high, the zener diode will conduct. The resulting high current through the fuse will cause
the fuse to blow. Excess supply voltage is the main reason why grounded barriers fail.

? Internal resistance. Every safety barrier has an internal resistance, Ri, that limits the current under fault
conditions. Ri also creates a voltage drop across the barrier. This drop can be calculated by applying Ohm's
law, V=IR. Not accounting for the voltage drop produces the most problems in the proper functioning of
intrinsically safe systems.

Thermocouple System Design Pointers

! Polarity. A thermocouple has two wires, each with a positive and negative polarity. Two single-channel
barriers, each with the proper polarity, could be used. Problems would occur if the positive leg to the
thermocouple were connected to the negative terminal of the barrier or vice versa. There are two possible
barrier choices for thermocouple circuits:

Thermocouple circuit with one

positive and one negative lead
1 standard DC barrier, positive polarity and
1 standard DC barrier, negative polarity

OR

1 double AC barrier

When barriers and thermocouples are being installed, the technician may forget which wire is positive and
which is negative. To avoid polarity problems on the terminals, a double AC barrier should be used. The wires
can be connected to either terminal and the circuit will function properly as long as thermocouple polarity is
maintained throughout.

? Rated voltage. A thermocouple produces a very small voltage (less than 0.1 V). It is connected to a
voltmeter which has a high impedance and which requires a very small current. Since the thermocouple

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produces such a small voltage, choose a double AC barrier with a higher rated nominal voltage (Vn). A survey
of most double AC barriers on the market shows that they are rated at low nominal voltages from 1 V and
higher. Select one between 1 and 10 V.

? Internal resistance. Since the mV signal has a very small current and is going to a high-impedance
voltmeter, the resistance of the barrier will not influence circuit function. A simple rule of thumb is that when a
signal is going to a high-impedance voltmeter, an internal barrier resistance of less than 1000 ohms will not
affect the mV signal. It usually is good practice, however, to select a barrier with a low resistance in case the
circuit is modified later.

? Barrier selection. For proper operation of thermocouples in hazardous areas, select safety barriers based
on the following parameters:

? Barrier type: double-channel AC barrier to avoid polarity problems

? Rated voltage: Barrier Vn > 1 V
? Internal resistance: barrier with lowest resistance (less than 110 ohms)

? Safety and installation check. Since the thermocouple is a simple device, it does not need third-party
approval. Make sure that the barrier has the proper approvals for hazardous area locations. The thermocouple
wires will be different from terminal connections on the barrier. Always use consistent wiring from the
thermocouple to the barrier and then to the control room. This will cancel any thermocouple effect caused by
the dissimilar metals on the barrier connection.

RTD System Design Pointers

RTDs come in 2-, 3-, and 4-wire versions. The 3-wire RTD is used in more than 80% of all applications. The 2-
wire version is not as accurate and is used mostly in the heating, ventilation, and air conditioning industry for
set-point temperature measurements. The 4-wire RTD provides the most accurate signal, but is more
expensive and requires one more extension wire to the process area.

Understanding RTD accuracy is essential in selecting the correct barrier. Many RTD measurements are in the
form of a Wheatstone bridge, whose output voltage is a function of the RTD resistance. The bridge requires
four connection wires, an external source, and three resistors that have a balanced temperature coefficient.
The RTD normally is separated from the bridge by a pair of extension wires.

Figure 2. Wheatstone bridge 2-wire RTD.

With a 2-wire RTD, the impedance of the barrier in series with the RTD will cause an imbalance on the bridge
and will affect the accuracy of the temperature reading. This effect can be minimized by using a third wire to
measure the voltage (refer to Fig. 3 for this discussion). If wires A and B are perfectly matched and if the
resistance in both channels of the barrier is the same, the impedance effects will cancel because each is in an
opposite leg of the bridge. The third wire, C, acts as a sense lead to the voltmeter.

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Figure 3. 3-wire RTD bridge.

Figure 4. 3-wire RTD bridge with barrier.

? Current loop A & B: Polarity. The current loop to the RTD has a positive and a negative polarity. Possible
solutions are similar to the thermocouple:

1 standard DC barrier, positive polarity and

1 standard DC barrier, negative polarity

OR

1 double AC barrier

Select the double AC barrier to avoid polarity problems. Because it is smaller, it also is less expensive.

? Current loop A & B: Rated voltage. The constant current amperage sent to the RTD typically is in the
micro amp (10-6) level. The maximum resistance of the most commonly used RTD, PT 100, is 390 ohms at
1560/C. The voltage drop across the RTD will be in mV, so the Vn of the RTD loop is similar to the
thermocouple. To be safe, select a barrier with a Vn greater than 1 V, similar to the Vn of the thermocouple
barrier.

? Current loop A & B: Internal resistance. The constant current source will have a rated maximum load or
burden (resistance load it can drive). Assume that this maximum load is 500 ohms and the maximum
resistance of the RTD at the highest temperature is 390 ohms. Knowing this information, the Ri of the barrier
can be calculated:

Control room resistance <= Barrier resistance + RTD resistance

500 ohms < Ri ohms + 390 ohms

Ri < 110 ohms

? Current loop A & B: Barrier selection. Use the same barrier that was used for the thermocouple circuit.

? Leg C to the voltmeter: Barrier selection. The RTD leg going to the voltmeter (C) is a millivolt signal
similar to the thermocouple circuit. The rated voltage, Vn, and internal resistance, Ri, of the barrier will have
the same parameters as the barriers used in the thermocouple and current loop of the RTD.

Selecting the correct barrier to make all thermocouples and RTDs intrinsically safe is not difficult. Use a
double-channel AC barrier with a rated voltage greater than 1 volt with the lowest internal resistance. The

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double-channel barrier is the lowest cost solution. The AC version will avoid any polarity problems. A barrier
with a rated voltage between 1 and 10 volts will provide a wide selection which have a low resistance and are
approved for the hazardous areas where the temperature sensors are located. This single barrier can then be
used to make all thermocouples and RTDs intrinsically safe. And don't forget, all thermocouples and RTDs are
simple devices, so they do not need third party approval to be intrinsically safe. When they are connected to
an approved intrinsically safe barrier, the circuits are intrinsically safe.

Figure 5. Intrinsically safe 3-wire RTD circuit.

Conclusion

Many temperature sensors are attached to 4-20 mA temperature transmitters, which comprise 22% of all
intrinsically safe applications. The next article in this series will show how to make these transmitters
intrinsically safe.

Return to the Intrinsically Safe Main Page

Copyright © 2000 Cooper Crouse-Hinds

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