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Intrinsic Safety Circuit Design

Making instruments intrinsically safe need not seem like a nightmare. Here, the basics of intrinsic safety circuit design are discussed.
Paul S. Babiarz

Intrinsically Safe Apparatus

Intrinsically Safe Applications(%) 32.0% 85.0% 15.0% 22.0% 13.0% 8.5% 4.5% 2.5% 2.0% 2.0% 13.5% 100.0%

Switching mechanical switches proximity switches 2-wire transmittersThermocouples & RTDs Load cells Solenoid valves Potentiometers LEDs I/P transducers Other devices Total field devices

3.12 of the ANSI/ISA-RP 12.6-1987 as any device which will neither generate nor store more than 1.2 volts, 0.1 amps, 25 mW or 20 µJ. Examples are simple contacts, thermocouples, RTDs, LEDs, noninductive potentiometers, and resistors. These simple devices do not need to beapproved as intrinsically safe. If they are connected to an approved intrinsically safe associated apparatus (barrier), the circuit is considered intrinsically safe. A nonsimple device can create or store levels of energy that exceed those listed above. Typical examples are transmitters, transducers, solenoid valves, and relays. When these devices are approved as intrinsically safe, under the entityconcept, they have the following entity parameters: Vmax (maximum voltage allowed); Imax (maximum current allowed); Ci (internal capacitance); and Li (internal inductance). The Vmax and Imax values are straightforward. Under a fault condition, excess voltage or current could be transferred to the intrinsically safe apparatus (field device). If the voltage or current exceeds the apparatus’ Vmax orImax, the device can heat up or spark and ignite the gases in the hazardous area. The Ci and Li values describe the device‘s ability to store energy in the form of internal capacitance and internal inductance.
Hazardous Area

Intrinsic safety prevents instruments and other low-voltage circuits in hazardous areas from releasing sufficient energy to ignite volatile gases. Although it is used widelyin Europe to safely install and operate instrumentation circuits in hazardous areas, it has caused much confusion in North American markets. Many users have heard of it and want to know more; however, most feel uncomfortable applying intrinsically safe products. One reason is that intrinsic safety has been a part of Section 504 of the National Electric Code only since 1990. In addition, the numberof different products on the market and seemingly endless calculations make applying intrinsic safety seem like an engineer’s nightmare. This is the first of a series of short articles that explain how to make the most common field devices (thermocouples, RTDs, contacts, solenoid valves, transmitters, and displays) intrinsically safe. We will begin with an introduction to the practical side ofintrinsic safety circuit design.

Figure 1. Current use of intrinsically safe apparatus in hazardous areas.

device, also known as a barrier or intrinsically safe associated apparatus; and the field wiring. When designing an intrinsically safe circuit, begin the analysis with the field device. This will determine the type of barrier that can be used so that the circuit functions properly undernormal operating conditions but still is safe under fault conditions. More than 85% of all intrinsically safe circuits involve commonly known instruments. Figure 1 shows the approximate use of intrinsically safe apparatus in hazardous areas. An intrinsically safe apparatus (field device) is classified either as a simple or nonsimple device. Simple apparatus is defined in paragraph
Safe AreaIntrinsically Safe Barrier
Current Limiting Resistor

Fuse

Input Voltage

Zener Diodes

Field Device

Start With The Field Device
All intrinsically safe circuits have three components: the field device, referred to as the intrinsically safe apparatus; the energy-limiting
Instrinsically Safe Ground

Figure 2. Barrier circuit

Z-131

5A

Limiting Energy To The Field Device
To...