Intrinsically-Safe (IS) Associated Apparatus with Field Connections Reduces Installation and Maintenance Costs

One way to prevent explosions and fires in hazardous areas is to use the intrinsically safe method of protection, which prevents measurement and control devices from having enough energy to cause a spark or thermal ignition. Significant savings, both initial installation and ongoing maintenance costs can be achieved by selecting associated apparatus as the Intrinsically-safe (IS) barrier in your system.

Field Device IS Classifications

Simple Apparatus include devices such as RTDs, T/Cs, LEDs, potentiometers and switches. They are electrical components which do not generate or store more than 1.5V, 100mA and 25mW or a passive component which does not dissipate more than 1.3W. Simple devices can be freely used without any agency certifications but do require an assessment for their maximum surface temperature and assigned a temperature classification (referred to as a T code).

Intrinsically-Safe Apparatus devices store electrical energy such as transmitters, I/P converters and solenoid valves. They may also be connected to simple apparatus in the hazardous field location. These devices must be certified as intrinsically safe apparatus and classified based on allowable hazardous locations, gas group and T code. Entity parameters for the device must also be provided and include the maximum voltage, current and power limits as well as the internal capacitance and inductance parameters of the device. These parameters are used in conjunction with the connecting cable parameters to calculate the maximum allowable cable lengths, loop voltage and current values for the system.

Barrier or Associated Apparatus

An IS system installation requires a barrier or associated apparatus interface between the field device and the control room equipment. Its function is to limit the energy to the hazardous area such that, even under a fault condition, there cannot be enough electrical or thermal energy released by the device to ignite an explosive atmosphere. They are designed for connection to simple or IS apparatus, and must be certified. There are two types of barriers that are most commonly used and a hybrid method where the barrier is integrated into the receiving device.

Zener Diode Barriers are simple passive devices comprised of Zener diodes, resistors and fuses that serve to limit the voltage, current, and power available to the hazardous area device. The design requires the use of a dedicated IS earth ground connection maintained at less than 1Ω and allows no grounding connections at the field devices. A common downside of using this approach is that the required earth ground has low noise rejection capability. This electrical interference can introduce stray and unwanted electrical noise components into the measurement circuit creating potentially significant measurement errors.

Isolated Barriers are active devices that incorporate galvanic isolation thus eliminating the requirement for an earth ground and the restriction for grounding of field devices.(Figure 1) They also provide a higher voltage to the field and devices. These barriers require operating power and are application specific with different models required for different applications (RTD, T/C, 4-20mA etc.)

Figure 1. A system using a safe area device with an external barrier and power supply.

Associated Apparatus incorporate a barrier into the safe area mounted receiving device or the control room equipment. The Moore Industries SPA2IS is an example of such a device that provides an isolating barrier within the alarm trip. This dramatically reduces the cost of purchase, installation and maintenance versus more traditional approaches that require a separate zener or isolating barrier. (Figures 2 and 3)

Figure 2. A Moore Industries SPA2IS associated apparatus incorporating the isolating barrier, temperature transmitter, temperature alarm and diagnostic alarming functions in a single device.

This alarm solution significantly reduces wiring, installation and maintenance costs. In addition, the SPA2IS is a combination alarm trip and temperature transmitter combination that includes built-in intrinsically-safe field connections for current/voltage, resistance temperature detectors (RTDs), and thermocouples.

Figure 3. A Moore Industries SPA2IS associated apparatus incorporating a spherical tank linearization measurement function, local pump control, Hi-Hi ESD, local indication, self diagnostics, and quad relay outputs for control and alarming.

The SPA2IS is powered by a universal AC/DC power supply and provides on/off control, warns of unwanted process conditions, alarms on rate of change, and assists with or performs emergency shutdowns. Very versatile, it accepts a wide variety of signal inputs from transmitters and temperature sensors and provides dual and quad independent and individually-configurable alarm relay outputs when a monitored process variable falls outside of user-set high and/or low limits.

Key features of the SPA2IS include:

  • Intrinsically-Safe field connections – Apply inputs from temperature sensors or transmitters located in hazardous areas without the need for a costly intrinsically-safe barrier.

  • Universal plant standard – With programmable input/output parameters and universal DC or AC power input, there’s no need to stock dozens of different alarm trips.

  • 20-bit input resolution – Delivers industry-best digital accuracy for both sensor (RTD and thermocouple) and analog (current/voltage) inputs.

  • Site- and PC-programmable – A choice of using front panel pushbuttons or Moore Industries’ Windows-based intelligent PC configuration software for fast and simple setup.

  • Large 5-digit process and status readout – A large display shows menu prompts during pushbutton configuration and, when the SPA2IS is in operation, shows the process variable, the output, or toggles between the two in selectable engineering units.

  • Combined alarm trip and transmitter – The analog output (-AO) option reduces costs and installation time when both alarm and transmitter functions are needed at the same location.

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