Wireless sensor networks frequently operate in hazardous areas where explosive atmospheres pose a safety risk to equipment and operators. The design of these systems must follow National Electrical Code (NEC), Canadian Electrical Code (CEC), and IEC/CENELEC installation requirements for Intrinsically Safe (I.S.) systems.
There are different categories of hazardous areas — this article will discuss design criteria for electronic circuits in hazardous areas designated Class I, Division 1 (C1D1). These are defined as environments where combustible gas, vapor, or mist is present or is expected to be present, for extended periods under normal operating conditions. An example is a tank level or oil well monitoring application that involves the storage of dangerous or volatile materials. The top of the oil tank or well head is the potentially hazardous environment. All components of a wireless telemetry system that operate in these types of potentially dangerous applications must be properly rated for intrinsically safe operation.
The goal of I.S. equipment design is to ensure that energy levels are low enough not to generate an arc, spark, or temperature that could ignite an explosive area, even under a fault condition. The electronics typically utilize zener diodes as voltage clamping devices to limit the voltages to safe levels, and current limiting resistors and fuses to limit maximum currents. The electronics must be designed from the ground up to meet the I.S. requirements.
Intrinsically Safe vs Explosion-Proof Systems
IS equipment differs from explosion-proof (XP) systems, in which a possible source of ignition is contained within the enclosure so as not to ignite the explosive environment. XP equipment does not require the same careful electronics design considerations as I.S. devices. But they do require the use of certified explosion-proof enclosures. In addition, all wiring must include explosion-proof conduit with appropriate seal-offs. XP devices must not be opened within a C1D1 hazardous area, which makes maintenance such as battery replacement more difficult. Additionally, access to internal buttons for configuration or setup is not possible.
While both appropriately rated I.S. and XP equipment are useable in a Class 1 Division 1 area, IS systems are typically cheaper and easier to install, as they do not require heavy enclosures and expensive conduit runs.
The Entity Concept
An I.S. approved apparatus, such as a sensor, may be connected to an I.S. approved associated apparatus such as a power source by following the rules for an “entity concept.” ISA-TR12.2-1995 defines the entity concept as follows.
The entity concept provides more flexibility in selecting equipment for an intrinsically safe system. The entity concept allows the user to identify acceptable combinations of intrinsically safe apparatus and associated apparatus that have not been examined as a system.
There is a set of safety or entity parameters for all I.S. equipment. Installers must first evaluate these entity parameters to ensure compatibility before use. I.S. rated equipment entity parameters are: voltage, current, power, capacitance, and inductance.
These values are defined as follows:
The entity concept states:
Ui or Vmax > Uo or Voc (Max input voltage > max output voltage)
Ii or Imax > Io or Isc (Max input current > max output current)
Pi > Po or Pt (Max input power > max output power)
Ca > Ci + Ccable (Max allowed capacitance > internal capacitance + cable capacitance)
Lo or La > Li + Lcable (Max allowed inductance > internal inductance + cable inductance)
These formulas determine whether two pieces of I.S. equipment can be connected while maintaining an I.S. rating. For example, the maximum voltage and current from the device providing the power (associated apparatus) must operate below the maximum rated voltage and current of the device being powered (I.S. apparatus). In determining ratings, refer to information listed on equipment markings or on a control drawing from the equipment manufacturer.
Example of an I.S. Entity Parameter Evaluation
Let's say, for instance, that you are considering using a C1D1 radio node from a remote monitoring system in combination with a guided radar sensor for the continuous measurement of flammable liquids inside a tank. (A C1D1 node is designed specifically for use in hazardous areas.) The node transmits data from sensors to a gateway that formats it for download to a computer, PLC, or Internet. (See Figure 1.)
As shown in Figure 2, a radar tank sensor (yellow unit) takes a level measurement and integrates with a radio node (white unit) that reads and sends data to a gateway for download into a SCADA system or local PLC. The hazardous location wireless node also powers the sensor, making the level monitoring system completely wireless.
Situated outside the C1D1 area, the gateway, PLC, or other polling devices do not need to meet C1D1 ratings. All equipment installed within the C1D1 area — such as the node and radar sensor — must be intrinsically safe to operate in the hazardous environment. An evaluation of the node and radar sensor ascertains their viability for this application. The installer is responsible for ensuring that equipment installed into a C1D1 area meets the requirements.
The following is a typical calculation:
Entity parameters of a SignalFire Sentinel HART node are:
Voc = 21 VDC, Isc = 111 mA, Pt = 0.8 W, Ca = 1.16 μF, La = 11.5 mH
Entity parameters for a VegaFlex 81 guided radar level sensor are:
Ui = 30 VDC, Li = 131 mA, Pi = 0.983 W, Ci=0 μF, Li = 5 μH
To evaluate the combination of these two pieces of equipment, the entity parameters must be compared.
Since the five entity parameters for these two I.S. apparatus meet the entity concept, they may be combined as an intrinsically-safe system.
I.S. Sensor Wiring
Installing wireless devices in a C1D1 zone close to sensors eliminates much of the wiring needed for a standard I.S. wired system. Wiring would only be needed for connecting sensors to a node. While I.S. wiring does not require rigid conduit with poured seals as with XP installations, ANSI/ISA-RP12.06.01 and NEC Article 504 standards list required practices for wiring I.S. equipment in classified locations. The following excerpt is from the NEC Article 504.
504.20 Wiring Methods. Intrinsically safe apparatus and wiring shall be permitted to be installed using any of the wiring methods suitable for unclassified locations, including Chapter 7 and Chapter 8. Sealing shall be as provided in 504.70, and separation shall be as provided in 504.30.
This statement means that any standard wiring practice for unclassified locations is permissible in I.S. installations with two exceptions. First, sealing applies when I.S. circuits run into a non-I.S. area to prevent gases from exiting into the safe location. (This does not apply if all equipment is intrinsically safe.) Second, I.S. wiring must be run in separate conduit or separated by at least 2” of air space from non-I.S. circuits, and I.S. wiring from different I.S. circuits that run together must have an insulation thickness of 0.01”.
Local and national codes require that any equipment installed into hazardous locations be appropriately rated and installed per code requirements to prevent an explosion that can destroy equipment or, even worse, harm workers. Using properly rated and installed C1D1 equipment in these locations ensures safety in the hazardous area.
With the right component classifications, you guard against potentially unsafe situations that can result in downtime and lost productivity. In areas where hazardous gases or vapors may be present it is important to ensure that appropriately rated equipment is used and properly installed. When in doubt, ask the manufacturer about the right equipment for a specific environment and ensure the installation is compliant to all local codes.