Wireless sensor networks frequently operate in hazardous areas where explosive atmospheres pose a safety risk to equipment and operators. A Class I, Division 1 hazardous area is defined as an environment 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 or well level monitoring application that involves the storage of dangerous or volatile materials. The top of the oil tank or well head presents a potentially hazardous environment. All the components of a wireless sensor control system operating in these types of potentially dangerous applications must be properly rated for safe operation.

Intrinsically safe (IS) equipment is designed so that energy levels are low enough not to generate an arc, spark, or temperature that could ignite an explosive area. IS equipment differs from explosion-proof (XP) systems where ignition is contained within an enclosure so as not to ignite the explosive environment. While both appropriately rated IS 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.

IS Equipment Installations

SignalFire IS-approved devices can be connected to other IS-approved apparatus such as a sensor. Before configuring sensors and other components to SignalFire IS equipment, installers must first evaluate entity parameters to ensure compatibility. All IS-rated equipment has four rating parameters:  voltage, current, capacitance, and inductance.  The entity concept states:

U1/Vmax > Uo/Voc (Max input voltage > max output voltage)

Ii/Imax > Io/Isc (Max input current > max output current)

Ca > Ci + Ccable (Max capacitance apparatus > capacitance + cable capacitance)

Lo/La > Li + Lcable (Max inductance > apparatus inductance = cable inductance)

These formulas determine that two pieces of IS equipment can be used together while maintaining an IS rating.  For example, the maximum voltage and current from the device providing the power must operate below the maximum rated voltage and current of the device being powered.  In determining ratings, information is listed on the equipment markings or available on a control drawing from the equipment manufacturer.

Example of an IS Entity Parameter Evaluation

Let’s say, for instance, that you are considering using a Sentinel HART node in combination with a VEGAFlex 81 guided radar sensor for the continuous measurement of flammable liquids inside a tank.

As shown in the graphic, the radar tank sensor (yellow unit) takes a level measurement and integrates with a SignalFire C1D1 Sentinel node (white unit) that sends data to a gateway for download into a SCADA system or local PLC.  The hazardous location node also powers the sensor, making the level monitoring system completely wireless.

Situated outside the C1D1 area, the gateway, PLC or other polling device do  not need to meet C1D1 ratings.   All equipment installed within the C1D1 area must be intrinsically safe to operate in the hazardous environment.  An evaluation of the Sentinel HART node and VEGAFlex 81 ascertain their viability for this application. The installer is responsible for ensuring that equipment installed into a C1D1 area meets these requirements.

Entity parameters of a Sentinel HART node are:

Uo/Voc = 21 VDC, Io/Isc = 111 mA, Ca = 1.16 uF, Lo/La = 11.5 mH

Entity parameters for a VegaFlex 81 are:

U1 = 30 VDC, Li = 131mA, Ci=0 uF, Li = 5uH

To evaluate the combination of these two pieces of equipment, the entity parameters must be compared: