Specifying Intrinsically Safe Remote Monitoring Sensor Systems

Sensors Insights by Josh Schadel

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.



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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/VmaxUo/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 figure 1, 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.

The yellow unit is the tank sensor that integrates with the system.
The yellow unit is the tank sensor
that integrates with the system.

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:


As the four entity parameters for these two IS apparatus are true, they may be combined as an intrinsically-safe system.


IS Wiring

Installing SignalFire wireless devices in a C1D1 zone close to sensors eliminates much of the wiring needed for a standard IS wired system.  Wiring is only required to connect sensors to the SignalFire Sentinel node.   While IS wiring does not require rigid conduit with poured seals as with XP installations, ANSI/ISA- RP12.06.01 standards outline recommended practices for wiring IS equipment in classified locations. 

The following are excerpts from this standard:

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 IS installations except in two cases.  First, sealing applies when IS circuits run into a non-IS area.  (This does not apply if all equipment is intrinsically safe.)  IS wiring must be separated from non-IS wiring by at least 2”. Second, IS wiring from different IS 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 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 potential unsafe situations that can result in downtime and lost productivity.   If unsure of the equipment installation location, invest in C1D1 equipment up front to ensure safe operations.

SignalFire discusses its Class 1 Division 1 wireless remote monitoring devices for hazardous and explosive environments in a video. More info about is available online about C1D1 wireless nodes.


About the author

Josh Schadel is the Director of Engineering at SignalFire Wireless Telemetry. The company designs and manufactures wireless telemetry products that interface to industry-standard sensors and communicate to/from a Gateway using its wireless mesh network.