To answer the question of whether you need to use an isolated barrier device with your sensor, it’s helpful to first answer the following two questions:
- What type of output does my sensor have?
- Is my application in a classified area, and if so, what is that classification?
Sensor output types
Sensors that operate as electronic switches typically have an ON state in which an electrical signal is sent and an OFF state in which that electrical signal is absent. A common electrical signal is the sensor’s power supply voltage, for example, 24 VDC or 120 VAC. This signal can be thought of as standard output since it’s what most people expect to see from a switch–voltage when “closed,” and no voltage when “open.”
In contrast to this voltage-switching output are sensors with NAMUR output. A device with this specialized output still has ON and OFF states, but because it does not include an amplifier circuit, the output is a low-level current change that allows the sensor to be used in hazardous areas where a higher output would have the potential to ignite gas or dust. In other words, while NAMUR-output sensors may operate on 24 VDC, they cannot provide a 24 VDC signal on their own. A separate amplifier is required.
Using sensors in classified areas
You can use both standard and NAMUR output sensors in certain hazardous locations, depending on the area classification and device approvals. For example, many standard voltage-switching sensors are considered non-incendive and are FM-approved for installation in Class I, Division 2 applications without a barrier.
You can also use NAMUR output sensors in such locations. However, their design is especially suited for Class I, Division 1 applications. In combination with an approved isolated barrier, NAMUR output sensors ensure intrinsic safety. In these cases, the barrier limits voltage and current levels to the sensor, which is located in the hazardous area. The barrier also amplifies the low current output of the sensor to a level where most controls recognize the signal (for example, 24 VDC or 120 VAC).
When to use an IS barrier and when not to use one
Getting back to our original question, when should you use an IS barrier? If you have a standard voltage switching sensor, then a barrier is not used. For one thing, most IS barriers provide a nominal 8 VDC to power a connected sensor. This amount of power is not adequate for standard electronic sensors, which typically require between 10 VDC … 30 VDC for operation. Secondly, simply adding an IS barrier will not make a standard output sensor intrinsically safe or approved for use in a Class I, Division 1 location.
On the other hand, a NAMUR output sensor in a Class I, Division 1 application calls for a barrier. These two devices are designed to operate together–the sensor in the classified area, and the barrier in the nonhazardous area–to maintain intrinsic safety. If the area is not classified, you can still use a barrier to provide an amplifier for the NAMUR output, but a barrier is not required–there are other simpler, less expensive amplifier devices available that you can use in these cases.
- Use an IS barrier if you have a NAMUR output sensor and need to maintain intrinsic safety in a hazardous location.
- An IS barrier is not used with a standard output sensor. An IS barrier is not required if you have a NAMUR output sensor and don’t need hazardous area approvals (use a simpler, less expensive solution).