April 22, 2015
There are two different protection methods when it comes to hazardous location certified sensors: intrinsically safe or explosion proof. Many operators simply choose the less expensive intrinsically safe option without knowing why you may want to consider an explosion proof sensor.
An intrinsically safe sensor has been designed with a low voltage and current to ensure that there is not enough energy to arc under failure conditions. The component temperatures are also kept low to ensure that gas in the atmosphere will not ignite.
An explosion proof sensor does not have a lot of the same restrictions. It is allowed to ignite the gas under failure conditions as long as the explosion is contained within the housing. The more rugged and dependable housing, along with the specialized testing and certification, is why these devices are usually a lot more expensive.
It all comes down to the rating of the controls you are using with the sensor and which areas each device is installed in.
If, for instance, you are using a plunger lift controller that is rated Class I, Zone 0 (Division 1), it has been designed and certified to have a safe amount of energy which will not ignite gas if the sensor were to fail. This controller can then be connected directly to the sensor without any additional components.
If you are using a controller that has more energy than allowed in the Zone 0 (Division 1) area, such as a remote terminal unit (RTU) rated for Class I, Zone 2 (Division 2), it will be installed in this less hazardous area. If the sensor is located in a more hazardous area, you must take steps to protect the sensor. These same steps must be taken any time energy can be delivered from a less hazardous area to a more hazardous area.
Figure 1 – Connecting across zones/divisions
If you wish to use an intrinsically safe sensor, then a barrier must be installed in the less hazardous (Zone 2,Division 2) area. A barrier is a device that limits the current and voltage so that the wiring entering the more hazardous area is safe. In this example, we take an RTU with Zone 2 rated outputs and make them safe for Zone 0/1. We are now able to connect to an intrinsically safe sensor that is rated for, and installed in the Zone 0 area.
Using an IS barrier adds cost, additional wiring and one more failure point. Many producers prefer to avoid using an intrinsically safe sensor and barrier because of these reasons. The much simpler (and typically less expensive) solution is to use an explosion proof sensor. Since we don’t need to constrain the voltage and current to Zone 1 levels, the barrier is not required. We instead rely on the sensor to contain an explosion under failure conditions.