Hazardous Atmospheres: Selecting Intrinsic Safety Barriers

In the last edition of 20/20 Insights we covered the concept of intrinsically safe (IS) systems and how their design limits the release of energy into a hazardous atmosphere to prevent ignition. Now we will look more closely at selecting the right combination of parts to produce a safe and reliable system.

Selection of an IS device or simple apparatus to be used in a hazardous area is fairly simple. For explosion protection, ensure that the device rating is equal to or better than the classification of the hazardous area (Class, Division, Group and Temperature Classification under the NEC or Category, Zone, Group, and Temperature Classification under the IEC). Then, as with any device, also make sure that its construction is suitable for any corrosive or other adverse environmental conditions that may be present.

Selecting an appropriate intrinsic safety barrier is often even simpler, since a conscientious supplier of IS solutions (such as Kele) will usually include a barrier recommendation and wiring diagram with the product in question. In the absence of such a recommendation, though, it is not an insurmountable chore to select an appropriate barrier under the “Entity Concept” described in ISA Technical Report TR-12.2-1995. Under this concept, the IS device to be applied in a hazardous location is labeled by its manufacturer with the maximum voltage (Vmax) and current (Imax) that can be applied to it without degrading its level of safety. The IS device is also labeled with the amount of internal capacitance (Ci) and inductance (Li) that it adds to the circuit.

Each intrinsic safety barrier is listed with the maximum voltage (Voc) and current (Isc) that can be passed through to the hazardous side terminals under fault conditions, and the maximum capacitance (Ca) and inductance (La) that can be safely connected to the hazardous side terminals. Finally, the cable capacitance (Ccable) and inductance (Lcable) can usually be gleaned from the cable manufacturer’s literature or technical support line. Or, if the exact cable parameters cannot be located, the default values recommended in ANSI/ISA-RP12.6 can be used (Ccable = 60 pF/ft or 197 pF/m and Lcable = 0.20 µH/ft or 0.67 µH/m).

For simple loops (i. e., two-wire IS devices that can function with one wire grounded), matching up a barrier with an IS device is as simple as making sure the following relationships are true:

  • IS Device IS Barrier
  • Vmax must be greater than or equal to Voc
  • Imax must be greater than or equal to Isc
  • Ci + Ccable must be less than or equal to Ca
  • Li + Lcable must be less than or equal to La

Note that this is for a simple system as described above, and that each ungrounded conductor in the hazardous area requires either a separate barrier or a multi-channel barrier designed for its purpose. More complex systems need more complex analysis, to make sure that no more than Vmax can be developed between any two conductors and that no more than Imax can be driven through any terminal in the IS device. Usually, the manufacturer or supplier of an IS device requiring more than one ungrounded conductor can provide a barrier recommendation that meets these criteria. Happily, most applications in the building automation industry can be handled with a simple loop. If grounding one side of a current loop is a problem, a signal isolator such as the DT-13 can be applied in the safe area as a solution.

For an example, consider a need to monitor a ventilator’s vibration level in a battery room. The project electrical engineer has classified the area as Class I, Division 1, Group B, and the hazardous material in question is hydrogen. Special corrosion protection is not required, and the cable run from the transmitter to the IS barrier will be about 200 feet (61 meters). Hydrogen’s autoignition temperature is 968°F (520°C), so our vibration transmitter need only have a temperature classification of T1 (842° F or 450°C) or lower. The Model 140T is a good choice. It is rated IS for Class I, Division 1, Groups A, B, C, and D, and its temperature classification is T6 (less than 185°F or 85°C). Its entity parameters are shown in Figure 2. The MTL7206 barrier was chosen since it is designed for loop powered 4-20mA devices, and its Voc and Isc parameters are less than or equal to the Vmax and Imax parameters of the Model 140T. In addition, its Ca and La ratings leave plenty of allowable cable capacitance and inductance for the length of our run. Refer to Figure 1 for the cable calculations
related to our example.

The shorter of these two results is the limiting distance, so our cable run from the MTL7206 barrier to the 140T vibration transmitter must not exceed 1,333 feet (406 meters). The estimated 200 foot (61 meter) run in our example is well within this limit. All that remains is to ensure proper grounding and installation according to NEC Article 504. As with all such systems, an IS drawing that shows all relevant entity parameters must accompany the installation (Figure 2 – Click image to enlarge).

With a little care and a little math, selection of IS barriers for simple loops can be straightforward.