In the previous post, I have talked about my proposed project. Hence my project primarily targets to improve the safety of the miners, it has to be ATEX compatible (in Europe) or intrinsically safe (in the USA). It looks like test centres provide their test in accordance with the multiple regulations so you can get the approval with one test for the most part of the world. In this week's blog post, I will talk about some regulations for electronics in explosive areas like mines. I don't have any experiences working in the safety-critical environments so please correct me if there is a mistake. The idea behind the intrinsically safe equipment is preventing the design to release enough energy to create an explosion. This can be done either by ignition or a hot surface. The lesson is that don't spark and don't heat up However, it is not that simple. Let's see some details of the standards.
"Simple device is defined 3.12 of the ANSI/ISA-RP 12.6-1987 as any device which will neither generate nor store more than 1.2 volts, 0.1 amps, 25 mW or 20 μJ." Simple devices can be used intrinsically safe and do not need to be approved. Therefore, LEDs, thermocouples may not need an approval. Unfortunately, my design is not in this category because it requires 3.3V or even 5V for sensors This means that this particular design should be tested in accreditated testing laboratories. This is way beyond the aim of the contest and my capabilities (at least for now ). Let's move on some other details. The energy level of any intrinsically safe device should always be below the ignition curve in Figure 1 where operating energy level is 1W or less. Fortunately, my design operates at 5V so my main design consideration is the whole system should be working under 1W. Still, this does not guarantee the intrinsic safety, it is just the necessary step.
Figure 1 - Ignition Curve
The idea behind the ignition curve is preventing to emit energy to create an ignition. Therefore another two important parameters, capacitance and inductance, play a role. Hence they can store energy, they should not store more than limitations. Let's say we overcome these problems and our system is obeying these rules. Is it intrinsically safe now? No, not yet. We only regulate the first but the most important rule limiting the energy. Another design consideration is fault tolerance. The design also should not exceed these limitations in a case of any fault. The design testing is done for the worst-case scenarios. "The probability of failure is irrelevant and is not considered." Maybe, the fault may occur once in million but that error can cause a catastrophic accident so the design should prevent exceeding limitations and it may require using fuses and Zenner diodes. The PCB design and encapsulation should be designed to prevent the environmental effects such as dust causing the short-circuit.
The surface temperature of the design also must be considered. The good design will solve this problem hence the power consumption is limited and the system is shut downed in a faulty condition. This is important because the gas or other substance may combust if the temperature limit is exceeded. If the design is working at the edge, an on-board temperature sensor may be required.
In order to prevent sparks, it is better to lock parts of the system to the each other. For example, batteries should be closed by screws so that people will not unplug and plug components inside the mine.
To summarise, intrinsically safe devices are mainly constrained by their energy consumption and temperature of the device is limited. They also designed to work under the harsh environment such as a dusty place. They are fault tolerant and do not exceed the limitation even the worst-case scenario. Obviously, they do not spark.
If we look at the parts of my design, I will use the development board and booster packs. Hence, there are extra components and system will be bulky, a PCB design is required. I am not sure in the given time frame, I can test the working prototype and design a PCB. However, I will limit the system power consumption to 1W and consider the surface temperature. I will also inform you if there is an obvious violation of the regulations in the prototype boards (if I can detect). If you have any suggestion, I look forward to hearing from you.
P.S. I may have errors hence this is my first design for safety critical environments. Please, correct me if there is an error.
The information above is particularly for portable devices so it does not include the grounding and intrinsically safe barriers,