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Safe and Sound
Blog Safe & Sound Wearables - Trackable Safety Helmet for Miners #2: Intrinsic Safety
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  • Author Author: msimon
  • Date Created: 23 Feb 2017 9:19 PM Date Created
  • Views 496 views
  • Likes 4 likes
  • Comments 2 comments
  • intrinsic_safety
  • safe&sound
  • ti
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Safe & Sound Wearables - Trackable Safety Helmet for Miners #2: Intrinsic Safety

msimon
msimon
23 Feb 2017

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 upimage 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 sensorsimage 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 image). 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[1]. 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.

 

image

     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[2]." 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,

 

 

References:

1 - Intrinsic Safety Circuit Design

2 - Essential Concepts of Intrinsic Safety

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  • msimon
    msimon over 8 years ago in reply to mcb1

    Hi Mark,

     

    You are right, I need an enclosure for sure but I am not sure it should be explosion-proof hence it is lower than the ignition curve. As far as I know, motors are high power and they should have an explosion-proof case and as you said they are not necessarily airtight. They should limit any explosion inside the box and cool down the air before realising the outside.

     

    If I can't achieve the 1W threshold (I am not sure the threshold value is correct hence some information is regarded as a secret and not shared), it needs an explosion-proof case which is mainly a mechanical design with intrinsically safe barriers to communicate with outside. However, my aim is to decrease power as much as I can and have a design like intrinsically safe smartphone.

     

    Thanks,

    Mehmet

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  • mcb1
    mcb1 over 8 years ago

    any device which will neither generate nor store more than 1.2 volts, 0.1 amps, 25 mW or 20 μJ.

    You may find that this is the energy level that is exposed to the explosive environment.

     

    When I worked with Fuel Dispensers, many of the higher energy parts were within a special housing.

    It was not airtight, but was designed to limit any internal explosion from propogating a flame to the external environment.

    This meant the number of threads and sizing was designed and tested to ensure it would allow gas to escape but at a pre-ignition temperature.

    Gallagher Fuel Systems' explosion proof box

    In the case of the older fuel dispensers, the feed to the display was through a limiting device to reduce the energy available.

    Motors were intrinsically safe with no fan and a run time of 30 mins to stop heat buildup.

     

     

     

    I'm sure your design can use a case and you specify that a production model uses an approved design.

    You may want to try to limit any wiring that passes out to a certain energy level (easily achievable nowdays).

     

     

    Mark

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