The primary reason behind requirement of intrinsic safety is that the circuitry in any device should cause hazardous situations. The intrinsic safety is more applied to devices which have higher currents flowing through them and which are exposed to external environment. The primary requirement is that any failure should not cause unsafe conditions like blowing up or causing problems to humans. As a part of intrinsic safety, the designer has to use a shield or a barrier to isolate sections which can cause hazardous situations. We generally see intrinsic safety being considered in industrial applications like that of motor, solenoid valves, etc., Now take the case of a simple gas lines laid underneath your local area, any leakage or exposure of such areas could cause serious blasts. so, these need to be have a barrier to be safe. this is one example which gives us a clear view of what intrinsic safety is.
From a designer point of view, what are the major points that have to be considered for intrinsic safety?
Designer has to identify the sections in his design which can generate or store energy beyond a certain limit and shield such devices. For example, a capacitor in a circuit stores energy. If the energy exceeds beyond 20uJ, then this need to shielded. This happens in all cases of power supply designers. So, in these cases, it is very important that the designer provides a separate cabin for the power supply in his system. Now generally, a doubt comes to the mind that if high energy devices are shielded, then can my system be intrinsically safe? Yes, indeed. Any other devices with lesser energy levels of storage or generation are automatically intrinsically safe. For example, resistors less than this power, LEDs and other devices are automatically intrinsically safe. Redundancy could sometimes be another factor to be intrinsically safe. when we discuss about the redundancy here, this gives a point to think, is intrinsically safe criteria is more about isolating the hazardous area from normal area using some mechanical means or electrical means? The answer is it could be both. For mechanical means, we have to use explosion proof enclosures or other means. Whatever the way it may be, isolation is very important.
Wiring is one major area which can cause the failure to spread to different regions on the board. For a circuit designer, isolating the ground between the hazardous area and normally operated area could be one solution to avoid damage and be intrinsically safe. Electrical engineer should also take following measures:
1. Monitor the temperature and include control mechanisms on the board to avoid over heating
2. Operate the circuit under as low voltages as possible
3. Avoid usage of high energy storage through capacitors or inductors
4. Isolate grounds wherever possible - Galvanic isolation
5. Current limiting mechanism - usage of Fuse, NTC
6. Over voltage protection - TVS, Zener diode usage
Typically, the hazardous areas are divided into zones. For example, "Zone 0" is the classification for most hazardous area and "Zone 1" is less hazardous than "Zone 0" where as "Zone 2" is occasionally hazardous. The intrinsic safety standards define systems as "ia", "ib" and "ic" and when classified as "ia", the system can be used in most hazardous "Zone 0". Similarly "ib" for "Zone 1" and "ic" for "Zone 2".
If your system need to be branded intrinsically safe, then it must be certified first. For this there are several certifications for different regions of the world.
1. United states - UL certification
When a designer, selects a relay for his application, he tends to check few parameters from the relay vendor and UL certification is one of the major specification.
Usage of UL certified part is a must. The standard for relays is UL508.
2. Europe - ATEX
3. Japan - TIIS
4. Other countries - IEC Ex (IEC 60079-11, IEC 60079-15)
Standards have defined a specific energy levels for each gas which can cause explosion. Like for example, a 20uJ of energy can cause ignition in the Hydrogen. This defines basically the energy limit for a system which is used in the hydrogen environment.
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