Questions about safety barriers
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I’ve been working on a project from before joining my current company recently; the design for this project was done by a design institute. While reviewing the documents, I encountered an issue related to safety barriers. First, the functions of the safety barrier are as follows; I also work at Baidu. 1. The safety barrier is a device that connects intrinsically safe instruments to non-intrinsically safe equipment ； 2. The safety barrier utilizes internal energy-limiting circuits to restrict hazardous signals between intrinsically safe instruments and non-intrinsically safe devices, thereby ensuring that the intrinsically safe properties of the field instruments are not compromised. For example, in the event of a short circuit in an intrinsically safe instrument, the safety barrier prevents the short-circuit current from becoming too high and posing a risk ； If a non-intrinsic safety device (such as a PLC card) experiences breakdown, the safety barrier ensures that hazardous voltages and currents are not transmitted to the intrinsic safety instruments on site, thereby preventing any danger. Secondly, after understanding the function of the safety barrier, in the specifications for the armored thermocouple: 1. In the specification sheet, the explosion protection rating is ExdIICT4, which indicates it is explosion-proof (as shown in the figure below). 2. In the list of measurement points, the “Intrinsic Safety” column corresponding to this thermocouple is marked with “Y”; I understand this to mean that the instrument is intrinsically safe. 3. In the instrument index table, this thermocouple is equipped with an analog safety barrier. I have 2 questions; please discuss them. 1: Even when thermocouples are not intrinsically safe, safety barriers are still used. In other words, the electrical signals sent to the thermocouples from the electrical cabinet are those that have been converted by a safety barrier. Can these converted signals enable the thermocouples to function properly? 2. Is his approach unnecessary? The system can still function with just the safety barrier thermocouples; it’s merely an additional safeguard that is also useful. Or it is also acceptable to not include a safety barrier. In other words, it’s fine not to add it; adding it makes it more secure, right?
From what I know about instrument selection these days, manufacturers such as Rosemount, Yokogawa, and E+H have some of their instruments certified for both explosion protection (for the device enclosure) and intrinsically safe operation (for the circuit components). Even when flameproof instruments are equipped with safety barriers, the intrinsically safe requirements are not met. Due to the energy-limiting function of these safety barriers, it may also be impossible to drive the field instruments, and in addition, an extra point of failure is created, which is unnecessary.
The reason for this confusion is that you have made a cognitive error (many people, including me, have experienced this): intrinsically safe is a system concept, not referring to instruments； By intrinsically safe instruments, we mean those that can meet the requirements of intrinsically safe systems; it does not refer to the instruments being capable of being intrinsically safe on their own. The basic functions of an intrinsically safe system can be summarized in two main aspects: 1. Preventing the transmission of hazardous energy into explosive areas ； 2. Do not generate hazardous energy in explosion-proof areas. The previous function mentioned above is also the main function of safety barriers; some control room instruments also possess this function ； The intrinsically safe instruments on site need to complete one more function. Here, there are mainly two requirements: first, the energy storage unit within the instrument must not release energy that reaches dangerous levels; second, its own energy consumption must be below the threshold of energy allowed to be sent into hazardous areas. .
In short, the safety barrier is added as an effective means of responding to emergencies; it prevents external lightning strikes or fluctuations in testing currents from causing damage, and also stops pulses from the on/off power supply inside from damaging instruments and cards.
A safety barrier is a device that separates hazardous areas from safe areas, thereby limiting the amount of electrical energy that can reach the hazardous area; this limited amount of energy is not sufficient to cause an explosion or fire in that area. Intrinsic safety refers to an intrinsic safety circuit, meaning that all components constituting the circuit are intrinsically safe. This includes the sensors and transmitters that make up the circuit, and the cables used for signal transmission should also be intrinsically safe cables. Intrinsic safety circuits follow the principle of isolation, that is, intrinsic safety circuits are separated from non-intrinsic safety circuits. Specifically, the cables should not be shared, nor should the grounding systems, wiring boxes, terminal strips, or cable trays. Cable trays are best to be separate; if that’s not possible, metal isolation panels for grounding should be added.
In other words, intrinsically safe instruments can also perform this function; it’s just that intrinsically safe designs do not generate any hazardous energy, while flameproof designs do generate such energy, but it is contained within the flameproof enclosure as a result. Of course, the risk remains higher than that of intrinsically safe devices, since there is still a possibility that hazardous energy could leak into the explosive environment. In other words, whether the instruments are intrinsically safe or flameproof, a safety barrier is necessary, as it prevents dangerous energy from being transmitted from the electrical cabinet to the explosion-proof area. You can understand it that way, right, senior?
That means explosion-proof instruments also require safety barriers; in other words, if the wires in the electrical cabinet lead to explosive areas, then these wires need to have their electrical energy limited. Is that how it should be understood? Senior
This post was last edited by Hao Liang Zhi Le on 2022-6-21 at 20:54. When the electrical energy of the instrument cannot be limited, isolation methods are used for explosion protection. For example, in the case of electromagnetic flowmeters, since a sufficient magnetic field strength needs to be generated, the current flowing through the coils of such flowmeters cannot be too low; therefore, it is not possible to limit the electrical energy consumption of the instrument. The principle of flameproof instruments is to keep explosive gases outside the instrument enclosure so that they do not enter the interior of the instrument. Even if sparks are generated inside the instrument due to a fault, it will not cause an explosion of explosive gases outside the enclosure. Even if there is a small amount of hazardous gas inside the instrument, and a spark generated by a fault causes a minor explosion within it, the thick casing of the instrument prevents any release of hazardous gases outside. Therefore, flameproof instruments have thick casings to resist minor explosions inside, and they must also have strict sealing measures to achieve isolation. Since it is not possible to limit the electrical energy at the site, flameproof instruments cannot use safety barriers. Because the safety barrier in use limits the electrical energy reaching the field, the instruments on site often fail to function properly.
You can’t say that. Although it’s possible that my perspective on you is different from yours. Flameproofing and intrinsically safe are two completely different explosion prevention strategies: The intrinsically safe approach involves eliminating all conditions that could lead to an explosion, which is why it is called intrinsically safe ； The strategy of explosion isolation is to limit the impact to a certain area in the event of an explosion. For intrinsically safe meters, many non-standard practices in the field usually do not lead to serious consequences ； For flameproof meters, improper installation and operation not in accordance with regulations can both lead to serious consequences. An explosion-proof system does not require a safety barrier. Of course, a higher safety factor is available, provided that it can be applied. .