Modeling, Simulation and Design of the Intrinsic Protection Using Safety Barriers

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1 Modeling, Simulation and Design of the Intrinsic Protection Using Safety Barriers MONICA LEBA, EMIL POP, BOGDAN SOCHIRCA, PETRE MARIAN VAMVU System Control, Applied Informatics and Computer Engineering Department, University of Petrosani, Romania, Abstract: In this paper the modeling, simulation and design of an intrinsic protection using safety barrier is presented. It is presented a short introduction regarding the intrinsic protection and the terminology used in explosive areas. If a device wants to communicate or acquire data from hazardous areas, it needs to be protected because if a fault appears there can lead to an explosion risk for the hazardous area. For this reason, there are designed dedicated safety barriers for every application. The mathematic model and the simulation results of a safety barrier using Zener diode, resistors and fuse are presented. Keywords: intrinsic protection, safety barriers, mathematical model, simulation. 1 Introduction It is known that the origins of the intrinsic protection date from the disaster of the coal mine in Senghenydd in Among the first who have made researches in this field were Wheeler and Thorthon who have proposed the technique of limiting the ignition energy called now intrinsic protection. From dictionaries the definition of the intrinsic word is by his nature or by his construction. The intrinsic protection is obtained by including some components in a circuit as an integrated part of the circuit design, which is intended to have intrinsic protection. The principle of protection with intrinsic safety is to ensure that the thermal levels that appear in an electric circuit which came in contact with the flammable gas to be limited under the level of the gas ignition. An electrical circuit or a part of it has intrinsic protection, if, in normal or fault functioning conditions, can t ignite an explosive area through spark or thermal effect. This definition suggests that spark and heat are allowed in an electric circuit in specified fault condition, but with the condition not to outrun the level that could initiate the ignition. These criteria and the separation criteria from other circuits, offer the integrity and good safety conditions. The possible faults are estimated through a serious examination of the faulting mechanism that can appear. The specific components are integrated in the circuit design in order to maintain the energy levels in safe parameters even under the incidence of a major fault produced in the circuit. These components are called safety components and are integrated in the circuit functional structure. To increase the safety, there are analyzed the possible faults of each of the circuit components in order to ensure the safety level even when these components break down one by one. Wheeler and Thorthon propose to investigate the voltage and current which cause the ignition. Empiric study quantizes the maximum values of the allowed voltage and current in the intrinsic circuits. This study covers the current and voltage levels and also the variation of current and voltage on the presence of inductances and capacitances. The explosion risk appears if two factors are present: the hazardous area and the ignition energy. The hazardous areas are classified in: Area 0 where the explosive atmosphere is permanent or for long periods of time. Area 1 where the appearance of the explosive atmosphere is probable, in normal functioning conditions of the technological installation (conveyer belt) which produces flammable dust. Area where it is unlikely the appearance of the explosive atmosphere in normal functioning conditions of the technological installation, but if it appears, it is probably rare and for a short period of time due to expected fault. The explosive atmosphere from area (surface industry) regarding the minimum ignition criteria (M.I.C), was divided in 3 subgroups: II A where the representative substance is propane and the minimum ignition energy is 30 μj II B where the representative substance is ethylene and the minimum ignition energy is 160 μj ISSN: ISBN:

2 II C where the representative substance is hydrogen and the minimum ignition energy is 40mJ. The most important concept for the intrinsic protection is the simple apparatus concept. In this category are included thermocouples, temperature dependent resistors, sensors, LEDs, switches, thermostats and others that can be used in a hazardous area without special validation if they can t accumulate more then 1.V, 0.1A, 0mJ, 5mW. The surface temperature of the simple apparatus in normal conditions or fault conditions must not exceed the ignition temperature of the surrounding atmosphere. The initiation energy called the dissipated energy (E) appears in case of electrical equipments especially in the presence of the inductance (L) and capacitance (C) with the value: 1 E = CU (1) or 1 E = LI () where U is the voltage [V] and I is the current [A]. In order to certify a device with intrinsic protection, the total dissipated or stored energy is necessary to be under the minimum ignition energy curves of the area where the devices are, as shown in fig.1. Simple apparatus: in this category it doesn t matter any fault condition because whatever fault appears, the circuit does not represent any dangerous potential for the hazardous environment, so these can be used in any hazardous environment. Ex ib : devices that can have one predictable fault, and even if a predictable fault appears, the circuit remains safe. If a second fault appears, the circuit is no longer a safe one. Ex ia : devices that can have two predictable faults, and even if two predictable faults appear the circuit remains safe, a third fault can make the circuit to be no longer a safe one. Fig.1. Minimum ignition curves: a) Resistive circuit I=f(U); b) Inductance L=f(I) for U=4V; c) Capacitance C=f(U) Problem Formulation In this section we present the principles and criteria for using the safety barriers protection..1 Intrinsic basic criteria The basic criteria for any circuit with intrinsic protection are: ISSN: ISBN:

3 limit the external circuit voltage which is transmitted in hazardous area; limit the current in the external circuit; limit the energy in fault conditions by introducing a power supply with fault identification; limit the stored energy in hazardous area circuit; limit the probability of overlapping the voltages or currents from external sources. Using high power capacitors in a circuit is recommended to be avoided, where it is possible. If a high power capacitor is necessary, there are used Zener diodes to limit from the rest of the circuit the applied voltage in fault conditions. The diode or Zener diode can be simple, double or triple, depending on the application. Using a series resistor will limit the charge/discharge rate and eliminate the danger of spark appearance. A parallel shunt resistor with a capacitor will allow controllable discharging in a controllable time. Many components of a circuit can be put together having the effect of a higher security, helping at heat dissipation, increasing the moisture resistance... Safety barrier protections One of the wide spread solutions for intrinsic protection is represented by safety barriers. The safety barriers are used in control of systems which operate with standard signals in current loop of 0-0mA and 10V. The barriers contain circuits with intrinsic protection used in the control of the intrinsic protection devices placed in hazardous areas. The safety barriers must be installed outside the hazardous area. Only approved intrinsic protection circuits are allowed to be mounted in hazardous area. The components of a safety barrier are: a resistor to limit the current, a Zener diode to limit the voltage and a fuse. If the supply voltage is increasing over a specified construction value as a result of a fault in the safe area, the diodes will start to conduct causing the fuse braking and by this preventing the transfer of the energy to the hazardous area. Considering the combination of resistor, Zener diode and safety fuse, if any two of these are to crack, than the circuit will stay safe. All of these components are considered safe, therefore to this assembly it is conferred the Ex ia classification. The probability to appear a fault of these components was estimated to approximately faults per year, so they are considered extremely reliable. There are two distinctive sets of information which describe the barrier characteristics: safety and operational. The safety characteristics offer the safety description of the barrier which represents the biggest voltage that can occur in a safe area during fault conditions. This information is necessary for the connection of the safety barriers to a circuit with intrinsic protection located in a hazardous area. The operational characteristics are those characteristics which are necessary for the barrier to work in good condition. The barrier must allow the passing of the signal without distorting the signal electric properties. Caution must be taken in choosing and designing the barrier to ensure a good functioning, but in the same time the whole system must remain safe from the intrinsic protection point of view. There are barriers with electronic protection to over-voltage. These are called semi-active barriers and are more expensive then the passive barriers. These are commonly used in the systems with battery where can exist a floating voltage which can exceed the maximum admitted voltage U max, but does not lead to the fuse braking. In fig. are represented different safety barriers. In the applications where the galvanic isolation is used, a grounding connection is not necessary, and in application where Zener diode safety barriers are used the grounding connection is necessary. The disadvantage of the safety barrier is that it requires a grounding connection. This imposes that the circuit located in the safe area and the circuit located in the hazardous area need to be linked to the same grounding circuit. The common effect is not harmful itself with the condition that the grounding connection of the intrinsic protection is made correctly. A fault between the circuit and the grounding may cause a spark or a thermal effect in the point where the fault appears. Where there are used more then one limiting circuits with the same grounding connection, the effect of loosing the grounding is the increase of the risk that some parasite channels would lead to the appearance of danger. A ground connection needs to be considered as a direct connection for the current to flow from the safe area to the hazardous area. Any current or voltage limiting method can not be applied in this connection. This is considered as a weakness of the design. ISSN: ISBN:

4 Fig.. Safety barriers with Zener diodes Fig.3. Galvanic isolator The grounding, divided between the circuits located in the safe and the hazardous areas is interrupted by electric isolation in a galvanic isolator. In this way the current can not flow between the two parts of the circuit. A different potential can appear on the isolator but with no potential danger for the safety of the circuit. The circuit from the hazardous area can be grounded without causing the current to flow through that point. A diagram of a safety barrier with galvanic isolation is presented in fig.3. The galvanic isolation is achieved by placing an isolation method of the electrical signal between the safe area and the hazardous area. From the electronic point of view, the circuits from the hazardous and from the safe areas have to communicate no matter the isolation system used. In this case it is used the galvanic isolation. Examples of galvanic isolators are: transformers, relays, and optocouplers. 3 Problem Solution In this paragraph we present the mathematical model of a safety barrier. We consider the barrier from the fig.4. ISSN: ISBN:

5 Fig.4. Equivalent design of a safety barrier We use the following notations: U i -Voltage applied to the barrier; F-Fuse; D z - Zener diode for voltage limiting purpose; R- Limiting current resistor; R s -.Load resistance; C-Load capacitance; U z -Zener diode opening voltage; I SC -fuse breaking current; I-Current barrier that flow in normal condition; θ(u i -U z )- Step distribution. Results the following input output equation: U i 1 R + R s 1 I = + Ui ISC R R Rs C R Rs C + s (3) Ui + θ( Ui Uz) R Based on this equation it is designed the mathematical model in MatLab-Simulink, with the model and simulation results from fig.5. Fig.5. Barrier: a) model; b) simulation results ISSN: ISBN:

6 4 Conclusions There are presented the principles of the intrinsic protection and the terminology used in explosive areas. There are presented the dedicated safety barriers for different applications. There is modeled, simulated and designed an intrinsic protection using safety barrier. The mathematic model and the simulation results for a safety barrier using Zener diode, resistors and fuse are determined. References [1] E.Pop, Automatizări în industria minieră. Editura Didactică şi Pedagogică, 1983 [] G.Bottrill, D. Cheyne, G. Vijayaraghavan, Electrical Equipement and Installations in Hazardous Area, Editura Elsevier, 005 [3] [4] * * *, Instalatii utilizate in mine grizutoase, INSEMEX, 007 ISSN: ISBN: