Intrinsic safety systems

Hence, attention focused on passive safety systems and on inherent or intrinsic safety systems. These needed fewer auxiliary systems, they were simpler, with a lower number of parts which could potentially fail, and they did not require as much operator intervention as active systems. 

Circuit capacitance and inductance, including the values of these parameters for interconnecting wiring, are integral parts of the overall analysis. It is not always possible to assure that the system will be maintained as designed with only approved intrinsically safe components and with circuits of the capacitance and inductance as originally installed. For this reason, intrinsically safe systems are used primarily at locations where there are sufficiently trained personnel to assure that the intrinsic safety of the system is always maintained. 

The intrinsic safety is however affected by both the process equipment and the properties of the chemical substances present in the process. Therefore also the index should reflect this fact. We have included parameters into the list (Table 5) to represent the process aspects of the inherent safety. These parameters are the type of equipment involved and the safety of process structure which describe the process configuration from a system point of view. Also a third parameter to describe the interaction (reactivity) of the chemicals present in the process has been included, since this is an obvious source of risk. 

With regard to hazardous substance substitution, the guiding principle of the intrinsic safety of chemical substances, products or application systems was examined within the scope of the SubChem project. An essential approach for implementation of the guiding principle of the intrinsically safe substance or the intrinsically safe application system, especially with regard to the health and ecological consequences, would be the use of chemicals of short ranges (chemicals with low environmental persistence and transport distance) e.g. by ... 

If a high quality equipotential bonding system cannot be put into practice or a steady supply voltage cannot be guaranteed, a galvanic isolation between intrinsically safe and non-intrinsically safe electrical circuits is recommended. Such apparatus, e.g. power supplies for transmitters or switching repeaters, are equivalent to safety barriers with respect to the philosophy of intrinsic safety. The additional feature may be seen in the galvanic isolation and functional characteristics which may be practicable. 

Currently HART transmitters and valve positioners are state of the art. They do not differ outwardly from their pure analogue predecessors. A wide range of varying functions and measuring principles is available with a HART interface. Numerous products are deliverable with a certificate for intrinsic safety, too. The same holds for safety barriers, multiplexers and remote 1/O systems which are suitable for HART communication as well. 

Currently, remote I/O systems of several manufacturers are available for operation in zone 1 or Division 1. To a large extent, these systems are based upon the technology of intrinsic safety. The power supply only is designed according to another type of protection, e.g. flameproof enclosure - d A remote I/O suitable for hazardous areas may be an integration of input (I) and output (O) assembly units of automation systems and of Ex i-isolators of classic design. The installation may be made in a hazardous area and supersede the classic field distribution box (terminal box). Standardized fieldbus systems may be used for data transmission from/to the automation system. These remote I/O systems for hazardous areas are characterized as follows ... 

During international fieldbus standardization at the start of the 1990s a physical layer had been specified as IEC 61158-2 with an option for intrinsic safety. In the meantime, two standardized fieldbus systems comply with said standard Profibus PA and Foundation Fieldbus HI. These are the most important characteristics ... 

Certificates are issued not only for individual apparatus. In the same way, an intrinsically safe system e.g. composed of a sensor, cable, power supply and meter amplifier, can be certified. This practice is customary in the Anglo-Saxon countries (United Kingdom, USA). Here, only those apparatus shall be planned and fitted which are listed or specified in the system certificate. So, a high level of safety results for the planner and user since the intrinsically safe system in its entirety (including all components) has been examined. When, however, another component has to be fitted, e.g. due to the nonavailability of a specified device, the system shall be certified again (see Section 6.9.6). 

Safety barriers (without galvanic isolation) ensure intrinsic safety only, when connected to the equipotential bonding system. 

The standard EN 50039 permits uncertified systems, too. In this case, type of protection intrinsic safety shall be established by knowledge of the data of all apparatus including those not certified and the cables. 

In the example of Fig. 6.233 a power supply for installation in zone 1 is protected according to flameproof enclosure - d The outputs (max. four) to supply the other system modules comply with intrinsic safety - i - with maximum values as follows ... 

Results have been published in Johannsmeyer, U. Investigations into the intrinsic safety of fieldbus systems, Report PTB-W 53, Physikalisch-Technische Bundesanstalt, Braunschweig and Berlin/Germany, 1994... 

A system specified to such a degree is intrinsically safe with a maximum of 32 field devices and two terminations. The proof of intrinsic safety is thereby reduced to a list of the apparatus installed and their certificate. The marking includes FISCO and the type of apparatus, e.g. power supply, field device, termination. 

Figure 6.239 Time-related and local power distribution on an AC fed cable. The integral of power over the time window tw must not exceed a certain limiting value in order to ensure the intrinsic safety of this system.

Of course, the concepts of intrinsic safety cannot be transferred to optical systems. The basic principle to limit power or energy, however, can be applied for optical radiation ... 

Low-level input sensors such as thermocouples, turbine meters, speed transmitters and a number of process analyzers complicate system selection. Signal conditioning equipment is required for low-level inputs to both the back-up system and computer data acquisition systems. If the system is to be installed as an intrinsically safe system, multiple grounding requirements conforming to intrinsic safety grounding must be referenced to the same safety ground to avoid multiple ground problems. 

Cables that must function as long as possible in safety control systems e.g., fire and gas detection systems, ventilation damper control and power systems, UPS, public address and communication systems, intrinsically safe systems. 

The need for transportability coupled with operator- and intrinsic safety considerations dictates the use of solid state sources operating at low voltage. Of these, Gunn oscillators are the least susceptible to noise and, if locked to a high stability source, are easy to stabilise and to scan in frequency. At lower frequencies they are both inexpensive and widely available as components of intruder alarm systems, but their cost rises rapidly with increasing frequency, and becomes uncomfortable beyond about 100 GHz. 

An important aspect for the safety of fusion reactors consists in the possibihty to decrease in future the decay heat and the radioactive products inventory. In fact, the use of materials with reduced or short-hved activation and with low tritium retention, together with a limited operation power density, would minimize the above mentioned safety problems, bringing the plant towards intrinsic safety conditions (for which no active systems are necessary). 

The PBMR is very interesting from the safety point of view even if the design of the system appears rather complex. The intrinsic safety characteristics declared seem to be feasible, under the condition that the detaU system design is submitted to an attentive safety analysis, and this inelndes surveillance systems for structures and components. 

As we progress into the new century there are both opportunities and challenges. Qpportunities present themselves in the form of (1) the potential of integrating system software safety with control engineering to more closely achieve a level of intrinsic safety and (2) the proliferation of system safety as a discipline in other parts of the world. The challenges we face include the... 

We define intrinsic safety as safety designed and built into a system. Yes, this is an overlap with system safety. The two concepts are converging. Intrinsic safety is certainly a noble goal and one that should be continually pursued. 

Doppler reactivity is the other important parameter in FBR cores because it assures the intrinsic safety characteristics of the core. The present cross-section adjustment method cannot treat the Doppler reactivity which is dominated by resonance-peak broadening of cross-sections. We have launched a new study to extend the applicability of the cross-section adjustment and design accuracy evaluation system to the Doppler effect. The basic method to evaluate sensitivity of self-shielding factors has been successfully derived from a generalized perturbation theory, and a prototype system to calculate the Doppler sensitivity is now under verification. 

Inherent (i.e. intrinsic) safety feature" is a specialized term used in this case to describe the fact that the reactor itself reacts to certain malfunctions without the actuation of active systems or external controlling interventions in such a way that no inadmissible or even dangerous situations can be reached. These reactions are governed by the laws of nature, i.e. they always function regardless of the condition of active systems. This means that they cannot malfunction or fail. The technical and nuclear physical design of the HTR-Module is such that the maximum fuel element temperature always stabilizes itself below 1600°C even in the case of assumed failure of all active shutdown and decay heat removal systems. 

Enhanced safety is provided primarily by intrinsic self-protective features of the reactor plant which rely upon increased safety margins and passive safety systems. 

Safety equipment and interface Fire/explosion with detection and protection, chemical reactivity, mitigation system, enclosure class, intrinsic safety circuits, etc. 

Intrinsic safety ground This is a grounding system with a dedicated conductot isolated from... 

A short discussion on earthing/grounding systems for IS has been covered in Chapter X. Here, the systems will be discussed in greater depth. In most of the cases, reference voltage point of computer and I. C systems and the barrier bus bar are linked, making the earth returns are combined. Combined earthing helps to avoid interference and ensures intrinsic safety. When isolators are used, the barrier bus bar may be omitted and the screens of the field wiring are connected to the 0 V rail of the system. Safety barriers shall be connected to the main electrical system earth or... 

Complete earthing system for intrinsic safety barrier and surge protection device. 

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