Temperature requirements, intrinsically safe

To contain the temperature of the electrical circuits within safe limits for a particular temperature class of the surroundings, the maximum current rating for a minimum size of a conductor is also stipulated in ICC 60079-1 I. The constructional requirements also stipulate the minimum clearances and creepage distances in air between the conducting parts of all the intrinsically safe L lectrical eirctiits. 

However, power reactors require significant amounts of reactivity (i.e., well above the amount needed to go prompt critical if added suddenly) that must be provided by movable control absorber devices (or removable poison dissolved in the piimaiy coolant) under the direction of a licensed operator and following jq>proved procedures during reactor start-up and the transition to equilibrium full-power operation. This positive reactivity is needed to compensate for losses associated widi increased core temperature, reduced coolant density including bubble void formation, and equilibrium fission product poison loads, especially Xe. Consequently, it is only possible to limit the amount of reactivity that could theoretically be inserted to small, intrinsically safe values when the reactor is already in the normal full-power operating mode with all movable control devices very near their maximum withdrawal positions (and when the dissolved poison concentration is close to zero). 

The requirement for use of IS devices does not restrict a designer. Rather it directs them to certain choices of equipment which have been designed for and are certified by lEC as available for use in hazardous areas with intrinsically safe barriers including 4—20 milliamp (mA) DC two-wire transmitters, thermocouples, resistance temperature detectors (RTDs), strain gages, pressure, flow, level switches, current/ pneumatic (l/P) converters, solenoid valves, proximity switches, infrared temperature sensors, potentiometers, EED indicating lights, and flowmeters with magnetic pickups. 

LHDC is used in a wide variety of applications but is particularly suited where there is a harsh environmental condition, a physical or hazardous maintenance access constraint to the protected area, and/or a requirement to cost-effective install detection in close proximity to the fire risk. The main benefits of LHDC systems are that they are effective in detecting a rise in temperature at any point along their length, and can be used in environments that may be potentially explosive and therefore require the use of intrinsically safe equipment (see Chapter 8). 

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