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Limit Sensors Most heated thermal systems have:•A heat source, either fuel fired or electrical resistance•A temperature controller•A temperature sensorThe sensor produces a signal valuebased on the temperature it is exposed to. The temperature controller interprets that signal into a value that is either above, below oron its predetermined set point. The controller will, in turn, create anoutput signal to command some device to turn the heat source on or off.As with any such heated thermalsystem,a failure in the sensor, tem-perature controller or heat-source controlling device will create an over- or under-temperaturecondition. These conditions can ruin product in process or pose a dangerto personnel and property. Limit circuits are used to avoid over-and under-temperature conditions. When they sense either condition,they may be programmed to sound analarm,and/or shut the heatedthermal system down. Whatever its application, the limit control systemmust be totally independent from the primary control system. Its only connection to the primary controlsystem is the ability to override it and:•Turn off the heat source in an over-temperature condition•Sound an alarm in an under-temperature conditionGenerally, the sensor used in a limitcontrol system need not be as accu-

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Sensor catalog - 7347 GeneralInformation

Think Safety

General Information When specifying any sensor or designing any sensor/instrumentation
Think Safety circuit, caution must be exercised. Itis important to comply with safetyrequirements, local and/or nationalelectrical codes, agency standards,considerations for use in toxic orexplosive environments and sound engineering practices. This section deals with safetyas it relates to thedangers presented by catastrophicfailures, fault conditions andhazardous environments. Operational Safety—Do Not Make It Dangerous The actual performance of a temper-ature sensor must take into accountafailure that could cause serious product, plant or user safetyproblems.If a sensor is used in a medicalapplication, with life or deathconsequences,careful attention mustbe paid to fitness for application withappropriate redundancies and/or alarms built into the product.If the sensor is used in an industrialenvironment where hazards exist, attention must be paid to protection methods (isolating sensors and wiring from explosive or flammable substances) to prevent failures or short circuits from becoming dan- gerous to personnel and property.It is a good engineering practice toalways consider the consequencesof a catastrophic failure and the affect it would have on people andproperty. Please institute theappropriate safeguardsto limit anydanger. When installing temperature sensorsin hazardous areas, circuits should be made intrinsically safe with “barriers”that prevent sparks andexcessive heaton the “safe” sidefrom reaching explosive gases orflammable materials. Many devices exist on the market tomeet this need. Some cost morethan others, but they have in common the use of a “buffer,” or“barrier” devicelocated in the sensorcircuit, between the sensor and instrument or temperature controller. If a fault condition should develop, it prevents current, in sufficientamounts, from reaching thehazardous area and causing sparks.All barrier devices have parametersthat will effect how it works and itseffect on the sensor circuit. These include:•Polarity, whether rated for AC or DC signals•Rated voltage, or working voltageofthe signal the device is designed to carry before it sensesa fault•Internal resistance, as the amountofresistance the barrier deviceinherently has will affect thestrength of the current signal it’sallowing to pass Designing Intrinsic Safety Circuits rate as the primary sensor. Its onlyfunction is to create a temperaturesignal that will allow a controller to determineif a preprogrammed over-or under-temperature conditionexists. Sensor longevity and agingmust be taken intoaccount if they willhave an unacceptableimpact on thelimit control system’s ability toaccurately determine an under- orover-temperature condition.Again, like with enclosures, there areagency standards for the design and construction of limit control systems,and their suitability for use. 3