Intrinsic safety protects your plant against explosions
Henry M. Morris
Explosions can be prevented by limiting the amount of electrical energy available in hazardous areas or by containing the situation using bulky, heavy devices called 'explosion-proof enclosures.' Limiting excess electrical parameters such as voltage and amperage (current) requires the use of energy-limiting devices known as 'intrinsically safe (IS) barriers.'
Explosion-proof enclosures prevent or control explosive situations with brute force. They are heavy containers designed to hold an explosion inside. Electrical devices within explosion-proof enclosures can operate at normal power levels. Even under fault conditions, an explosion or fire may not occur because there simply isn't enough air within the sealed container to support combustion. If an explosion happens, the housings are strong enough to contain it.
Intrinsically safe barriers are more elegant. They limit the levels of power available in the protected area. If a spark or excess electrical heat cannot occur, neither can a fire or explosion. IS barriers eliminate the bulky enclosures, the use of conduit and expensive enclosure/conduit seals, and the associated installation costs.
Implementing intrinsic safety
While most users in Europe have used IS for many years, IS wasn't adopted as a part of the U.S. National Electrical Code until 1990 (Section 504). Implementing IS requires more intimate knowledge, and careful selection, of the devices involved than specifying explosion-proof enclosures. The need to perform many calculations to implement IS has turned off many engineers.
Three components comprise an intrinsically safe circuit: the target device, the intrinsically safe barrier, and the wiring.
Devices within the protected area are either simple (RTDs, LEDs, contacts, thermocouples, resistors, etc.) or complex (transmitters, solenoids, relays, transducers, etc.). Complex devices may store excess energy, and thus should be certified 'intrinsically safe' by a third party, such as Underwriters Laboratories (Northbrook, Ill.), Canadian Standards Assn. (Rexdale, Ontario, Canada), FactoryMutual (Norwood, Mass.), etc.
The simplest form of intrinsic safety barrier employs a resistor to limit current, at least two Zener diodes to limit voltage, and a fuse. The resistor limits the current to a specific value known as the short-circuit current. The Zener diodes limit the voltage to a value referred to as the open circuit voltage. The fuse will blow when the diode conducts. This interrupts the circuit, preventing the diode from burning, which could allow excess voltage to reach the hazardous area. There are always at least two Zener diodes in parallel in each intrinsically safe barrier. If one diode fails, the other acts as a backup providing safe operation and complete protection.
Selection of the proper intrinsic safety barrier requires calculating the open-circuit voltage and the short-circuit current. For complex devices, its necessary to also calculate the allowed capacitance value and the allowed inductance value. Results are then compared to 'ignition curves.'
Because different materials can be ignited by different levels of energy, Ignition Curves have been calculated for a wide variety of materials and can be obtained as Standards 3610 and 3611 from Factory Mutual, 1151 Boston-Providence Tpke., Norwood, MA 02062; Tel: 617/762-4300, Fax: 617/762-9375, or www.factorymutual.com.
The weak link
Intrinsic safety can be compromised at some point in time after the initial commissioning. This usually happens when an unrelated failure prompts improper maintenance or repair of wiring. Sometimes, to get things back up and running after a shutdown, temporary shortcuts are used. Left in place, these shortcuts can render intrinsic safety designs useless. Unfortunately, the effect of these shortcuts sometimes goes unrecognized--until its too late.
It's essential that good wiring documentation be created at installation, and that all future changes be noted as implemented to ensure intrinsic safety protection remains intact after repair.
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