The Definition and Concept of Intrinsic Safety
Intrinsic safety is a technique applied in electrical and electronic equipment, mainly field equipment, to prevent the ignition of gas and/or dust when the electrical or electronic equipment is in use in a precarious area (Ericson 228). Precarious areas have high concentrations of gases that are flammable or dust. Such concentrations are common in mines and petrochemical refineries. It is usual for electrical and electronic equipment to generate tiny electrical sparks in connectors, switches, and motor brushes, among other places. Electronic equipments may also heat up when in use. Electrical arcing is a common occurrence in electrical and electronic equipment. The sparks, heating, and electric arcing can be sources of ignition for inflammable gases and dust. The purpose of intrinsic safety is to safeguard against such ignitions.
However, intrinsic safety is just one of the several techniques available for solving the problem. Some of the other techniques available include oil immersion, sand and powder filling, explosion-proof exposures, venting, and hermetic sealing. Intrinsic safety is the most realistic and appropriate method that prevent the device from initiating an explosion. There are therefore three potential ignition mechanisms that intrinsic safety aims to safeguard against heating, sparking, and electrical arcing.
The method of intrinsic safety is applied to handheld electrical and electronic devices. Usually, the power consumption for the devices is usually low; intrinsic safety is applied to low currents and voltages, or low power.
Components of an Intrinsically Safe Device
An intrinsically safe device is a device that relies on the technique of intrinsic safety to limit and prevent explosions from occurring in hazardous areas. Every intrinsically safe device is made up of three components: the field device called intrinsically safe apparatus; the energy-limiting device called intrinsically safe associated; and the field wiring.
The Field Device
A field device, or an intrinsically safe apparatus, is categorized as either a simple device or a nonsimple device.
A simple apparatus is any device that cannot store or generate an amount more than 0.1A, 1.2V, 25mW or 20μJ (Anand 581). Some of the available simple devices include Light Emitter Diodes (LED’s), thermocouples, non-inductive potentiometers, Resistance Temperature Detectors (RTD’s), and resistors. The safety of these simple devices does not need authentication and approval. If incorporated into an approved intrinsically safe apparatus, the resulting circuit is automatically safe.
On the other hand, a nonsimple device is one that can either store or generate currents, voltages, and wattages that are higher than those defined for simple devices. Examples of nonsimple devices are solenoid valves, relays, transducers, and transmitters. Nonsimple devices that have been authenticated as intrinsically safe have the following parameters: maximum current, Imax; maximum voltage, Vmax; internal inductance, Li; and internal capacitance, Ci. The Imax and Vmax values are the values of current and voltage respectively, above which if transferred to the field device, heating, sparking or electric arcing may occur, and cause an explosion. The values Li and Ci describe the device’s energy storage ability in terms of internal inductance and internal capacitance respectively.
The Barrier: Restricting the Amount of Energy Available to the Field Device
For intrinsic safety to be successfully accomplished, it is necessary that the energy that is made available to the field device is limited to values below the specified maximum values explained earlier. Otherwise, an ignition, hence explosion, may occur. A barrier or a device that limits energy, also called an intrinsically safe associated apparatus, is used to check and regulate the amount of energy that is supplied to or finds its way to the field device.
The circuit above shows the basic circuit that is used for an intrinsically safe associated apparatus, or barrier.
The circuit has three electronic elements that are responsible for the restriction of current and voltage that is made available to the field device: a fuse, a resistor, and two zener diodes. (There can be more than two Zener diodes.) The resistor restricts the amount of current to a value referred to as short circuit current, Isc. Isc is the amount of current that would flow if the terminals of the circuit meant for the field device were joined together. Not more than that amount of current can flow through the field device. The zener diode makes sure that the voltage does not exceed the open-circuit voltage; Voc. Voc is the value of the voltage that would be measured if the voltmeter was placed on the terminals connected to the field device, without the field device in place. If the voltage exceeds the diode’s breakdown voltage, the diode may begin to conduct and eventually be permanently damaged. In the event the diode starts to conduct, the fuse will blow. This action stops the diode burning. If the diode is damaged, the excess voltage will reach the hazardous zone and consequently cause and explosion. More than one zener diode is used in order to allow the circuit to continue to work in case one of the diodes is damaged. (There are many more circuits that use other or additional electronic elements).
The designing of the circuit and the analysis of the final circuit design has to take into account the relevant parameters as shown below
(Intrinsically associated apparatus)
(Intrinsically safe apparatus)
|Allowed capacitance Ca ≥ Ci Internal Capacitance|
|Allowed inductance La ≥ Li Internal Inductance|
|Open circuit voltage Voc ≤ Vmax Maximum Voltage|
|Short circuit current Ioc ≤ Imax Maximum current|
Determining the Parameter Specifications for the Circuit
The maximum values for voltage and current needed for the designing of the barrier circuit is determined from a graph of ignition curves, such as the one shown above. The selection of the specification for the maximum voltage and maximum current will first depend on what type of inflammable gas is present in the hazardous area, after which a point that associates a given voltage with a given current is chosen along the curve of representative of the graph. For example, if the gas has been determined as propane, a point is chosen on the graph labeled propane from which the maximum voltage and current is picked.
Application of Intrinsic Safety
Intrinsic safety is applied in equipment that are used is areas where there are inflammable gases or dust. Without intrinsic safety, sparking, heating or electric arcing can occur in the device and thus ignite gases or dust in the precarious area.
Industries associated with hazardous locations where intrinsic safety should be applied include munitions, wastewater, pharmaceutical, cosmetics, plastics, paint spray, and brewing, among others. These industries either use or generate inflammable gases. There would be a threat of explosion if intrinsic safety was not used.
Besides the ignition of gases, the explosion can also result from the ignition of dust though with a level of difficulty higher than gases. Almost all metallic dust and organic products (food products included) can be readily ignited. An explosion at a point in a cloud or layer of dust can initiate a series of chain explosions, resulting in a rolling explosion and cause devastation. Locations or industries that generate dust are therefore considered hazardous. Some of the industries that produce dust as their by-products include printing, grain, posh mill, and mining. Natural calamities, such as volcanic eruptions can also accumulate dust in the atmosphere, making in hazardous. Intrinsic safety is applied in such situations.
Approval of Intrinsic Safety
Intrinsic safety equipment can be certified by either of two methods: a parameters approval or a systems approval. With parameters approval, each device in the equipment is evaluated separately and assigned a set of safety parameters. In the case of systems approval, every component is approved and the entire system evaluated.
Advantages of Intrinsic Safety
Compared to the other available techniques, intrinsic safety has several advantages as outlined in Polke, Epple, and Heim (177). Intrinsic safety is the least expensive, easiest to install and safest technique available. Shock hazards are eliminated because only very low energies are required, allowing for a safe calibration and maintenance of field instruments without having to turn off the power. Intrinsic safely has an accommodative to newer technology, making it a favorite. In comparison to traditional protection methods, the technique significantly saves labor because of lack of bolted enclosures or conduits, among others.
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