IS System Drawing Introduction to Intrinsic Safety
There are many places where an explosive mixture of air and gas or vapour is or may be present continuously, intermittently or as a result of an accident. These are defined as hazardous areas by BS EN 50014:1998, Electrical apparatus for potentially explosive atmospheres – General requirements. Hazardous areas are common in petroleum and chemical engineering plants and in factories processing and storing gases, solvents, paints and other volatile substances. Electrical equipment for use in these areas needs to be designed so that it cannot ignite an explosive mixture, not only in normal operation but also in fault conditions. There are a number of methods available to achieve this – oil immersion, pressurised apparatus and powder filling, for example, but the two most common used are flameproof enclosures and intrinsic safety. Flameproof equipment is contained in a box so strong that an internal explosion will neither damage the box nor be transmitted outside the box. The surface must remain cool enough not to ignite the explosive mixture. When flameproof equipment is interconnected, flameproof wiring must be used. This method is most valuable when high power levels are unavoidable but it is not acceptable for areas in which an explosive gas/air mixture may be continuously present or present for long periods. For this reason these
flame detectors are made intrinsically safe rather than flameproof. Intrinsically safe equipment operates at such low power and with such small amounts of stored energy that it is incapable of causing ignition: Hazardous Area Safe Area Safe Area Apparatus
Unspecified except that it must not be supplied from nor contain under normal or abnormal conditions a source of potential exceeding 250V RMS or 250V DC with respect to earth. Flame Detector Barrier i + 1 + +
24V DC Normal (Break supply to resetif detector set to latch) -
2
-
i
= See detector datasheet for details o In normal conditions o With a single fault (for ib type of protection code) o With any combination of two faults (for ia type of protection code) In any of these conditions every component must remain cool enough not to ignite gases for which it is approved. See Table 2
Flame Detector + 1 + Barrier +
24V DC Normal (Break supply to resetif detector set to latch) -
2
--
+
(2 channel) -
-
+ 6578
-
Zone
Fault Relay Fire Relay Flame Detector NOTE 1 Each Barrier fed circuit must be a separate circuit and must not be connected with any other electrical circuit. NOTE 2 The electrical circuit in the hazardous area must be capable of withstanding an AC test voltage of 500 volts RMS to earth or frame of the a + 1
Classification of Hazardous Areas
-
2 EN 50014 states that electrical apparatus for potentially explosive atmospheres is divided into: 6578 Group I : Electrical apparatus for mines susceptible to fire damp;
pp aratus for one minute. NOTE 3 The installation must comply with national installation requirements (for example to EN 60079-14) NOTE 4 The capacitance and either the inductance or the inductance to resistance (L/R) ratio of the hazardous area cables must not exceed the maximum permissible parameters for the required groups IIA, IIB and IIC. Fault Relay Fire Relay EOL Group II : Electrical apparatus for places with a potentially explosive atmosphere, other than mines susceptible to fire damp. These flame detectors are designed to meet the requirements of Group
Refer to note 6 II apparatus. For the type of protection “ i ” intrinsically safe, Group II is subdivided into Equipment Categories, Type of Explosive Atmosphere (Table 1), Type of Protection Code (Table 2), Temperature Class (Table 3) and Gas Group (Table 4).
Detector Input Parameters Terminal 1 with respect to terminal 2 Terminal 3 with respect to terminal 4 Ui = 30V Ii = 100mA Pi = 0.65W Ci = 0.03µF Li = 0 Terminal 5 with respect to terminal 6 Terminal 7 with respect to terminal 8 Ui = 30V Ii = 100mA NOTE 5 The cable may be separate cables or a twin pair contained in a type ‘A’ or a type ‘B’ multicore cable (as defined in clause 5.3 of EN50 039). Provided that the peak voltage of any circuit contained within the muticore does not exceed 60 volts. NOTE 6 If required a loading resistor of not less than 3k 0.5 watt and having a surface area between 20cm² and 10cm² may be connected between the terminals of any circuit, but not between circuits. 2 11