Testing intrinsically safe circuits

Textile motors Crane motors Determining the size of motor Sugar centrifuge motors Motors for deep-well pumps Motors for agricultural application Surface-cooled motors Torque motors or actuator motors Vibration and noise level Service factors Motors for hazardous locations Specification of motors for Zone 0 locations Specification of motors for Zone I locations Motors for Zone 2 locations Motors for mines, collieries and quarries Intrinsically safe circuits, type Ex. f Testing and certifying authorities Additional requirements for ciritical installations Motors for thermal power station auxiliaries Selection of a special-purpose motor... 

Intrinsically Safe Circuits A circuit which any spark or thermal effect, produced either normally or in specified fault conditions, is incapable, under the test conditions prescribed in this standard, of causing ignition of a mixture of flammable or combustible material in air in its most easily ignited concentration."(3)... 

Part 3 Spark test apparatus for intrinsically safe circuits... 

Generally, cables for intrinsically safe circuits are subject to voltage tests. Cables intended for installation in chemical plants (Group II application) shall be tested with at least 500 Vrms AC between (Fig. 6.227(a)) ... 

Figure 6.236 may give an impression of the test points to be selected in an intrinsically safe circuit. 

Intrinsic safety ia, ib, ic (note 2) EN 60079- 11 The equipment used in the explosion hazard zone only contains intrinsically safe circuits. A circuit is intrinsically safe if neither a spark nor a thermal effect can cause the ignition of a standard explosible atmosphere. The corresponding tests are carried out under standardized conditions, which comprise both normal operation and certain faulty states. Measurement and control devices, communication devices, sensors, actors... 

Establishing a procurement policy, which includes a pre-procurement risk assessment, will ensure that correct selection of plant and equipment for use in hazardous areas (where a flammable or explosive atmosphere has the potential to cause a fire or explosion) will minimise the risk relating to such equipment. BS EN 50020 2002, Electrical apparatus for potentially explosive atmospheres - Intrinsic safety i , specifies the construction and testing of intrinsically safe apparatus, intended for use in potentially explosive atmospheres and for associated apparatus, which is intended for connection to intrinsically safe circuits which enter such atmospheres. This document may well form the basis from which an organisation establishes its procurement policy for such equipment. 

Intrinsic Safety. Static electrical concepts such as minimum ignition energy do not directly apply when assessing the safety of electrical circuits such as radios, flashlights and instmmentation. Intrinsically safe electrical equipment is usually available which has been subjected to fault analysis and testing. The equipment must be certified for the flammable atmosphere in which it will be used (NFPA 497). Refer to texts on Intrinsic Safety such as [63]. 

The qualification being that it must fail safe not only in its normal mode of operation, but, also, under specific modes of failure. Therefore, it is not enough to state that the circuit is of low voltage, and because of this is intrinsically safe. This is only half of the requirement. To qualify as intrinsically safe, the circuit must also fail in such a way as to be incapable of causing ignition, and further, it must be either tested or analyzed according to prescribe methods. 

Spark test apparatus applied to intrinsically safe electrical circuits. 

Coaxial cables used for data highways must be tested using sine-wave reflective testing techniques. Circuits involving intrinsically safe (IS) instmmentation must be tested (e.g., loop impedance, inductance, L/R [Inductance/Resistance] ratio) in accordance with the manufacturer s instmctions and approved by the customer. 

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