Stahl - Catalog - STAHL - #36

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2/5 2 Automation 02.07.2008 Safety Barriers Series 9001, 9002, 9004 2. Safety consideration The safe maximum values of an individual safety barrier (single- or dual-channel) are determined by the certification: • Maximum voltage Uo • Maximum current Io • Maximum power Po • Maximum permissible capacity Co • Maximum permissible inductance Lo It is to be tested however, if the permissible safe maximum values of the intrinsically safe apparatus (field apparatus in the hazardous area) are maintained by the selected safety barrier. 09938E02 Interconnection of Safety Barriers If several safety barriers are interconnected, possible current and / or voltage addition is to be taken into consideration from the safety point of view (example 1 and 2). The maximum values for Uo and Io permissible for an interconnection as well as the resulting permissible maximum values for Co and Lo for the various explosion groups can be referred to in the ignition curves (see EN 60079-11). Example 1 Interconnection of two safety barriers for positive potential. From a safety point of view a current addition results, i.e. Io = Io1 + Io2 The new voltage Uo is assumed to be the higher of the two values Uo1 and Uo2, thus Uo = max. (Uo1, Uo2) 09941E02 Example 2 Interconnection of two safety barriers for positive and negative potential. From a safety point of view a voltage addition results, i.e. Uo = Uo1 + Uo2 The new current Io is assumed to be the higher of the two values Io1 and Io2, thus Io = max. (Io1, Io2) 09942E02 Selection Criteria - Function and Safety Ex i PA/ + + Uo 1 Uo 2 I o 1 I o 2 Io=Io1+Io2 Safety Barriers Ex i PA/ + - Uo 1 Uo 2 I o 1 I o 2 Uo= Uo1+ Uo 2 Safety Barriers