1、Intrinsically Safe SystemAssessment Using theEntity ConceptApproved 2 January 1995ISATR12.21995TECHNICAL REPORTCopyright 1995 by the Instrument Society of America. All rights reserved. Printed in the UnitedStates of America. No part of this publication may be reproduced, stored in a retrieval system
2、, ortransmitted in any form or by any means (electronic, mechanical, photocopying, recording, orotherwise), without the prior written permission of the publisher.ISA67 Alexander DriveP.O. Box 12277Research Triangle Park, North Carolina 27709ISA-TR12.2, Intrinsically Safe System Assessment Using the
3、Entity ConceptISBN: 1-55617-559-0ISA-TR12.2-1995 3PrefaceThis preface is included for informational purposes and is not part of ISA-TR12.2.This technical report has been prepared as part of the service of ISA, the international society for measurement and control, toward a goal of uniformity in the
4、field of instrumentation. To be of real value, this document should not be static, but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms, and asks that they be addressed to the Secretary, Standards and Practices Board, International Society for M
5、easurement and Control, 67 Alexander Drive, P. O. Box 12277, Research Triangle Park, NC 27709, Telephone (919) 990-9227, Fax (919) 549-8288, e-mail: standardsisa.org.The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and th
6、e International System of Units (SI) in particular, in the preparation of instrumentation standards, recommended practices, and technical reports. The Department is further aware of the benefits to U.S.A. users of ISA standards of incorporating suitable references to the SI (and the metric system) i
7、n their business and professional dealings with other countries. Toward this end, this Department will endeavor to introduce SI-acceptable metric units in all new and revised standards to the greatest extent possible. The Metric Practice Guide, which has been published by the Institute of Electrical
8、 and Electronics Engineers as ANSI/IEEE Std. 268-1992, and future revisions, will be the reference guide for definitions, symbols, abbreviations, and conversion factors.It is the policy of ISA to encourage and welcome the participation of all concerned individuals and interests in the development of
9、 ISA documents. Participation in the ISA standards-making process by an individual in no way constitutes endorsement by the employer of that individual, of ISA, or of any of the standards, recommended practices, and technical reports that ISA develops.The information contained in the preface, footno
10、tes, and appendices is included for information only and is not a part of the standard.The SP12.2 committee wishes to give special recognition to Mr. Brad Larson for his outstanding contribution to the development of this document.The following people served as members of ISA Subcommittee SP12.2:NAM
11、E COMPANY*R. Weinzler Eastman Kodak Company*D. Bishop Chevron USA Production Company*N. Abbatiello Eastman Kodak CompanyR. Amin Dow Corning, Inc.P. Austen Electronic Design Control*P. Babiarz Crouse-Hinds CompanyA. Bartkus Underwriters Labs, Inc.*W. Calder III Foxboro CompanyU. Dugar Mobil Chemical
12、CompanyA. Engler Applied Automation, Inc.*One vote per company4 ISA-TR12.2-1995NAME COMPANYT. Feindel R. Stahl, Inc.W. Fiske Electrical Testing LabsG. Havens Bently Nevada CorporationS. Jackson Elcon Instruments, Inc.B. Larson Pepperl + Fuchs, Inc.D. Li Canadian Standards AssociationE. Magison Honey
13、well, Inc.R. Masek Bailey Controls CompanyD. Martell Dastech, Inc.F. McGowan Factory Mutual Research Corporation*A. Mobley 3M Company*S. Norako Crouse-Hinds Company*E. Olson 3M CompanyC. Oudar MTL, Inc.A. Page III MSHA Certification CenterN. Posey Union Carbide Corporation*M. Rains Foxboro CompanyT.
14、 Schnaare Rosemount, Inc.C. Short, Jr. Conco, Inc.L. Truscott Motorola, Inc.The following people served as members of ISA Committee SP12:NAME COMPANY*F. McGowan, Chairman Factory Mutual Research Corporation J. Bossert, Co-Chairman Hazloc, Inc.D. Bishop, Managing Director Chevron USA Production Compa
15、ny*N. Abbatiello Eastman Kodak CompanyW. Alexander Mine Safety Appliance Company*P. Babiarz Crouse-Hinds Company*A. Ballard Crouse-Hinds CompanyA. Bartkus Underwriters Labs, Inc.G. Bentinck E. I. du Pont de Nemours, Inc.K. Blayden Upjohn CompanyR. Brodin Fisher Controls International, Inc.M. Buettne
16、r Ralston Purina CompanyR. Buschart PC Imax= Maximum current that may be applied safely to the intrinsically safe apparatus;Ci= Internal unprotected capacitance of the intrinsically safe apparatus; andLi= Internal unprotected inductance of the intrinsically safe apparatus.Each channel of associated
17、apparatus is assigned the following parameters:Voc= Maximum open-circuit voltage that can appear across the intrinsicallysafe connections of the associated apparatus under fault conditions;Isc= Maximum short-circuit current that can be drawn from the intrinsicallysafe connections of the associated a
18、pparatus under fault conditions;Ca= Maximum capacitance that can be connected safely to the associatedapparatus; andLa= Maximum inductance that can be connected safely to the associated apparatus.6.1 Entity parameter specificationVmaxand Imaxare specified by the manufacturer of the intrinsically saf
19、e apparatus.They are used for comparison to associated apparatus parameters that have been determined under specified fault conditions. Thus, the values of Vmaxand Imaxdo not necessarily bear any relationship to the normal operating voltage and current of the intrinsically safe apparatus.For the par
20、ameters to be useful, values high enough to allow interconnection with appropriate associated apparatus must be selected. The values of Vmaxand Imaxare limited only to the maximum voltage and current that the intrinsically safe apparatus can receive and remain intrinsically safe, based on stored ene
21、rgy and thermal considerations.12 ISA-TR12.2-1995The Vmaxand Imaxvalues specified for a given intrinsically safe apparatus, taken together and compared to the ignition curves (Reference: ANSI/UL913), may fall in the ignition-capable area of the curve. This is not a problem, since any NRTL-approved a
22、ssociated apparatus to which the intrinsically safe apparatus might be connected will always have Vocand Iscparameters that are not ignition capable. For example, an intrinsically safe apparatus with low Ciand Livalues and properly rated components could realistically have a Vmaxof 45 volts and an I
23、maxof 350 mA. At 45 volts, 350 mA is well into the ignition-capable area of the ignition curve. However, based on the ignition curve for Groups A and B, an associated apparatus with a Vocof 45 volts would have an Iscof no more than 45 mA, and an associated apparatus with an Iscof 350 mA would have a
24、 Vocof no more than 19 volts. The connection of either associated apparatus to the intrinsically safe apparatus would form an intrinsically safe system, since in both cases Vmax Vocand Imax Isc. An additional parameter that optionally may be assigned to either the intrinsically safe apparatus or ass
25、ociated apparatus is Pmax. When assigned to the intrinsically safe apparatus, Pmaxis the maximum power that may be applied safely to the intrinsically safe apparatus. When assigned to the associated apparatus, Pmaxis the maximum power that can be delivered under specified fault conditions by the ass
26、ociated apparatus. The use of the Pmaxparameter can provide additional flexibility in the use of the entity concept. For example, when Pmaxis specified for the intrinsically safe apparatus, the NRTL examination of thermal effects in the intrinsically safe apparatus is conducted using the value of Pm
27、ax. In this case Vmaxand Imaxare not limited by thermal effects, only by stored energy, and thus may have considerably higher limits. When Pmaxis not specified, the Vmaxand Imaxparameters will be used in the examination of thermal effects. This may have the effect of limiting the value of Vmaxand Im
28、axto values that do not allow interconnection with the desired associated apparatus. When Pmaxis specified for an intrinsically safe apparatus, it must be connected to an associated apparatus that has an equal or lower Pmax.7 Entity concept control drawingsEntity parameters may be found on the contr
29、ol drawing supplied by the apparatus manufacturer, along with other pertinent information regarding proper connections, conditions of use, etc. Figure 2 is an example of a control drawing for an intrinsically safe apparatus. Figure 3 is an example of a control drawing for an associated apparatus.Fig
30、ure 2 Control drawing for an intrinsically safe apparatus examined under the entity conceptNon-Hazardous LocationClass II, Division 1, Groups E,F,GClass I, Division 1, Groups A,B,C,DClass III, Division 1Hazardous (Classified) Location+ACME InstrumentsModel ABCV=30VmaxmaxiiI = 300mAC = 0.02 FL = 0.15
31、mH1243Any Approved AssociatedApparatus with EntityConcept Parameters:V 0.02 F + CL 0.15mH + LaacablecableocscThe installation must be in accordance with the , NFPA 70, Article 504, andANSI/ISA-RP12.6.National Electrical CodeRISA-TR12.2-1995 13Figure 3 Control drawing for an associated apparatus exam
32、ined under the entity concept8 Assessing the intrinsic safety of simple loopsThe interconnection of the apparatus depicted in Figures 2 and 3 is a two-wire loop with grounded returni.e., a transmitter with single-channel grounded zener barrier. This is an example of the simplest and most easily unde
33、rstood combination. Assessing the intrinsic safety of this system is a matter of comparing the entity parameters as follows:Vmaxmust be greater than or equal to Voc;Imaxmust be greater than or equal to Isc;Ci+ Ccablemust be less than or equal to Ca; andLi+ Lcablemust be less than or equal to La.Note
34、 that the capacitance and inductance of the cable must be added to that of the intrinsically safe apparatus. If the actual cable capacitance and inductance values are known, they may be used. If unknown, ANSI/ISA-RP12.6 recommends the following default values:Cable capacitance - 60 pF/foot; and Cabl
35、e inductance - 0.20 H/foot.Inserting the values from Figures 2 and 3, the above conditions are met as follows:Vmax(30 Volts) must be greater than or equal to Voc(28 Volts);Imax(300 mA) must be greater than or equal to Isc(93 mA);Ci(0.02 F) is less than Ca(0.13 F) by 0.11 F; andLi(0.15 mH) is less th
36、an La(3.7 mH) by 3.55 mH.14 ISA-TR12.2-1995A maximum of 1,833 feet of cable (0.11 F/60 pF per foot) can be used due to capacitance limitations and a maximum of 17,750 feet of cable (3.55 mH/0.20 H per foot) can be used due to inductance limitations. The shorter length (1,833 feet) can be used. There
37、fore, the interconnection of these two devices is intrinsically safe with as much as 1,833 feet of cable. 9 Loop operation considerationsThe comparison of entity parameters assures only that the interconnection of the intrinsically safe apparatus and associated apparatus is intrinsically safe. Nothi
38、ng can be inferred from a successful entity parameter match about the operational suitability of the interconnection. The user still must evaluate the operating specifications of each apparatus to ensure that the combination will operate as desired.10 Special considerations for multi-channel associa
39、ted apparatus loopsThere are many intrinsically safe installations for which a single-channel associated apparatus with grounded return is not appropriate. An example of this is when it is not desirable to connect either conductor in a two-wire loop to ground, as is often the case with low-level ana
40、log signals, or in which the intrinsically safe apparatus has multiple connections. In these cases it is necessary to limit the total energy from any combination of conductors entering the hazardous location to an intrinsically safe level. Figures 4 and 5 depict a two-wire loop with neither conducto
41、r connected to intrinsic safety ground. Figure 4 uses two, single-channel associated apparatus, and Figure 5 uses one, two-channel associated apparatus.Figure 4 Two-wire loop with neither conductor connected to ground using two single-channel associated apparatusISB Inc.Model 4564321ISB Inc.Model 12
42、3Non-Hazardous LocationClass I, Division 1, Groups A,B,C,DClass II, Division 1, Groups E,F,GClass III, Division 1Hazardous (Classified) LocationI.S. GroundI.S. Ground4321+ACME InstrumentsModel ABCISA-TR12.2-1995 15Figure 5 Two-wire loop with neither conductor connected to ground using one dual-chann
43、el associated apparatusThe intrinsic safety of the system cannot be assessed if: (1) only the entity parameters of each associated apparatus in Figure 4 are known, or (2) only the entity parameters of each channel of the associated apparatus in Figure 5 are known. An assessment cannot be made becaus
44、e the individual Vocparameters address only the maximum voltage of each conductor with respect to ground. The maximum voltage between the two conductors is not known and cannot be derived from the individual Vmaxparameters. Therefore, it is necessary for the manufacturer of the associated apparatus
45、to provide a control drawing giving the entity parameters for the specific connection.One NRTL, Factory Mutual Research Corporation, uses the parameter Vtto describe the maximum voltage between any two channels, and the parameter Itto describe the sum of the currents from all channels of the given c
46、ombination. Other NRTLs continue to use Vocand Iscand assure that the control drawing is clear as to which combination of terminals each parameter applies. In any case, the user must assure that the entity parameters used are taken from control drawings that correctly describe the loop configuration
47、. Once this is done, the entity concept assessment of the intrinsic safety of the loop is performed in exactly the same manner as previously described. When the maximum voltage and current of the apparatus (as shown in Figure 6) are indicated on the control drawing by Vtand It, these values should b
48、e used in the parameter comparison in lieu of Vocand Isc. However, the Vtand Itparameters are independent from each other and cannot be used to determine the maximum power (Pmax) that can be delivered by the combination of channels. The entity parameter comparisons for the loop shown in Figure 5 are
49、 correctly made using the values from Figures 2 and 6, as follows:Vmax(30 V) is greater than Vt(29.2 V);Imax(300 mA) is greater than It(287 mA);Ci(0.02 F) is less than Ca(0.11 F) by 0.09 F; andLi (0.15 mH) is less than La(0.48 mH) by 0.33 mH.The connection is intrinsically safe with as much as 1,500 feet of cable using the default cable parameter of 60 pF/foot (0.09 F/60 pF per foot = 1,500 feet).ISB Inc.Model 234Non-Hazardous LocationClass I, Division 1, Groups A,B,C,DClass II, Division 1, Groups E,F,GClass III, Division 1Hazardous (Classified) Location11I.S. Gr
