1、UL COPYRIGHTED MATERIAL NOT AUTHORIZED FOR FURTHER REPRODUCTION ORDISTRIBUTION WITHOUT PERMISSION FROM ULUNDERWRITERS LABORATORIES INC. CERTIFICATION REQUIREMENT DECISIONThis Certification Requirement Decision is prepared and published by Underwriters Laboratories Inc.(UL). It is normative for the a
2、pplicable UL Product Certification Program(s); however, it is currently notpart of the UL Standard(s) referenced below.Product Category (CCN): XACN, XAAK, XABE, XACD, XACI, XACS, XACX, XATJ,XABJ, XADH, XADM, XAPX, SDFYStandard Number: UL 60730-1Edition Date: September 30, 2009Edition Number: 4Sectio
3、n / Paragraph Reference: Clause 11.13Subject: Assessment of zero cross circuits, power regulation circuits, overcurrentprotection, and similar circuits or componentsDECISION:11.13.1.101 DV D2 If an electronic circuit/component is relied upon to prevent a hazard under normal orabnormal operation of t
4、he product, that electronic circuit/component is considered to be providing aprotective function. Such components/circuits shall comply with the relevant requirements for protectivecontrols unless it can be shown through a fault assessment and tests of clause 27 that failure ormalfunction of the pro
5、tective circuit/component will not lead to a hazardous condition of the control i.e.,loss of the protective function.Note Examples of such circuits could be zero cross circuits, power regulation circuits, overcurrent protection, etc.RATIONALE FOR DECISION:This Certification Requirement Decision is i
6、ntended to document present practice in the application ofthese requirements.To illustrate the safety concern and rationale for this requirement, consider a zero-cross circuit used in acontrol. The main function of this circuit is to precisely control the switching of the output relay within asmall
7、window of the zero-cross of the voltage supply under normal and abnormal operating conditions. Byreliably performing this function, the relay is forced to operate at fairly low voltage and current levels. Thisis done so that relays with relatively low load ratings = may be used to control relatively
8、 large loads;thereby reducing cost per unit and optimizing packaging size of the control.The design of these zero-cross circuits vary in complexity from simple RC timing circuits to complexclosed-loop, real-time, self-adjusting circuits. In all cases, the analysis to establish the failure modes ofth
9、ese circuits is the same. It starts with a thorough hazard and fault analysis from the sensing element, tothe logic solver and finally to the actuator.In the case of a zero-cross circuit, there are essentially three failure modes of this function; namely:a) function is enabled (stuck short)b) functi
10、on is disabled (stuck open)c) function operates outside of its designed limits undetected (drift).UL COPYRIGHTED MATERIAL NOT AUTHORIZED FOR FURTHER REPRODUCTION ORDISTRIBUTION WITHOUT PERMISSION FROM ULA failure analysis of the zero-cross circuit with respect to electrical safety (electric shock, f
11、ire and casualtyhazards) needs to be performed to ensure that the above failure modes are satisfied by inherent designof the control, under normal and single component failure of the zero-cross circuit (which includes thedetection circuit.) For example, in item c), if the limit of the function drift
12、s from its designed values and thisfault is not detected, the relay would be switching constantly at a particular phase angle higher than itsdesigned limit undetected for the life of the product. This condition which would lead to thermal andelectrical stress of the relay, potentially leading to a h
13、azard since the relay is undersized and notevaluated for this condition during its certification.In this fault condition, it is quite tedious to establish the “worst-case” phase angle for the system but it canbe done by trial and error and could be one of the options that can be exercised to demonst
14、rate electricalsafety of the control. Alternatively, for the above reason, manufacturers tend to rely on electronicsafeguards (HW and SW components) in their design to mitigate this fault condition and others in areliable and consistent manner. Consequently, such safeguards shall be evaluated for th
15、eir reliability inaccordance with the requirements for protective controls.Copyright 2012 Underwriters Laboratories Inc.UL, in performing its functions in accordance with its objectives, does not guarantee or warrant thecorrectness of Certification Requirement Decisions it may issue or that they wil
16、l be recognized or adoptedby anyone. Certification Requirement Decisions are the opinion of Underwriters Laboratories Inc. inpractically applying the requirements of the standard. They do not represent formal interpretations of thestandard under American National Standards Institute (ANSI) processes
17、. UL shall not be responsible toanyone for the use of or reliance upon Certification Requirement Decisions by anyone. UL shall not incurany obligation or liability for damages, including consequential damages, arising out of or in connectionwith the use or reliance upon Certification Requirement Decisions. The electronic version of theCertification Requirement Decision is the current version and previously printed copies may be outdated.This document is published as a service to ULs certification customersSTANDARD NUMBER: UL 60730-1 -2-
