1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-18a71 Center Point of Contact: KSCa71 Submitted by: Wil HarkinsSubject: Automatic Transfer Switches (ATS) In Critical Applications Practice: This practice provides proven techniques for enhancing the reliability of Automat
2、ic Transfer Switches (ATS) used in critical applications. Systems which require the use of ATS may be optimized for fail-safe operation using worst-case design techniques and good maintainability/preventive maintenance practices. The probability of internal ATS failures which could result in loss of
3、 power to the load can be minimized by giving particular attention to the ATS transfer methods, power-switch types used, and regular attention to the health of the equipment.Programs that Certify Usage: This practice has been used on Kennedy Space Center Shuttle Landing Facility 60 Hertz Power Syste
4、m supporting critical landing aids.Center to Contact for Information: KSCImplementation Method: This Lesson Learned is based on Reliability Practice No. PD-ED-1256; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:The major benefit of these desig
5、n considerations is the greater assurance that loss of power to critical loads and the resulting consequences will not occur. Achieving optimum reliability is of paramount Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-importance in systems that pro
6、tect life and property. Along with the increase in the reliability of the ATS that is achieved, usually little or no additional design cost is required.Implementation Method:The implementation of these techniques has resulted in the elimination of critical failure modes in Automatic Transfer Switche
7、s used in the KSC Shuttle Landing Facility 60 Hertz Power System supporting critical landing aids. Increased system reliability and reduced maintainability costs are inherent features of these practices.Technical Rationale:For critical 60 Hertz Power applications which must use Automatic Transfer Sw
8、itches (ATS) to provide redundant power sources, it is important to examine the following three design considerations which serve to minimize the probability of ATS failures that could result in loss of power to the load.The first of the three considerations involves the use of ATSs which are design
9、ed such that either molded-case circuit-breakers or molded-case switches can be used interchangeably as the switching devices. Only the latter should be used to avoid the possibility of a premature-trip of the circuit-breaker causing total loss of power to the critical load, or depending on the cond
10、itions used to cause the ATS to switch, the ATS could transfer prematurely. Whereas a premature transfer is not a critical failure-mode, any unnecessary transfer of an ATS supporting a critical load is undesirable because of the possibility of a system malfunction occurring due to transients or glit
11、ches caused by the switch-over. The potential for loss of power to the critical load caused by an open switching device in the ATS brings about the necessity to investigate the second consideration.The second consideration pertains to the technique used to cause the ATS to transfer to its alternate
12、power source. Many ATSs employ a voltage-sensor circuit(s) to detect the loss of preferred input power. When the voltage sensor detects the absence of preferred input power, the transfer is initiated and the ATS switches the load to the alternate power source. In the event that the ATS switching dev
13、ice fails “open“ as described in the previous paragraph, no transfer would occur because the voltage-sensor at the preferred input would not have detected any loss of input power. The result would again be loss of power to the load. Hence, the only sure way to guard against this failure-mode is to c
14、onfigure the ATS to sense loss of voltage at the “load“ terminals of the ATS instead of at the “preferred“ power input terminals. Thus, any internal ATS failure which would otherwise cause loss of power to the load will be neutralized.The intent of providing an ATS in critical power circuits is to e
15、liminate single failures that would cause loss of power to the load. Eliminating the potential for these two types of internal ATS failure-modes significantly reduces the chances of the ATS itself becoming a single-fail point.Provided by IHSNot for ResaleNo reproduction or networking permitted witho
16、ut license from IHS-,-,-The third consideration is maintaining the good working order of the ATS, i.e. maintainability and preventative maintenance. Because the ATS is used only for emergency transfer of power, and since maintenance could require taking down the system, regular maintenance may be re
17、peatedly postponed or ignored until a real-time catastrophe reveals that the ATS will not transfer because of a maintenance related failure. Adequate preventative maintenance should not be substituted for convenience. Where possible, scheduled down-time should be used to complete an appropriate main
18、tenance plan. In systems where down-time is not a convenient option, a hardware method of bypassing the ATS to facilitate maintenance should be installed. Designing the system for maintainability allows the required preventative maintenance to be performed without interfering with normal system oper
19、ation.With the appropriate hardware configuration and preventative maintenance plan in place, optimum reliability can be achieved without any substantial increase in installation or operating cost. Any cost incurred, if any at all, to implement these safeguards is assuredly returned by the avoidance
20、 of a single failure of the ATS under worst-case conditions.Impact of Non-Practice: The consequences of non-practice can lead ultimately to loss of life, and secondarily property. The very nature of the critical systems which drive the requirement to use ATS to guard against loss of power demand maximum reliability.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-18a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
