May, 2020


www.us- tech.com Fail-Safe vs. Maintenance Bypass Switches Continued from previous page


contacts will cause the same type of momentary load interruption that occurs when transferring a comparably sized transfer switch.


Maintenance Bypass Switch Design To bypass to either the normal or emergency


source with a maintenance bypass switch, the ATS must be in the source position to which the opera- tor wishes to bypass. If it is not, then it must be electrically or manually transferred to that source before it can be safely bypassed. Some designs require feeder breakers


to be opened first to accomplish the trans- fer. Or, the manual bypass switch must be bypassed to whatever source position the ATS is in, the ATS must be isolated, then the manual bypass switch must be bypassed a second time to the desired alter- nate source. In an emergency situation, these addi-


tional steps can significantly increase the time required to bypass the ATS and restore power to the load. In addition, if the ATS portion of the unit has been damaged or intentionally locked in the opposite posi- tion, a manual transfer could be, at best, extremely hazardous or, at worst, impossi- ble. In this situation, operating the bypass switch could trigger an extended electrical outage (ironically, a situation it was designed to prevent) and/or present a seri- ous safety hazard to personnel. An advantage is that this switch


allows bypass of the ATS section from either normal source to load or emergency source to load without load interruption, but only if the ATS is connected to the source to which the operator wishes to bypass. Otherwise, the operator must first manually transfer the ATS to this source and experience a load interruption.


Disadvantages: l


Figure 2: maintenance bypass switches do not


Designed primarily to allow mainte- nance without an interruption of load,


experience any interruption when bypassing, but also require numerous steps.


maintenance bypass switches do not meet the specification requirement: “Bypass of load to either the normal or emergency power source with complete isolation of the automatic transfer switch shall be possible regardless of the position or condition of the automatic transfer switch.”


l If for some reason the automatic transfer


switch portion of the unit has been rendered mechanically or electrically inoperable while in the opposite position, bypass becomes impossible


and the bypass/isolation switch is left useless. It is also important to consider that, in accordance with UL l008, manual operation of the automatic transfer switch is hazardous unless the manual operating handle of the transfer switch is exter- nal to the enclosure (a feature not typically speci- fied).


l To prevent the possibility of a direct short cir-


cuit (by inadvertently connecting both sources together), the operation of most maintenance bypass switches is inhibited by an elabo- rate arrangement of electrical and mechan- ical interlocks that prevents the operator from bypassing a source to load while the transfer switch is in the alternate source (see Figure 2). The more elaborate these safeguards, the greater the possibility of malfunction or operator error.


With its additional (break-before-


mate) load contacts, the fail-safe bypass switch design reduces the number of steps and, therefore, the time required to bypass the ATS. This design allows the bypass switch to be thrown at any time — regard- less of the position or condition of the ATS — dramatically simplifying the switch’s opera- tion in emergencies and virtually eliminat- ing the chance of operator error. Fail-safe bypass switches should be


considered when simple, fast and foolproof bypassing is needed for emergency outage situations and is the overriding concern. Maintenance bypass switches should be considered when maintenance is the over- riding concern. Both types are advanta- geous in different use cases. When design- ing or updating a power control system, it is important to choose the ATS/bypass switch type best suited to the particular use case. Contact: Russelectric, A Siemens


Business, 99 Industrial Park Road, South Shore Park, Hingham, MA 02043 % 781-749-6000 E-mail: info@russelectric.com Web: www.russelectric.com r


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