All ATS consist of a Power Switching Assembly (PSA), Control Unit Assembly
(CUA) and the enclosure. Three most common types of ATS by the type of the Power Switching Assembly used are: contactor based, circuit breaker based and solid state.
a) Contactor based ATS
Contactor based ATS is the most common type by the number of units installed.
Some of the advantages of the contactor based ATS are low cost and availability.
The contactors are known as the devices designed for frequent switching of the load current and commonly used as a part of the motor starters. However many contactor based ATS manufacturers do not use traditional contactors in their PSA. Therefore most contactor based ATS are not rated for the number of switching operations (with or without load) as traditional contactors. Majority of the contactor based ATS manufacturers design their PSA to meet the requirements of the UL 1008 standard.
Traditional contactor base ATS is neither capable nor intended to interrupt a fault current. Therefore UL 1008 standard specifies the minimum fault currents the switch should be able to withstand without damage for a time period of at least 3 cycles. (50 mS)
Power systems with the contactor based ATS must have a circuit breaker or a fuse upstream from the ATS (for each power source: normal and emergency) for the purpose of short circuit protection. See figure 1 below:
If the ATS is rated to be able to withstand the available fault current for only 3 cycles, the upstream protective device shall interrupt the fault in less than 3 cycles. This means that there is no practical way of coordinating the upstream short circuit protective device with any of the short circuit protective devices downstream from the ATS. As a result, a short circuit anywhere in the critical circuit fed by the ATS will cause operation of the upstream short circuit protective device with the consequent loss of power all the critical circuits fed by the ATS.
At the same time, if the fault is in one of the critical circuits protected by a feeder circuit breaker with the instantaneous protection, the feeder circuit breaker may clear the fault.
Operation of the short circuit protective device upstream from the ATS will look to the ATS as a loss of the present source and will cause the ATS to transfer the critical load to the alternative power source. If the fault in the critical load circuit was cleared by the down stream feeder circuit breaker, than the remaining critical load circuits will be energized by the alternate power source. . If the fault in the critical load circuit was not cleared by the downstream feeder circuit breaker due to the fault location, tripping time, uncoordinated tripping setpoints or feeder circuit breaker malfunction, transferring of the faulted critical load circuit to the alternate power source will cause further damage to the faulted circuit as well as tripping of the alternate power source upstream from the ATS
b) Circuit breaker based ATS
Circuit breaker based ATS typically uses two interlocked, electrically operated circuit breakers as the heart of its PSA. The circuit breaker types, typically used in the ATS PSA are designed to comply with the UL 489. There are two major types of the circuit breakers, which fall in this category. The industry calls them molded case circuit breakers and insulated case circuit breakers. Molded case circuit breakers are the most basic type of circuit breakers, commonly found in low voltage power systems. The trip units typically include overload protection and short circuit protection (instantaneous setting of approximately 5 to 10 times frame rating). Insulated case circuit breakers are designed with stored energy mechanism and spring charging motor for remote closing, and are commonly available in draw-out construction.
The circuit breakers used in the PSA assembly may be with or without the trip unit. Even when used without the trip unit, most circuit breakers will trip and interrupt the fault, when the level of fault current exceeds the withstand rating of the circuit breaker. This self-protecting feature is one of the advantages of a circuit breaker based PSA over a contactor based PSA.
Generally, a circuit breaker based ATS acts the same as a contactor based ATS.
Circuit breaker based ATS can integrate the functions of an ATS and normal and alternate sources short circuit protective devices when supplied with the trip units.
If the time delays to transfer setpoints are short, and under certain power system conditions it is possible that the ATS will try to transfer during a fault. In that case a circuit breaker type ATS will be able to successfully disconnect the faulted load and complete the transfer, since the circuit breakers are rated to interrupt fault current. A contactor based ATS is likely to fail under the same scenario, since the PSA contactors are not rated to interrupt fault.
c) Solid state ATS
Solid state ATS PSA is based on the use of the heavy duty Silicon-Controlled Rectifiers (SCR). Use of SCRs in conjunction with the microprocessor based control unit allows the solid state ATS manufacturers to sense the failure of the normal source and transfer to the alternate source in less than ¼ of a cycle. This transfer time is fast enough to not affect the operation of the common computer equipment.
Due to the solid state nature of the SCR based PSA it is common to see an isolation and bypass circuit breakers on each source side and load side of the ATS. The isolation circuit breakers allow for visible disconnection of the SCR units from each source and load in case of the SCR units failure or need for maintenance. After the SCR unit has been fully disconnected by the isolation circuit breakers, one of the bypass circuit breakers can be closed to directly energize the load by the selected power source.
For a simple wiring control diagram you can refer to Wiring Riddle No.3
Listing of most common types of currently available automatic transfer switching devices, description of their operation and overview of their suitability for various applications.
Advanced Power Technologies, Inc.www.aptinc.net