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What is Automatic Transfer Switch?

Publish Time: 2022-08-01 11:29:57     Author: DELIXI

Automatic Transfer Switching Equipment (ATSE) is mainly applicable to emergency power supply systems whose rated voltage does not exceed 1000V ac or 1500V dc. It interrupts power supply to loads during power conversion.

The national product standard for Automatic transfer switch (ATSE) was issued in October 2002 and implemented in April 2003. But dual-power systems have been used for decades before that. In the case of double power conversion, the combination of double contactor and double circuit breaker is mostly used - and this mode of use is still used today.

Each type of product has its own definition, function, role, performance standards and typical applications, (mechanical) contacter is defined as: only one resting position, can be switched on, load and break the current under normal circuit conditions (including overload operation conditions) a non-manual operation of mechanical switching appliances; Its function is to frequently connect and break the main circuit and large capacity control circuit at a distance; Product manufacturing standard GB14048.4-2003. (mechanical) circuit breaker is defined as: can be connected, load and break the current under normal circuit conditions, can also be connected in the specified abnormal circuit (such as short circuit), often in a certain time to break the current of a mechanical switching appliances; The function of circuit breaker is to automatically cut off the fault line, protect the load equipment and distribution line, can also be used to make up for frequent switch on and off the circuit and control the motor start/stop; Product manufacturing standard GB14048.2-2001.

Contactors are mostly used in control loops, while circuit breakers are used to protect loops, but two contactors or a combination of two circuit breakers can be used to complete the dual power conversion function after reasonable mechanical/electrical interlocking.

With double contactor to complete the function of double power supply conversion, the use history has been for several decades, its performance is not evaluated here, but there is a point worth noting: after the combination of contactor only complete the function of power conversion, no longer has the function of line control. If the circuit needs to start the motor and other equipment, still need to increase the control of the contactor!

Dual circuit breakers to complete the dual power conversion function has been used for decades. Unlike the dual contactor, the two circuit breakers were given dual tasks - power conversion and line protection - from the beginning.

1. Basic function - dual power conversion.

In the GB/T14048.11-2008 standard, transfer switch is defined as: the electrical appliance that converts one or more load circuits from one power supply to another power supply; An automatic transfer switching appliance (ATSE) is an appliance consisting of one (or several) transfer switching appliances and other necessary appliances for monitoring the power supply circuit and automatically switching one or several load circuits from one power supply to another. That is, between the two power supply sources, choose one that meets the standard to continuously supply power to the load, which is the most basic function of ATSE.

2. Additional function - Protection function.

According to GB/T14048.11-2008 standard, ATSE is divided into PC class and CB class, PC class is only to complete the basic function of ATSE - dual power conversion; In addition to the conversion function, CB ATSE is also given the function of over current (short circuit) protection. In fact, CB ATSE is a product with double circuit breaker combination to complete the power conversion function.

PC grade or CB grade for ATSE has been the focus of debate in the building electrical industry in recent years, which will determine whether the power supply and distribution system is equipped with additional protection components and increase the cost. Whether it is a CB-class ATSE or a combination of two circuit breakers for the dual power conversion function, it is essential to ensure that the operating mechanism has reliable electrical and mechanical interlocking to prevent simultaneous connection of normal and standby power supplies. The electrical interlocking required here is relatively easy to understand, but how do you determine a reliable mechanical interlocking?

According to the structure classification, the circuit breaker has the frame circuit breaker, the plastic case circuit breaker. The construction of the switch contacts is the same regardless of the type of circuit breaker (Figure 1). The live body, including the moving and static contacts, is sealed inside the switch, which means that the contacts of the circuit breaker cannot be seen. So how do double circuit breakers do mechanical interlocking?

The molded case circuit breaker uses a mechanical drive/interlocking mechanism to fix the lever of the molded case circuit breaker so that one breaker lever is up (where the circuit breaker is closed) and the other breaker lever is down (where the circuit breaker is disconnected) to achieve mechanical interlocking. However, in actual use, we found that the lever of the circuit breaker is not so reliable, which can not truly reflect the position of the switch contact. If the interlocking lever between the lever and the switch contact is broken, it is possible that the movement of the lever does not drive the movement of the switch contact, and the reliable mechanical interlocking of the two circuit breakers cannot be achieved at all.

The widely used frame switch and its interlocking mechanism, steel spring or steel rod, can complete the protection and power conversion function. Similarly, the dynamic and static contacts of the frame circuit breaker are sealed in the frame and cannot be reached by either the steel spring or the steel rod - even if they are reached, it is quite dangerous! Therefore, the actual interlock of this external rigid interlock is the position node that reflects the switch state. The circuit breaker has three positions: "ON", "OFF", and "TRIP", and a position node will respond to each position. This position feedback can be transmitted by electric signal ----, which is often used as the electrical interlocking of double circuit breakers. It can also be used as the mechanical interlocking of double circuit breakers through the displacement feedback of rigid elements such as steel rod and steel wire spring. However, there is an interlocking relationship between the position node reflecting the switch state and the switch moving contact, which is not directly connected to the switch contact. If the interlocking relationship fails, the position node cannot truly feedback the position of the switch moving contact. Therefore, the mechanical interlock made at this location node cannot be reliably guaranteed not to connect two power supplies at the same time!

Many designers and users will find this approach unreliable. Therefore, the dual role of double circuit breaker protection and conversion is actually at the expense of the safety and reliability of conversion.
When using double circuit breakers for power conversion, several problems should be noted:

1) Effective isolation of circuit breakers.

Circuit breakers are classified into suitable for isolation () and unsuitable for isolation () according to whether they are suitable for isolation. Because the circuit breaker is a protective element, the contact structure is in the form of quick split and quick closing, and the gap between moving and static contacts is small. So isolation is a weakness. Our common distribution systems are as follows:

Increased by isolating switch, is can't determine the isolation function of circuit breaker, afraid of circuit breaker display position for the separation of state, and the switch contact actual (or isolate), no separate personnel load operation and maintenance is to get an electric shock accident, thus adding isolation effect of isolation of circuit switch, to ensure the separation of the line. Another solution is to use a circuit breaker with an explicit isolation function.

Double power supply conversion, must not be the two power supply short together, therefore also requires the conversion of electrical switches ---- whether the circuit breaker, contactor, mechanical switch, must have a very clear switch contact separation (isolation) and then input requirements. Again, we cannot determine the contact isolation function of conventional circuit breakers, so the choice of dual circuit breaker combination power conversion, or the use of CB-class ATSE switches, should choose the circuit breaker with isolation function.

2) Circuit breaker maintenance problems.

Compared with the fuse, the biggest advantage of the circuit breaker is the repeated use, when the short circuit fault is eliminated, the circuit breaker can be reset to continue to use. The circuit breaker has two technical specifications: rated current and short-circuit breaking/switching current. After the short circuit current impact, although the tripping device acts quickly to protect the circuit and circuit breaker switch, the switch contact still passes the short circuit current impact, and the contact surface will be cauterized, reducing the performance of the switch. That is, the circuit breaker in the short circuit impact, the rated current does not change, can continue to use, but the short circuit breaking capacity will be reduced, lost the original design performance. Therefore, the circuit breaker is also to overhaul and maintenance.
If the circuit breaker is used as a dual power conversion, with a fixed interlocking mechanism, a fixed, plug-in circuit breaker cannot break the interlocking mechanism and maintain a single circuit breaker. If the protective circuit breaker needs to be repaired or replaced, disconnect both power supplies upstream of the ATSE, resulting in a total power failure of the double-circuit power supply. In the case of a drawer breaker, if the switch can be easily removed and changed, the interlock between two devices will be more unstable.

3) Protection type of circuit breaker.

Circuit breakers are classified into electrical models and power distribution types, as well as over current, over load, and ground fault protection types. In the power supply system, different types of protective circuit breakers are selected according to the actual load requirements. But these special position of the circuit breaker, in the completion of the dual power conversion function, there will be many drawbacks. For example, if the transfer switch at the front of the fire pump uses CB grade products and the circuit breaker has overload protection, the fire pump cannot operate under overload when a fire occurs, otherwise the two circuit breakers will jump off, thus limiting the use of the fire pump.

How to select circuit breaker and its setting value in power supply and distribution system is a complex work; The choice of circuit breaker at the same time also consider the problem of double power conversion - normal use, short circuit, over load, voltage/frequency loss, two power supply in parallel, equipment linkage and many other problems, which makes it difficult for designers and product manufacturers to choose. Therefore, it is recommended that ATSE should not have protection function.
Therefore, we do not recommend the use of CB-class ATSE or dual circuit breakers for conversion and protection.

3. Additional features - Isolation capabilities.

Gb14048.3-2002 /IEC60947-3:2001 is a national product standard for low-voltage switchgear and control equipment - switches, isolators, isolators and fuses. When talking about electrical isolation of power supply and distribution lines, it is important to understand this product standard. Because we use isolators or isolators to complete the electrical isolation of the line.

In this product standard, first understand the relevant definition:

(Mechanical) switch: a mechanical switching device that can turn on, carry, and break current under normal circuit conditions (including specified overload working conditions) and carry current for a specified period of time under specified abnormal circuit conditions (such as short circuit);

Isolator: in the disconnected state can meet the requirements of the isolation function of mechanical switching appliances;

Disconnector: A switch that meets the isolation requirements of the disconnector in the off state.

ATSE product standards have been issued for nearly 6 years, but there are still many problems in the design and use, and some accidents and losses have been caused. Imperfect national product standards are one reason. For example, there is no provision of ATSE legend, so that the design drawings appear in many ways of expression, but also cause the builder, the user of many understanding. Also easy to cause the role of ATSE, the function of chaos.

Then there is not enough awareness. Many people know the number of ATSE national standard, but the main content and the selection of ATSE main technical indicators are not clear, so the application of circuit breaker, disisolating switch technical parameters to requirements, more and more people confused circuit breaker, disisolating switch, ATSE. Besides, the manufacturers of ATSE should provide detailed and real technical parameters to designers and users, and give correct guidance to design and users. After all, ATSE is still in a development stage in China.


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