Automatic switch of two inputs: device, circuits and installation

Organizing uninterrupted power supply is a critical task for any facility, be it a country house, an industrial workshop or a server room. Automatic switch of two inputs (AVR) is a device that takes on the function of instantly switching the load from the main source of energy to a backup one when the voltage in the network disappears. Unlike manual switches, which require human presence and intervention, automation operates in a fraction of a second, preventing equipment downtime and data loss.

The operating principle of such systems is based on constant monitoring of the input voltage parameters by the controller. If AVR controller detects when parameters exceed acceptable limits (phase loss, imbalance, voltage surge), it issues a command to open the contacts of the main input and close the backup circuit. For private households, this often means switching to diesel or gasoline generator, and on an industrial scale - switching to a second supply line from the transformer substation. The reliability of the entire system directly depends on the quality of the assembly of the shield and the correctness of the selected components.

In modern energy consumption conditions, the use automatic entry of reserve ceased to be an excess and became a necessity. Refrigerators, heating systems, pumping stations and security systems require the constant presence of electricity. A critical parameter when choosing a circuit is the switching time, which for sensitive electronics should not exceed 100-200 milliseconds. In this article we will analyze in detail the design of contactor and relay circuits, the features of choosing equipment and the nuances of setting time delays.

Operating principle and control logic of ATS

The fundamental basis of any work automatic shifter is a logical block that compares the current state of the network with reference values. In simple circuits, this role is performed by voltage control relays that monitor the presence of a phase. More complex systems built on the basis of specialized AVR controllers, are able to analyze the frequency of the current, the level of harmonics and even the temperature of the generator engine before starting. The operating logic is based on the priority of the first input: while the network is present, the load is powered from it, and the generator is in standby mode.

The switching process is never instantaneous in a physical sense, since time intervals must be observed to protect the equipment. When the network fails, the controller waits for a set time (usually 1-5 seconds) to eliminate false alarms during short-term failures. This is followed by a command to start the generator, warm it up, and only after the output voltage has stabilized does the load switch to reserve input. The reverse process, when the network appears again, is also accompanied by a delay and checking the stability of the parameters.

⚠️ Attention: Never set the minimum generator turn-on delay to less than 10 seconds after it starts. The engine must warm up and the generator rotor reach its rated speed, otherwise the supply of cold current can damage both the alternator itself and the connected pump or compressor motors.

An important element of the logic is the prevention of counter-inclusion. This is a condition in which the contacts of the main and reserve inputs are closed simultaneously, resulting in a short circuit between the network and the generator. To avoid such an emergency, use mechanical locking contactors and electrical interlocking in control circuits. Even if the electronics fail, the mechanical stop will prevent both contactors from closing at the same time.

Key circuit components: contactors and relays

The heart of the power unit of the AVR is contactors (or magnetic starters). They are the ones who physically open and close the circuit under load. When selecting contactors for a panel assembly, it is necessary to take into account the rated load current, application category (AC-1 for resistive loads, AC-3 for motors) and wear resistance. For powerful consumers, circuits with two contactors are often used, where each is responsible for its own input, and their control coils are mutually blocked.

The second most important element is phase control relay (RKF) or voltage relay. These devices act as the “eyes” of the system. They send a signal to the contactor coil only when the network parameters are normal. If the RKF detects an accident, it opens the power supply circuit of the main contactor coil, causing it to fall off. In generator circuits they are often used time relay, which form a pause before starting the starter and a pause before switching to the generator.

Intermediate relays can be used to switch small control currents and organize complex logical circuits. They allow you to amplify the signal from a low-current controller to the level necessary to turn on a powerful contactor coil. Also, control circuits must contain circuit breakers to protect control circuits and power lines from overloads and short circuit currents.

• 🔌 Contactors: must have a current reserve of at least 30% of the maximum load to prevent contact sticking.
• ⏱️ Time relay: necessary to form pauses for warming up the generator and delaying the return to the network.
• 👁️ Phase control relay: provide monitoring of voltage presence and correct phase rotation.
• 🛡️ Automatic circuit breakers: protect cables and contactors themselves from overcurrents.

Comparison of contactor and relay circuits

When designing an uninterruptible power supply system, the owner is faced with a choice between a classic relay contactor circuit and the use of a specialized controller. Contactor circuits (often called "on contactors") are built around two cross-locking contactors and a voltage monitoring relay. This is a simple, cheap and reliable solution that does not require programming. The logic here is “hardwired” into the physical switching of wires.

Schemes based on AVR controllers (for example, devices such as ATS, BZVK or programmable logic controllers) offer much more flexible configuration. In them you can set the exact delay time, voltage response thresholds, and “Generator-Network” or “Network-Network” operating modes. Such systems often have a digital display showing the current status and can send notifications to the owner's phone. However, they are more difficult to set up and more expensive to purchase.

The choice between these two approaches depends on the requirements of the site. For a small home with a simple generator, a reliable contactor assembly with a minimum set of automation. For boiler rooms, server or production lines, where accuracy and the ability to integrate into the Smart Home system are important, without programmable controller can't get by. Errors in circuit type selection can result in either unnecessary costs or insufficient equipment protection.

Comparison parameter	Contactor circuit (RCN + Contactors)	Circuit on controller (ATS)
Component cost	Low / Medium	High
Difficulty setting up	Minimum (twists on the relay)	Requires programming/menu setup
Logic flexibility	Rigid, depends on connections	High, customizable timers and thresholds
Reliability	High (less electronics)	Depends on the quality of the controller
Indication	Light (light bulbs)	Digital (display, remote access)

Is it possible to combine contactors and controller?

Yes, this is the best option. The controller acts as a “brain”, controlling the contactor coils. This gives flexibility in setting up the controller and reliability of power switching with contactors.

Power calculation and selection of equipment ratings

Correct power calculation is the key to durability automatic shifter. The rated current of contactors and circuit breakers must exceed the total current consumption of all devices that will operate simultaneously. However, simply adding up the currents is not enough. Starting currents must be taken into account, especially if the load contains electric motors (pumps, compressors, machines). The inrush current can exceed the rated current by 3-7 times, which can cause false operation of the protection or welding of contacts.

When choosing a cable cross-section for connecting inputs, they are also guided by the load current, but with a margin. The cable running from the generator to the ATS panel must withstand a current equal to the maximum power of the generator. Usage copper conductors preferable to aluminum in panel assemblies due to lower oxidation and better conductivity in small sections. All connections must be tightened with the torque recommended by the manufacturer and checked after the first warm-up.

Particular attention should be paid to the protection class of the shield itself. If automatic switch installed in a garage or boiler room, where dust and moisture are possible, the housing must have a degree of protection of at least IP54. For residential premises, IP30 or IP40 is sufficient if the shield is built into a niche. Overheating of contacts is a common problem, so a dense arrangement of elements without taking into account heat dissipation is unacceptable.

Installation and setting of time intervals

Installation automatic entry of reserve requires adherence to a strict sequence of actions. First, the power part is installed: input circuit breakers, contactors and busbars. Then the control circuit is assembled. Correct phasing is critical. If you switch a three-phase load (for example, a 380V well pump), then when switching to a generator, the phase rotation should remain the same. An error in the connection will cause the motors to rotate in the opposite direction, which can damage them.

Time intervals are configured on the time relay or in the controller menu. There are several key delays. First - switch-on delay (Time to Start), which waits a few seconds after a network failure before starting the generator. Second - warming up (Warm-up), generator operating time without load (usually 10-30 seconds). Third - switching delayto allow the generator to stabilize speed. And the fourth - return delay to the network to make sure that the city electricity did not disappear for a couple of seconds.

After assembly and initial setup, load testing is required. The disconnection of the main input is simulated, and the time until voltage appears at the ATS output is measured. For active loads (light, heaters), long pauses are acceptable; for computer equipment and boilers with electronics, a time of less than 0.2-0.5 seconds is critical (a UPS may be required here). If the switching time is long, it is necessary to reduce delays in the settings, but not at the expense of safety.

⚠️ Attention: When setting up a three-phase ATS, always check the phase rotation at the generator output relative to the network input. If the phases are reversed, three-phase motors will run in reverse, causing damage to pumps, fans and compressors in a fraction of a second.

Typical errors during assembly and operation

One of the most common mistakes is ignoring mechanical lock. Electricians often rely only on cross-connecting contactor coils. However, if one of the contactors “sticks” mechanically (the contacts are welded), the electrical interlock will not work, and the second contactor will try to turn on, creating a powerful short circuit between the network and the generator. A mechanical locking lever installed between the contactors will physically prevent the second contactor from closing.

Another mistake is the wrong choice of class of circuit breakers. At the input, circuit breakers with characteristic “C” are often installed, which are designed for starting currents of motors. For protecting cables and electronics, circuit breakers with characteristics “B” or specialized ones are better suited residual current devices (UZO/Difavtomat), selected by leakage current. Grounding is also often forgotten: the housing of the ATS switchboard and the zero bus of the generator must be correctly connected to the ground loop.

Operation automatic switch requires periodic maintenance. Once every six months, it is recommended to carry out a visual inspection for melting, dust and foreign odors. Once a year, preventive broaching of the contacts should be carried out, since the metal expands and contracts due to heating and cooling cycles, weakening the clamp. It is also necessary to conduct a test run of the system once a month, simulating a network outage, to ensure that the automation is working.

Frequently asked questions (FAQ)

Is it possible to use a regular automatic switch instead of contactors?

Yes, there are motorized switches (ATS based on a switch) that are controlled by an electric drive. They take up less space than two contactors and have higher switching reliability under load, but are significantly more expensive than a pair of contactors.

Is it necessary to ground the generator zero in the ATS circuit?

This depends on your home's grounding system (TN-C-S or TT). In most cases, for private houses (TT system), the generator neutral must be isolated from the network neutral, and the generator housing ground is connected to the local circuit. In a TN-C-S system, it may be necessary to switch the zero along with the phases (4-pole ATS).

Why are the contactors humming in the ATS switchboard?

A humming sound indicates that the contactor magnetic circuit is not fully closed. Reasons: the spring has weakened, debris has gotten between the magnetic circuit contacts, low voltage in the control circuit or coil wear. This requires replacing the contactor, since the humming will lead to overheating and eventual combustion of the coil.

What power reserve is needed for a generator when using an ATS?

The generator power should be 20-30% higher than the total power of connected consumers. This reserve is necessary to compensate for the starting currents of the engines and prevent the generator from operating at the limit of its capabilities, which extends its life.