Generator changeover panels

The main duty of the generator changeover panels [1] is  to transfer the load to the backup generator [4]. A generator changeover panel may also command the backup generator to start [2], based on the electrical parameters monitored on the primary supply. The generator changeover panels isolate the backup generator from the electric utility when the generators are on and providing temporary power [3]. The control capability of generator changeover panels may be manual only, or a combination of automatic and manual. The most advanced generator changeover panels may be equipped with advanced controllers and remotely controlled via GSM or TCP-IP as indicated in the image.

You can monitor, from your office, your entire fleet of backup generators installed in various  areas  by using the Bernini Design generator changeover panels. Application areas include: industrial, Commercial & retail,  Light Industrial, Municipal, Hospitals and Telecoms.

Imagine, for example that you have a fleet of back up generators installed in a chain of retail stores. The worst scenario is a wide-scale power outage that affect a big area including many of your clients. In this case, the  generator changeover panels will instruct the backup generators to start. Once the  generator changeover panels see that the generators are ready to provide electric power, the  generator changeover panels break the retail stores connection to the electric utility and connect the generators to the retail stores main electrical panels.

The generator supplies power to the retail store electric load, but it is not connected to the electric utility line. As a matter of fact it is mandatory to isolate the generator from the distribution system to protect the generators from overload in powering loads. In the meantime the generator changeover panels protect workers from back-feeding. On the screen of the computer you can monitor all generators. You can instruct , from your office, qualified personeel in case of a generator fails to start.  

When utility power returns, for a pre-set time, in stable conditions, each generator changeover panel will start to transfer the retail store back to utility power and command the generator to turn off. From your computer you can follow the situation gradually returning normal. One after one the electrical system in each retail store will be finally connected to the utility power.

Safety of generator changeover panels

A generator changeover panel works in a ‘break-before-make transfer mode’. A break-before-make transfer switch opens contact with one source (Power utility for example) of power before it makes contact with the other (Generator for example). It prevents back-feeding from a backup generator back into the utility power, for example. This solution is often called ATS (Automatic Transfer Switch). During the split second of the power transfer the current is interrupted. Is like you handle a three position circuit breaker, with utility power on one side, the generator on the other, and "off" position in the middle. Before making any connection, you are required to ‘cross’ the position ‘OFF’ (that is ‘no connection’). The internal transfer switch controller checks the electrical parameters before making a connection. The automatic transfer switch controller opens the circuit in case of severe troubles as well.


Automatic changeover switch circuit diagram

An automatic transfer switch works in conjunction with your backup generator to ensure you have electrical power to your business premises at all times. During a power outage, the automatic transfer switch transfers the load to the generator, and then transfers it back once utility power has been restored. The controller (in this example Be28) ensures that all this is done smoothly and with virtually no downtime.  The automatic changeover switch for generator comes with a built-in controller, which monitors the parameters of power and detects when there are any interruptions or when an electrical parameter, frequency for example, is out of the preset limit. The controller will then automatically start the engine when necessary via the KFH relay. The contact of the relay will instruct the Auto Start Module to start the engine. Once the generator has reached the correct frequency and voltage, the automatic changeover transfers the load from the utility to the generator. This happens immediately after a power failure or based upon your requirements (it could be delayed up to 24 hours). Once utility power has been restored, the Automatic changeover will switch the load back automatically. After a time delay, the engine is then stopped. All items that allow the above described functions are included in the wiring diagram on the left. The controller monitors the electrical parameters of the power utility and generator via the terminals R-S-T-L1-L2-L3. The drawing indicates a 3-phase system. But you can connect the panel is single phase as well. The changeover switch is made by using two electrically separated circuit breakers or so called ‘contactors’. These devices are not allowed to close simultaneously. They are allowed to open simultaneously or only one at a time. There is a particular mechanism that does not allow to both breakers to be in a ‘circuit closed position’. This device is called ‘mechanical interlock’. A contactor is an electrically controlled switch, really similar to a relay except with higher current and voltage ratings. A contactor is controlled by a circuit that energize the solenoid. In the wiring diagram the CONTACTOR of the generator is indicated as KG and the CONTACTOR of the power utility is indicated as KM.  Each contactor in its own ‘closed position’ opens the auxiliary contact. This contacts being in series with the coil of the other contactor adds more safety to the automatic changeover. This connection is called ‘electrical interlock’.  So, we can say that by using the ‘mechanical interlock’ and  ‘electrical interlock’ we are sure that it is not possible to connect the generator to the power utlity. The only possible allowed connections are:


The automatic transfer switch controller features a particular programmable timer named ‘CHANGE OVER DELAY’. In other words, you can program the switching time between GENERATOR to LOAD and POWER UTILITY to LOAD. Normally this time is set at two seconds. This means that all the time you switch a power source you will have a ‘black-out’ in your load for about two seconds. The coils that operate the contactors are controlled by two auxiliary relays: KMC & KGC. Click on the Generator automatic transfer switch wiring diagram to enlarge the image.

Automatic changeover switch for generator


Automatic changeover switch for generator

In this drawing you can see how intuitive is the connection of an automatic changeover switch for generator. In the drawing a 3-phase connection is shown together with a pre-heat element. This equipment will maintain the body of the engine warm; about 30-40 degree Celsius. This prevent the fluid in the engine to freeze and will guarantee a fast response time of the generator especially in winter. In critical situation you require immediately the full load from your generator. If the engine is cold, you are required to warm up the engine by running it off load for a few minutes. Once you start with a reasonable  good temperature you can demand full load practically immediately. Despite the fact that current sensors can provide Over Load & Over Current Protection (by correctly programming the automatic transfer switch controller) it is recommended to use a magnetic circuit breaker for the protection of the generator. In this way you prevent severe damage in case the automatic transfer switch controller fails to protect the system. The installation of an automatic changeover switch  must be carried out by qualified personnel only. The installation must be up to code. An electrical code is a set of regulations for the electric wiring and electrical installations. The purpose of an electrical code is to provide standards to ensure electrical wiring systems that are safe and unlikely to produce either electric shock or fire. Electrical codes are ruled by national and international technical organizations, and adopted as law to make them enforceable.

Here you can see a typical guide line   and a typical  Electrical Panel Permit Requirements 

Automatic generator changeover switch price price list








€ 599,00





€ 810,00





€ 924,00





€ 945,00





€ 1138,00





€ 1534,00





€ 1833,00