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Correctly Choose The Switch Of Diesel Generator Set

Sep. 03, 2021

When choosing an automatic transfer switch for a diesel generator, users need to consider many different factors, the most important of which is size. Careful selection of automatic transfer switches is important to ensure maximum reliability and sufficient capacity in normal and emergency situations.


1.Different types of loads.

The industry classifies automatic transfer switch loads. The listed loads include total system load, motor load, discharge lamp load, restrictive load and incandescent lamp load. In addition, it is also required that all transfer switches of diesel generators should be clearly marked to indicate the type of load they can handle. The "Total System Load" panel indicates that the switch can be used for the various loads described in the previous paragraph. However, when handling incandescent lamp (tungsten-based filament) loads, the total load should not exceed 30%, unless the transfer switch is specifically used to transfer a higher percentage of power to the incandescent lamp.


Generally, most transfer switches used in old generators can handle the total system load. It is best to check the markings because some total system loads are marked with "resistance only, tungsten only, etc.". By selecting the total system load from the beginning of the project, engineers can greatly reduce the hassle of overall project management.


2. Voltage rating.

The automatic transfer switches of second-hand generators are unique in their power distribution system because they are one of the few electrical devices designed to connect two asynchronous power sources. For example, this may mean that the voltage applied to the insulated side of the device may actually be as high as 960 volts in a 480 volt AC system. A well-designed UL transfer switch will provide sufficient spacing and insulation to cope with the increased voltage stress.


It is because of the increased stress on the unit that the spacing in the transfer switch should be less than the spacing shown in UL1008 table 22.1, regardless of the components used as part of the transfer switch.


The rated voltage of the AC system is usually 120, 208, 240, 480, 600 volts, single-phase or multi-phase. The standard frequency is 50 or 60 Hz. If required, automatic transfer switches can be used for other voltages and frequencies, which also includes DC.


diesel engine

3. A component of the continuous rated current onan automatic transfer switch.

The standard expectation for continuous load automatic transfer switches is that the switch should be able to hold the maximum for three hours or more. The difference between transfer switches and other emergency equipment is that they must continuously deliver current to critical loads, whether from normal power sources or emergency power sources. The backup engine generator set usually supplies power only in emergency situations.


The automatic transfer switch for diesel generators is manufactured to meet the continuous current rating of 30-4000 amperes. Generally, the most commonly used ampere ratings include 30, 40, 70, 80, 100, 150, 225, 260, 400, 600, 800, 1000, 1200, 1600, 2000, 3000, and 4000 amperes.


Modern transfer switch technology can carry 100% of the rated current at an ambient temperature of 40°C. Transfer switches containing integrated overcurrent protection devices may be limited to continuous load currents that do not exceed 80% of the switch rating. In modern switches, there are appropriate control measures to ensure that no more than 80% of the load is continuous. However, in older equipment, the probability of system failure exceeds 80%.


The project engineer should predict the future load demand during the planning process. Not all projects require pre-consideration of future load requirements, but it is usually recommended to select a transfer switch with a continuous current rating equal to the total expected load.


To calculate the continuous rating of the transfer switch, the amperage required by all loads must be totaled. To determine the load current of tungsten (incandescent) lamps and electric heaters, the total wattage must be calculated. The load current of mercury vapor, fluorescent lamp vapor, and sodium vapor lamp must be based on the current consumed by each ballast or autotransformer, not the total wattage of the lamp. The motor full load current only determines the motor load, and cannot accurately measure other types of loads. When determining the size of the transfer switch for the total system load listed by UL, there is no need to consider the locked rotor and the motor inrush current. In most cases, there is no need to degrade the transfer switch when used at ambient temperatures up to 40°C, this includes switches installed in switchboards or separate enclosures.


4.Find the ampere rating.

The following example formula involves the need to find the ampere rating of the total system load:

Automatic transfer switch rated for the total system load, used for 208/120V, three-phase, four-wire circuits, including the following three-phase balanced loads.


115kW heating load


64kW tungsten lamp load


Three 10HP motors, each 32amps


Total load=320+178+96=594 amps


Please note when checking the tungsten wire load, since the load does not exceed 30% of the total load, please select a three-pole switch with a rated current of at least 600 amperes. The relationship between typical diesel generator line current and kW rating can be found in UL1008. For the previous example, a 200kW generator may be suitable for this application, provided it can handle the inrush current.



Jiangsu Starlight Electricity Equipments Co.,Ltd. is a diesel generator set manufacturer with more than 40 years of production experience. Its main brands include Yuchai generator sets, Cummins generator sets, Volvo generator sets, etc., and 64 sales and service departments nationwide. Users provide one-stop service of design, supply, debugging and maintenance. Welcome contact us by email sales@dieselgeneratortech.com

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