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Emergency Standby Power


emergency standby power

With more and more electrical equipment being used in modern buildings, diesel genset has been widely used as standby power supply to ensure power supply reliability when the mains is unable to provide two independent power supplies or the power consumer is classified as a primary load.

According to relevant electrical codes and national standards, Class I high-rise buildings should be supplied as a primary load. The primary load generally includes: TV stations, radio stations, civil airports, railway stations, banks, hospitals of county (district) level or above, weather stations of municipal (regional) level or above, important office buildings, important laboratories of universities and academies of sciences, major museums and computing centers, etc.

Primary load requires that power supply system shall ensure a continuous power supply both in normal operation and in emergency. Therefore, two independent power supplies shall be provided for primary load. 

One or both of these two power supplies come from the mains and a standby power supply, i.e., diesel genset, shall be equipped. A dual-power automatic transfer switch (ATS) is used to automatically transfer the power supply to the load. 

Generally, standby genset shall be always ready for firefighting systems, evacuation lighting and elevators of high-rise buildings as well as control systems and important communication systems of automatic production lines. For this reason, genset is equipped with automatic control system to guarantee automatic power supply in emergency. System diagram is generally as follows: two standby gensets in parallel

Soft load transfer may be used for important loads at the places with higher electricity demand. 

That is, when the frequently-used power supply fails, the genset will be started and switched to supply the power; when the frequently-used power supply resumes, the genset will be automatically synchronized to the mains and the load will be transferred to the frequently-used power supply, and then the genset will be disengaged to cool down and stop. System diagram is generally as follows:

One genset in parallel with the mains

Dual-power automatic transfer switch (ATS), the switching device, is essential for standby genset. It is used for switching the frequently-used power supply and the standby power supply. ATS can be selected according to the operation types and load types as shown in the table below: wherein Operation A refers to frequent switching and Operation B refers to non-frequent switching.

Current properties

Operation type

Typical application

Operation A

Operation B



AC-31 B

Non-inductive or slightly inductive load



Mixed resistive and inductive loads, including moderate overload.



Total system load, including cage motor and resistive load.



Motor load or mixed loads including motor load, resistive load and incandescent lamp load of less than 30%.

AC-35 A

AC-35 B

Discharge lamp load

AC-36 A

AC-36 B

Incandescent lamp load


DC-31 A

DC-31 B

Resistive load

DC-33 A

DC-33 B

Motor load or mixed loads including motor load

DC-36 A

DC-36 B

Incandescent lamp load

Standard ATS typically has 2 connected power supplies interlocked electrically and mechanically to ensure that these two power supplies will not be switched on simultaneously. In some occasions, bypass type ATS may be used in order to test the switching operation of ATS periodically without interruption. The bypass type ATS is divided into withdrawable integrated type and assembled type. 

The former one only requires one dual-power switch and one frame to meet operating requirements, but the cost is relatively high. While the latter one is made up of two standard dual-power switches, which can be made into single bypass or double bypasses. However, its failure rate is increased with the increasing of the bypass quantity.

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