How to Select Protective Device in DC Electrical System


DC Circuit Breaker | Source: ABB

For the correct sizing of a circuit breaker in a direct current network, some electrical parameters which characterize the device itself must be evaluated.

Rated operational voltage Ve 

This represents the value of the application voltage of the equipment and to which all the other equipment parameters are referred. 

Rated uninterrupted current Iu 

This represents the value of current that the equipment can carry for an indefinite time (uninterrupted duty). This parameter is used to define the size of the circuit breaker. 

Related Article: 

Rated current In 

This represents the value of the current of the trip unit mounted on the circuit breaker and determines the protected characteristic of the circuit breaker itself according to the available settings of the trip unit. This current is often referred to as the rated current of the load protected by the circuit breaker itself.

Rated ultimate short-circuit breaking capacity Icu 

The rated ultimate short-circuit breaking capacity of a circuit breaker is the maximum short-circuit current value which the circuit breaker can break twice (in accordance with the sequence O – t – CO) at the corresponding rated operational voltage. After the opening and closing sequence the circuit breaker is not required to carry its rated current. 

Rated service short-circuit breaking capacity Ics 

The rated service short-circuit breaking capacity of a circuit breaker is the maximum short-circuit current value which the circuit breaker can break three times, in accordance with a sequence of opening and closing operations (O - t - CO - t – CO), at a defined rated operational voltage (Ve) and at a defined time constant (for direct current). After this sequence, the circuit breaker is required to carry its rated current. 

Rated short-time withstands current Icw 

The rated short-time withstand current is the current that the circuit breaker in the closed position can carry during a specified short time under prescribed conditions of use and behavior. The circuit breaker shall be able to carry this current during the associated short-time delay in order to ensure discrimination between the circuit breakers in series.

Sizing Circuit Breakers in DC Electrical System

In the previous pages, the main electrical characteristics needed to choose the correct circuit breaker have been defined so that protection of the plant is guaranteed. To size the circuit breaker, it is necessary to know the following characteristics of the network: 

  1. The type of network (see:  Type of DC Network: Network with the middle point of the supply source connected to ground and other DC Network System), to define the connection of the circuit breaker poles according to the possible fault conditions;
  2. The rated voltage of a plant (Vn), to define the operational voltage (Ve) depending on the pole connection by verifying the relation: Vn≤ Ve;
  3. The short-circuit current at the installation point of the circuit breaker (Ik), to define the circuit breaker version (depending on the connection of the poles) by verifying the relation Ik ≤ Icu (at the reference rated operational voltages Ve). 
  4. The rated current absorbed by the load (Ib), to define the rated current (In) of the thermal-magnetic trip unit or of the DC electronic trip unit by verifying the relation Ib≤ In.

The following diagram summarizes the choices for correct sizing of the circuit breaker in relation to the characteristics of the plant.

Sizing Circuit Breaker 


  • ABB Circuit Breakers for Direct Current Applications | pp. 20-23
  • Publisher: ABB
  • Download Here

No comments:

Select Topics

electric protection Electrical Design power system protection Electrical Safety Fault Analysis Electrical Machines protective relaying circuit breaker electrical protection Electrical Equipment Technical Topics Electrical Installation Power System BS7671 short circuit analysis DC Circuit Earthing System Transformer power system analysis what Direct Current System Energy Efficiency Generator IEC standard Manual Resources Transmission Lines Unbalanced Fault Analysis electrical motor electrical testing grid automation power system automation smart grid tutorial video ebook how motor control substation automation symmetrical components AC Machines Advance Circuit Theory IEC 60364 Renewable Energy Voltage Drop Calculation current transformer electrical grounding schneider electric Circuit Analysis fuse generator protection power system stability quiz switchboard transformer protection ABB Manuals AC Circuit Busbar DC Machines GE Whitepapers General Electric Line to Line Fault National Electrical Code arc flash earth fault loop impedance electric vehicle electrical wiring power plant power system operation selective coordination switchgear video tutorial 3D printing ABB AREVA AUS/NZ 3000 Assignment help Busway Current Nomenclatures Electricity Spot Market G3 technology IEEE C37.2 IEEE/ANSI Device Numbers MiCom NFPA 70E Philippine Electrical Code Terms of use Theoretical UFES VFD ampacity battery building wiring capacitor circuit breaker curve cooling system cooper bussman disruptive technologies electrical earthing electrical harmonics energy industry energy savings engineering education iec 61850 inspection checklist learning process bus protective bonding single line to ground fault transmission line protection variable frequency drive voltage compensation voltage transformer voltage unbalance