Type of DC Network: Network with the middle point of the supply source connected to ground

 

DC Network with the middle point of the supply  source connected to ground | Source: ABB

This type of network is obtained by connecting the middle point of the battery to the ground. This type of connection reduces the value of static overvoltages, which could otherwise be present at a full voltage in an insulated plant. 

Grounding Solution | Source: ABB

The main disadvantage of this connection, if compared with other types, is that a fault between a terminal and ground gives rise to a fault current at a voltage V/2.

Related Article: 

• Type of DC Network: Network insulated from ground
• Type of DC Network: Network with one terminal grounded

Fault types in a network with the middle point connected to ground

Fault A 

The fault between the two terminals is a short-circuit current fed by the full voltage V. The breaking capacity of the circuit breaker shall be sized according to the short-circuit current relevant to such a fault.

Sample Fault: Fault at A

Fault B

The fault between the terminal and ground sets up a short circuit current lower than the fault between the two terminals as it is supplied by a voltage equal to V/2 depending on the soil resistance.

Sample Fault: Fault at B

Fault C

In this case, the fault is analogous to the previous case but it involves the negative terminal.

Sample Fault: Fault at C

With this type of network, the fault which affects the version of the circuit breaker and the connection of the poles is fault A (between the two terminals). However, the fault between a terminal and ground should also be taken into consideration because a current could flow at a voltage equal to V/2. 

In a network with the middle point of the supply connected to the ground, the circuit breaker must be connected on both terminals.

Source: 

• ABB Circuit Breakers for Direct Current Applications | pp. 18-19
• Publisher: ABB
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