Example On How To Design a Low Voltage Switchboard

Low voltage switchboard construction

This technical article has the aim of helping the panel builder and the designer in the construction of ABB SACE ArTu low voltage switchboard. To this purpose, starting from the single-line diagram of a plant, it is possible to arrive – by selecting the suitable components – to the construction of an switchboard and to the relevant declaration of conformity with the Std. IEC 61439-2.

Characteristics of the low voltage switchboard, according to the specification:

• “not separated” switchboard;
• IP 65;
• exposed wall-mounted.

Single-line diagram

Let us suppose that realization of a main distribution switchboard is required, to be placed immediately on the load side of a 2000kVA MV/LV transformer. Three 850A outgoing feeders from this switchboard supply other distribution switchboard, but they are not dealt with.

Due to reasons of selectivity with other circuit breakers of switchboards on the load side, air circuit breakers have been chosen branched from the busbars. The main distribution busbar short-circuit current is 48 kA.

Selection of the circuit breakers and of the conductors external to the switchboard

Circuit breakers //

As shown in the single-line diagram, the circuit breakers chosen are:

• 1 pc. of Emax E3N3200 PR121/P-LSI In 3200 (main circuit breaker of the switchboard QF1);
• 3 pcs. of Emax E1N1250 PR121/P-LSI In 1250 (circuit breakers for the three outgoing feeders QF2, QF3, QF4).

Conductors:

• Incoming, from the transformer:
  – 1 bus duct with Iz = 3150 A; L = 5 m
• Outgoing from the switchboard, hypothesizing overhead installation on perforated trays, there are three low voltage cables:
  – 1 cable L = 20m 3 x (3 x 120) Iz = 876,3 A;
  – 1 cable L = 70m 3 x (3 x 120) Iz = 876,3 A;
  – 1 cable L = 100m 3 x (3 x 120) Iz = 876,3 A.

Switchboard front, distribution system and metalwork structure

With regard to the positioning of the equipment, it has been decided to locate the main circuit breaker in one column, and the three outgoing feeders in another one. Since the power supply comes from below, it has been decided to position the circuit breaker QF1 at the bottom.

The switchboard assembly is of “not-separated” type. A possible layout of the busbars and of the circuit breakers is shown in the following figure:

Distribution system:

As regards the busbars inside the switchboard, by first approximation, they are selected according to the size of the circuit breaker:

Main distribution busbar system

Circuit breaker QF1
(From the “Distribution Switchgear – General catalogue”)

BA2000 In=3200 A (IP65); I_cw max =100 kA

To get an I_cw value suitable to the short-circuit current of the plant:
– 5 busbar holders PB3201 at a maximum distance of 425mm (I_cw=50 kA) must be positioned.

Branch busbars of the circuit breakers

Circuit breakers QF2, QF3, QF4
(From the “Distribution Switchgear – General catalogue”)

BA1250 In= 1250 A (IP65) I_cw max = 75 kA

To get an I_cw value suitable to the short-circuit current of the plant:
– 5 busbar holders PB1601 at a maximum distance of 425mm (I_cw=50 kA) must be positioned.

Joining pieces between circuit breakers and busbars
(circuit breakers QF2, QF3, QF4)

Table 11.2 of clause 11.4 of the Technical Application Paper shows the cross-sectional areas of the busbars for the connection of the circuit breakers:

• E3N323200 – A cross-sectional area 3 x (100 x 10)
• E1N121250 – A cross-sectional area 1 x (80 x 10)

Moreover, according to the terminal types, the maximum anchoring distance of the first anchor plate, shown at clause11.3 of the Technical Application Paper, shall be respected.

Joints for busbars //

As indicated in the “Distribution switchgear. General catalogue” the following joints are necessary:

• Joint from 3200 busbar to 3200 busbar, T joint, AD1073
• Joint from 3200 busbar to 1250 busbar, AD1078

Earthing busbar //

As shown on page 44 and 45 of this technical Application Paper, the earthing busbar shall have a minimum cross-sectional area equal to 1⁄4 of the cross-section of the main busbars. Therefore a bar 50×10 has been chosen.

Metalwork structure

As regards the metalwork structure, an ArTu K series switchboard with door (IP 65) is used. In order to house the circuit breakers, the vertical busbar system and the outgoing cables the following is used:

• 2 columns for the circuit breakers;
• 2 cable containers, one for the busbar system and one for the outgoing cables.

For a correct selection of the structure it is advisable to consult the “Distribution switchgear. General catalogue” where:

• to house Emax E1-E2-E3 circuit breakers a switchboard assembly with 800mm depth and 600mm width and one installation kit KE3215 are required.

The cable container has obviously 800mm depth and 300mm width. In the general catalogue for distribution switchgear the fixing crosspieces for busbars with shaped section can be found:

• For the 3200 A horizontal busbars (BA2000) the selected type of installation is number 5, for which the correct choice is two components TV6221 and one TV8011;
• For the 3200 A vertical busbars (BA2000) the selected type of installation is number 2, for which the correct choice is TV8101 component;
• For the 1250 A horizontal busbars (BA1250) the selected type of installation is number 5, for which the correct choice is two components TV6221 and one TV8011.

As specified in the general catalogue for distribution switchgear, the metalwork structure shall be completed by the side-by-side kits (AD1014).

References //

1. Guidelines to the construction of a low-voltage assembly complying with the Standards IEC 61439 Part 1 and Part 2 – ABB
2. Distribution switchgear General catalogue // ArTu K – ABB