PM825 PM825-1 ABB 3BSE010796R1 Off the shelf programmable controller

PM825 PM825-1 ABB 3BSE010796R1 Off the shelf programmable controller

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The short-circuit current at the terminals of an accumulator battery may be

supplied by the battery manufacturer, or may be calculated using the following

formula:

where:

• UMax is the maximum flashover voltage (no-load voltage);

• Ri

 is the internal resistance of the elements forming the battery.

The internal resistance is usually supplied by the manufacturer, but may be

calculated from the discharge characteristics obtained through a test such as

detailed by IEC 60896 – 1 or IEC 60896 – 2.

For example, a battery of 12.84 V and internal resistance of 0.005 Ω gives a

short-circuit current at the terminals of 2568 A.

Under short-circuit conditions the current increases very rapidly in the initial

moments, reaches a peak and then decreases with the discharge voltage of

the battery. Naturally, this high value of the fault current causes intense heating

inside the battery, due to the internal resistance, and may lead to explosion.

Therefore it is very important to prevent and / or minimize short-circuit currents

in direct currents systems supplied by accumulator batteries.

Criteria for the selection of circuit-breakers

For the correct selection of a circuit-breaker for the protection of a direct current

network, the following factors must be considered:

1.the load current, according to which the size of the breaker and the setting

for the thermo-magnetic over-current release can be determined;

2.the rated plant voltage, according to which the number of poles to be

connected in series is determined, thus the breaking capacity of the device

can also be increased;

3.the prospective short-circuit current at the point of installation of the breaker

influencing the choice of the breaker;

4.the type of network, more specifically the type of earthing connection.

Note: in case of using of four pole circuit-breakers, the neutral must be at 100%