Is there any standard to consider the situation when load break switches are equipped with fuse-links?
Yes, IEC 60 420 gives guidance for the combination of a fuse-link according IEC 60 282-1 and a load break switch according IEC 60 265-1.
What is the task of IEC 60 420?
This standard helps to select the fuse rating by the criteria of the transfer current of the switch, the inrush current of the transformer, the maximum short circuit current of the circuit and the time/current-curve of the fuse-link.
The rated transfer current of the switch must be higher than the transfer current of the fuse-link. Both values have to be compared at the same time, the so-called opening time of the switch.
The rated transfer current of the switch must be higher than the maximum primary short-circuit current. This current is calculated by the reactance of the transformer (4 - 6 %). The inrush current of the transformer must be smaller than the time/current-value of the fuse-link at about 100 ms. Typical inrush currents are between six and twelve times of the transformer rated current.
The fuse-switch combination has to withstand the fuse-link power loss at loading currents. Additionally the conditions of IEC 60 787 (Transformer protection) have to be observed.
What does the rated transfer current mean?
The rated transfer current at the switch opening time indicates the limit of the task either of the fuse-link or the load break switch. Fault currents above that limit are interrupted by the fuse-links. Below that point, the switch itself interrupts fault currents. At currents below the rated transfer currents, it is the main job of the fuse-link striker pin to actuate the switch by tripping a 3pole opening mechanism. The switch manufacturer gives the value of the rated transfer current. Typical values are at 10 kV 800 to 1600 A and at 20 kV 600 to 1200.
This time also marks in combination with the rated transfer current the limit of the task either of the fuse-link or the load break switch. Typical values are between 30 and 200 ms..
