A load break switch is designed to make and break normal load current and to carry, but not break, short-circuit current. It sits between a disconnector, which cannot interrupt load at all, and a circuit breaker, which can interrupt fault current. In ring main units and compact secondary substations the load switch is the everyday operating device, while a fuse or an upstream breaker clears faults.
The FKN12-12 combines a three-position load break switch (closed – open – earthed) with a set of high-voltage current-limiting fuses in one compact, gas- or air-insulated unit. The switch is typically an SF6 or vacuum type that quenches the arc during load breaking; the fuses provide the short-circuit protection. An interlock ensures the switch cannot be turned to the earthed position while the fuses are live, and that the fuse compartment cannot be opened until the switch is earthed.
| Parameter | Typical value |
|---|---|
| Rated voltage | 12 kV |
| Rated current | 630 A (switch), fuse rating per application |
| Rated short-time withstand | 20 kA / 4 s |
| Load breaking capacity | ≥ 630 A at 0.7 power factor |
| Mechanical life | ≥ 3000 operations |
Correct protection depends on the fuse clearing any fault above the switch's making and load-breaking capability. The fuse must be selected so that a transformer-secondary fault is cleared by the fuse before the upstream device trips, and so that the fuse does not operate on transformer inrush. Where two transformers are fed from one switch, the fuses must be individually coordinated.
Can a load switch replace a circuit breaker?
No. It interrupts load current only. Fault current must be cleared by a fuse or an upstream breaker.
Why combine a switch with fuses instead of using a breaker?
Cost and compactness. For a small distribution transformer the switch-plus-fuse combination gives adequate protection in a fraction of the space and cost of a breaker cubicle.
