In the Is limiter, a current limiting fuse is placed in parallel with a solid conductor (5000 A), thus avoiding thin fuse elements in the circuit. When a short circuit current is detected by its steep increase (di/dt) an explosive charge is detonated that ruptures the main conductor and the arc voltage will commutate the fault current into the parallel fuse that will in turn interrupt the current in less than 1 ms.
The Is limiters are applied when the available short-circuit current exceeds that of individual components or network sections. Examples:
- Parallel service of transformers. the secondary busbars are connected through an Is limiter. This device separates the two transformers and the contribution to the fault current at the secondary side is limited to one transformer only.
- In parallel with a current limiting reactor. In the undisturbed situation, the normal current is carried by the Is limiter and losses across the reactor are avoided. In the case of a fault, the Is limiter blows and the fault current is limited by the reactor.
- In the connection from the network to local generation centres.
Photo from ABB catalogue shown Is limiters connected in parallel with a reactor.