Is-limiters are frequently used in the coupling between two systems or parts of systems whose short circuit withstand capability would not be sufficient with parallel connection via a circuit breaker.

Instead of designing the two systems for double the short-circuit current, an Is-limiter is installed in the coupling. In the case of a fault, it limits the peak short-circuit current at the very first rise. It separates the systems electrically into two parts before the short-circuit current can endanger system components. After separation, only the faulty part of the system still feeds the short circuit, which is selectively shut down by the relevant circuit breaker. The peak short-circuit current thus does not become higher than the short-circuit current from a single transformer at any point in the switchgear. When the Is-limiter trips, the voltage in the part of the system not affected by the short circuit only drops for fractions of a millisecond at most.

Even sensitive loads are thus protected from voltage dips. The Is-limiter is therefore also highly suitable for installation in the coupling between an ”unprotected” an a ”protected” system.