Is-limiter — pyrotechnic fault current limiter for bus-tie coupling with parallel transformers

The [guide on Icw/Icm — short-time withstand current and making capacity](/guides/nen-3140/vermogensschakelaar-icw-icm-kortstondige-stroomsterkte-maakvermogen) covers how a circuit breaker must be rated for the prospective short-circuit current at its installation point. This article covers a specific situation where that prospective short-circuit current suddenly becomes higher than the existing switchgear can handle, because of a design change — and a device that solves that problem without replacing the switchgear itself: the Is-limiter.

The problem: bus-tie coupling raises the short-circuit current

In a low-voltage switchboard with two (or more) transformers, each feeding its own busbar section, a bus-tie breaker between the two busbar sections can be closed — for example, for operational reliability, so that if one transformer fails the other can supply both busbar sections. However, as long as the bus-tie is closed, both transformers jointly contribute to any short circuit on either busbar section: the prospective short-circuit current for a fault on busbar section A is then no longer just transformer A's contribution, but the sum of the contributions from both transformer A and transformer B via the closed tie.

See the [guide on short-circuit current calculation using the IEC 60909 impedance method](/guides/nen-1010/kortsluitstroomberekening-iec-60909-impedantiemethode) for how such a combined contribution is calculated. The practical consequence: the prospective short-circuit current with the bus-tie closed can be significantly — sometimes well above the short-circuit rating (Icw/Icm) of the existing circuit breakers, busbars, and other switchgear — higher than the short-circuit current of either transformer alone.

The conventional solution and why it isn't always practical

The conventional solution is to replace the entire switchgear — busbars, circuit breakers, protection relays — with equipment rated for a higher short-circuit capacity that can handle the full combined short-circuit current. In an existing installation where the bus-tie must only later be kept permanently closed (for example after a change in operating philosophy, or after an expansion with a second transformer), this is a costly and time-consuming intervention that often requires a prolonged shutdown of the entire switchboard.

How an Is-limiter solves this: interrupting before the first current peak

An Is-limiter is a pyrotechnic switching element placed specifically in the bus-tie connection itself, which, upon detecting a short-circuit current that exceeds a preset pickup value, physically interrupts the bus-tie connection within a few milliseconds — well before the first current peak (the first peak of the asymmetrical short-circuit current) is reached — by explosively severing a conductor and extinguishing the resulting arc almost instantly in a dedicated quenching medium.

Because the interruption takes place before the short-circuit current reaches its peak value, the rest of the installation — the existing busbars and switchgear on either side of the coupling — "sees" a short-circuit current that remains limited to roughly the level of a single transformer, instead of the full, unimpeded sum of both transformers. As a result, the existing switchgear does not need to be over-rated for the combined short-circuit current, even though the bus-tie remains closed during normal operation.

Note: an Is-limiter is fundamentally different from a circuit breaker or fuse — it is designed to limit the rise of the current itself before the peak value is reached, not to simply trip after a fault like a regular protective device. This makes it suitable as an additional current-limiting device in the bus-tie connection, not as a replacement for the regular discriminative protection of the outgoing feeders.

The key limitation: single-use operation

The pyrotechnic operating principle of an Is-limiter means that the device is physically consumed upon activation and must be replaced before the bus-tie can be put back into service — unlike a circuit breaker, which can simply be reset and re-closed after the fault has been cleared. This is a deliberate design trade-off: the Is-limiter is expected to operate only rarely (only during an actual short circuit with the bus-tie closed), and the cost of occasional replacement often outweighs the savings from not having to replace the full switchgear.

Practical relevance

When assessing a switchboard with two or more transformers operated in parallel and a bus-tie that must remain permanently closed, an Is-limiter is an option worth considering when the combined prospective short-circuit current exceeds the short-circuit rating of the existing switchgear — especially in an existing installation where full replacement of the switchgear would be disproportionately costly or operationally infeasible. When applying it, verify that the pickup value of the Is-limiter is correctly matched both to the normal operating current through the bus-tie (no unwanted activation during normal load switching) and to the actual short-circuit contribution of the second transformer.

Common mistakes

Permanently closing a bus-tie after an expansion with a second transformer without recalculating the combined prospective short-circuit current, unknowingly loading the existing switchgear beyond its short-circuit rating.
Treating an Is-limiter as a replacement for regular discriminative protection of the outgoing feeders — the Is-limiter specifically limits the combined contribution via the bus-tie during a busbar fault, not the normal discrimination between feeders.
Not keeping a replacement unit in stock, so the bus-tie stays out of service longer than necessary after an activation.
Setting the pickup value incorrectly relative to the normal operating current through the tie, causing an unwanted activation (and thus unnecessary replacement cost) during a normal, non-faulted load current.