Circuit-breaker trip settings — L, S, I and G in the LSI(G) protection curve

The MCCB versus MCB guide covers the difference between a miniature circuit-breaker (MCB, fixed characteristic) and a moulded-case/air circuit-breaker (MCCB/ACB, IEC 60947-2). This article takes it a step further: a circuit-breaker with an electronic trip unit is not just "bigger", it is also adjustable — and that setting determines whether the protection works correctly just as much as the choice of breaker itself.

The four protection functions: L, S, I, G

An electronic trip unit on a circuit-breaker combines up to four independently adjustable protection functions, denoted by the letters that also appear in the product name (LSI or LSIG):

• L — Long time (thermal/overload): Ir is the long-time pickup setting, expressed as a factor of the breaker's rated current (for example Ir = 0.8× In). tr is the associated time delay, which defines the inverse-time curve (the higher the overload, the shorter the trip time) and which must, among other things, tolerate motor starting current without tripping unnecessarily.
• S — Short time (short-circuit, delayed): Isd is the short-time pickup setting, set as a multiple of Ir (typically in the order of 1.5 to 10× Ir). tsd is the associated, short adjustable time delay (typically tens to hundreds of milliseconds), which is specifically used to achieve selectivity with a downstream breaker.
• I — Instantaneous: Ii is the threshold above which the breaker trips without intentional delay — intended as a last-resort safeguard for a severe, close-in short circuit, regardless of the configured S delay.
• G — Ground fault: Ig is the pickup current for a ground fault (typically lower than the phase protection thresholds, to detect an earth fault that the regular overcurrent protection would not trip on), with its own time delay tg.

Note: not every circuit-breaker has all four functions — a basic LSI breaker has no separate ground-fault protection (G); this is then implemented with a separate RCD/earth-fault relay, or the G function is deliberately omitted because ground-fault protection is already provided elsewhere in the installation (for example at circuit level).

Why the setting matters as much as the choice of breaker

Two circuit-breakers with identical Icu/Ics ratings (see the MCCB versus MCB guide) can behave completely differently in practice if their LSI(G) curves are configured differently. Selectivity between a main breaker and a downstream breaker is achieved by deliberately setting the upstream breaker's short-time delay (tsd) longer than the total trip time of the downstream breaker — so that a fault in the downstream circuit trips only the downstream breaker, giving the main breaker time to "wait and see". An incorrectly set tsd (too short, or equal to that of the downstream breaker) undoes this selectivity, even if both breakers are otherwise well matched in rated values and short-circuit withstand.

Practical relevance

At handover, inspection or reassessment of a distribution board with circuit-breakers, it must be checked not only whether the breaker itself (rated current, Icu/Ics) fits its position in the installation, but also whether the actually configured Ir/tr, Isd/tsd, Ii and (if present) Ig/tg values match the design calculation — a breaker left at "factory default" is rarely correct for the specific installation.

Common mistakes

1. Assuming a correctly selected circuit-breaker automatically protects correctly — without the right Ir/Isd/Ii/Ig setting per the design calculation, even the correctly chosen breaker will not function as intended.
2. Leaving the factory default setting unchanged (often the highest Ir and longest tsd) at commissioning, without tuning it to the actual load and the desired selectivity with other breakers in the installation.
3. Setting tsd equal or too close between two breakers in series — for genuine selectivity the upstream breaker must have a clearly longer tsd than the total trip time (including its own switching time) of the downstream breaker.

Related

• Circuit-breaker (MCCB) versus miniature circuit-breaker (MCB)
• Distribution boards & selectivity — §536
• RCD selectivity — type S cascading