Discrimination Between Upstream and Downstream Protection

The 63 A type C MCB magnetic threshold is 5-10x In = 315-630 A. At a 4.2 kA fault the MCB operates instantaneously, clearing in well under 0.1 s.\n\nThe 150 A MCCB instantaneous setting is 1500 A nominal, with +/-20% tolerance, so it may pick up as low as 1200 A. A 4.2 kA fault is far above that, so the MCCB also sees the fault as instantaneous.\n\nBoth devices operate in their instantaneous bands with no intentional time delay between them, so the upstream device trips on the downstream fault. This is a genuine discrimination failure: the fault current exceeds the upstream instantaneous threshold and there is no time or current margin. The fact that the fault current (4.2 kA) is above the MCCB's minimum instantaneous pickup (1200 A) is the root cause, not a device defect.

Fix 1 - raise the MCCB instantaneous setting or add a short-time delay. If the MCCB has an adjustable short-time band, set it to a delay of 0.1-0.2 s at a pickup above 4.2 kA so the downstream MCB clears first. This preserves the MCCB's fault-clearing capability while creating a time margin. Check that the MCCB still clears a main-board fault (9.5 kA) within the required disconnection time and that the cable's withstand (k^2 t) is not exceeded during the delay.\n\nFix 2 - replace the 63 A MCB with a current-limiting device or a downstream MCCB with a lower instantaneous threshold and a higher breaking capacity, and/or increase the cable size to the lighting panel to raise the fault current so it is more decisively in the MCB's instantaneous band while remaining below the upstream setting. This is more expensive and requires re-cabling.\n\nComparison: Fix 1 is cheaper and quicker but relies on the MCCB having an adjustable short-time element and on the cable withstand being adequate for the added delay. Fix 2 is more robust but disruptive. In either case, verify the arrangement by calculation against the actual device curves, not by rule of thumb, and confirm the MCB's breaking capacity exceeds 4.2 kA.

(1) The actual manufacturer's time-current curves for both devices, including tolerances, not just nominal settings. (2) The MCCB's short-time and instantaneous adjustability and its breaking capacity versus 9.5 kA. (3) The cable size, length and insulation type to the lighting panel, to check k^2 t withstand for any added time delay. (4) Whether the 4.2 kA figure is the minimum or maximum prospective fault current - discrimination must hold at the minimum, and the maximum must not exceed breaking capacities. (5) Whether the MCCB is the sole upstream device or whether a further device exists, since discrimination is a chain property. (6) Confirmation that the 63 A MCB's breaking capacity (often 6 kA for MCBs) exceeds 4.2 kA with margin. Sign-off without the curves and the minimum fault current would be unsafe.