Q: What ratio is needed for fuse discrimination?

A ratio of about 2:1 is the rule of thumb — upstream fuse rating ≥ 2× the downstream fuse (or downstream MCB) for BS 88 fuses. This ensures the upstream fuse's pre-arc energy exceeds the downstream device's total let-through, so the downstream device clears first. Always confirm with manufacturer I²t curves for critical work.

Question 1

What is discrimination (selectivity)?

Accepted Answer

Discrimination — also called selectivity — means that for a fault, only the protective device nearest the fault operates, leaving the rest of the installation energised (BS 7671 Reg 536.4). Without it, an upstream device can trip and take out healthy circuits as well.

Question 2

How do you achieve discrimination between two MCBs?

Accepted Answer

Same-type MCBs need an upstream-to-downstream rating ratio of at least 1.6:1 for thermal (overload) discrimination. However, above the downstream device's instantaneous trip threshold (3–5× In for Type B), both MCBs can trip together — so MCB/MCB discrimination is only assured up to that current. A fuse or higher-type MCB upstream improves it.

Question 3

What ratio is needed for fuse discrimination?

Accepted Answer

A ratio of about 2:1 is the rule of thumb — upstream fuse rating ≥ 2× the downstream fuse (or downstream MCB) for BS 88 fuses. This ensures the upstream fuse's pre-arc energy exceeds the downstream device's total let-through, so the downstream device clears first. Always confirm with manufacturer I²t curves for critical work.

Question 4

Why is a fuse better than an MCB for upstream discrimination?

Accepted Answer

Fuses have a smoothly inverse time/current characteristic, so a higher-rated fuse upstream coordinates well with a downstream MCB across the whole fault range. MCBs have a fixed instantaneous trip band, so two MCBs both enter instantaneous tripping at high fault currents and lose discrimination.

Discrimination / Selectivity Checker