§536 — Distribution boards & selectivity between protective devices

Selectivity (also called discrimination) means that, in the event of a fault, only the protective device closest to the fault trips — the rest of the installation stays in operation. Without selectivity, a short circuit in a single circuit can sometimes take down the whole main group or even the entire building.

Note on status: the selectivity provisions in §536 are, in the Netherlands, informative, not mandatory. NEN 1010 does require (via the division into circuits) that an installation be built up so that a fault in one circuit does not unnecessarily drag down other circuits — selectivity is the practical implementation of that requirement, not a stand-alone statutory obligation.

Why split an installation into circuits?

Limits the consequences of a single fault to one part of the installation.
Allows maintenance on one circuit without shutting down the rest.
Simplifies fault-finding: a tripped circuit points directly to the problem area.

The 1.6:1 rule of thumb for fuses

For fuses according to NEN-EN-IEC 60269-1 of the same type (e.g. gG), with a rated current ≥ 16 A, the following applies: full selectivity between two fuses connected in series is guaranteed if the ratio between the rated currents is ≥ 1.6 : 1 (for example, a 25 A fuse upstream of a 16 A fuse).

For miniature circuit breakers (MCBs) this rule of thumb cannot simply be applied — circuit breakers of the same type/brand must be checked against the manufacturer's selectivity tables, especially for short-circuit currents. A larger current difference between two circuit breakers increases the likelihood of selectivity, but does not automatically guarantee it the way it does for fuses.

Types of selectivity

Type	How it works
Current selectivity	The upstream protective device has a higher trip current — works mainly for lower fault currents.
Time selectivity	The upstream protective device trips with a delay (see also selective type S RCD, §531).
Energy selectivity (I²t)	Based on the let-through energy of fuse links — the downstream fuse melts before the upstream device lets through energy that would cause damage.
Full vs. partial selectivity	Full = selective up to the maximum short-circuit current; partial = selective up to a certain current value, above which both devices trip.

Distribution board layout

Practical layout rules that are common in new-build dwellings:

At least 2 residual current devices of 30 mA, so that a leakage current does not de-energize the entire dwelling.
Maximum 4 final circuits per RCD — a rule of thumb to limit cumulative leakage current (§531: must stay below 30% of I∆n) and the "domino effect" in the event of a fault.
A main switch upstream of all circuits, directly accessible.
Circuit layout such that comparable loads (lighting separate from socket outlets, kitchen separate from the rest) don't let a fault unnecessarily radiate into unrelated rooms.

Common mistakes

Combining circuit breakers from different brands and assuming they are selective — selectivity tables are brand- and type-specific, and are never simply interchangeable.
Too few circuits — a dwelling with only 1 RCD for the entire installation has no selectivity whatsoever: any leakage current shuts everything down.
Confusing overload selectivity with short-circuit selectivity — two circuit breakers can be selective at low overload currents but not at a short circuit close to the source.
Applying the 1.6:1 rule to circuit breakers instead of only to fuses of the same type — this leads to false confidence.

§531 — Residual Current Devices (RCD): selective type-S RCD as a form of time selectivity.
MCB B/C/D characteristics: tripping characteristics that affect selectivity calculations.
Fuse links: I²t values for energy selectivity.