Discrimination / Selectivity
Selectivity means that in a fault only the protective device closest to the fault opens, while the main protection at the upstream level stays closed. That way a fault in a single circuit does not stop the whole system. It is achieved by leaving sufficient current and time difference between levels.
In a non-selective system, a short circuit in a single lamp blows the main fuse and the entire machine stops. In a selective system only that lamp's 5 A fuse opens, and the control and driver keep working.
- Current difference: a clear ampere difference is left between the lower and upper level; values very close to each other spoil selectivity.
- Time characteristic: using a fast characteristic at the lower level and a delayed one at the upper level strengthens selectivity.
- Limit in short circuit: at very high short-circuit currents both levels may open at once; selectivity is generally guaranteed only up to a certain current level.
In caravan, construction machine and agricultural vehicle installations, plan this principle from the start: split the lighting, control, driver and valve circuits onto separate fuses. Otherwise a short to chassis in a single cable can immobilise the vehicle completely. Marking the levels on the diagram also speeds up fault finding in the field.
Context in which this term is used: Installation, redundancy and field continuity