Selectivity
Discrimination
Arranging protective devices so only the one nearest a fault operates and everything else stays on. Older books and plenty of people on site call it discrimination. It can be total, or partial up to a stated , so check the makers' curves or selectivity tables rather than just comparing ratings.
Using Selectivity in context
Reviewed
A distinction worth remembering
Two circuit breakers with the same nominal current can respond differently to short high-current events. A different curve can affect the fault-current conditions needed for rapid disconnection. Changing the curve is a design decision, not a casual cure for tripping.
Questions to take further
- How can I revise Selectivity beyond the definition?
- Explain what Selectivity describes, compare it with a related term and use it in a topic question. State any assumptions and what evidence you would need before applying the idea to real work.
Sources
- IET: RCD selection, types and testing — Residual-current waveforms, device selection and the distinction from overcurrent protection.
- IET: earthing and bonding FAQs — Protective bonding, earthing arrangements and loop impedance.
Where Selectivity fits in the learning path
The Design Calculations section covers these units:
- Sizing a Cable, Every Factor
- Volt Drop and Zs in Depth
- Fault Current, the Adiabatic Check and Device Choice
- Submains, Three-Phase Boards and Demand
Curriculum mapping: C&G 2365 Unit 305.
See this section in the full curriculum
This section is on the Plus path. Lesson availability depends on content review and your learning path.
Read, practise, return
1. Read it in context
- MCB Types B, C and DInstallation
- RCD TypesInstallation
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
Follow a topic above to explore how the term is used, then use the learning entry below to find your path.
3. Return to the meaning
After reading or practising, explain the idea in your own words, then check what you remembered.
Recall a related term
Which term fits this definition? Say it to yourself, then reveal it. This is a memory prompt, with no score or saved result.
Any current above the value a circuit is designed to carry. It comes in two flavours with different time scales: overload, where a healthy circuit is simply asked for too much for too long, and fault current, where a fault such as a short circuit can drive a far larger current that has to be cut off quickly. Protective devices are chosen to handle both.
Overcurrent
Read the Overcurrent entryCompare related terms
Open a meaning to compare its role with Selectivity, or follow its full entry.
Overcurrent
Any current above the value a circuit is designed to carry. It comes in two flavours with different time scales: overload, where a healthy circuit is simply asked for too much for too long, and fault current, where a fault such as a short circuit can drive a far larger current that has to be cut off quickly. Protective devices are chosen to handle both.
Read the Overcurrent entryMCB — Miniature Circuit Breaker
An automatic switch that turns a circuit off when too much current flows, whether a slow overload or a sudden short circuit. It has replaced the rewirable fuse in modern boards. Each one is chosen by its current rating, such as 6 A for lights or 32 A for sockets, and by its type: B for most home circuits, C or D where equipment draws a big surge as it starts.
Read the MCB entryRCD — Residual Current Device
A protective device that detects an imbalance between the currents in the live conductors passing through it. In a single-phase circuit this usually means comparing line and neutral current. It disconnects when the residual current meets its operating conditions. A 30 mA RCD can provide additional shock protection, but it does not prevent every kind of electric shock or replace safe isolation.
Read the RCD entrySubmain
A cable feeding one distribution board from another, such as a run from the main board to a board in an outbuilding. Submain is the everyday word: BS 7671's Part 2 calls it a distribution circuit. Because it carries a whole board's load, it's sized for that board's demand, and its protective device has to work with the devices below it.
Read the Submain entryA definition is a start. Using it in a real question is where it sticks.