Adiabatic equation
The calculation that checks a conductor can carry fault without overheating. For a protective conductor it's written S = √(I²t) ÷ k, which gives the minimum cross-sectional area. It treats all the fault's heat as staying in the conductor, and BS 7671 applies it to disconnection times up to 5 s. Take t from the device's curve at the actual fault current, not from the maximum disconnection time, and below about 0.1 s use the maker's let-through energy in place of I²t.
Using Adiabatic equation in context
Reviewed
A distinction worth remembering
A useful revision comparison is a short cable in open air against the same load supplied over a long route through insulation. Installation conditions, voltage drop and fault protection can change the selection. Learn the meaning of each input before treating a calculation as a design decision.
Questions to take further
- How can I revise Adiabatic equation beyond the definition?
- Explain what Adiabatic equation 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
- Open University: relationships between electrical quantities — The relationship between voltage, current and resistance and the limits of an ohmic model.
- IET: current BS 7671 edition and updates — BS 7671:2018+A4:2026 and the main subject areas updated by Amendment 4.
- IET: earthing and bonding FAQs — Protective bonding, earthing arrangements and loop impedance.
Where Adiabatic equation 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
- Cable SizingInstallation
- Prospective Fault CurrentTesting & Inspection
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
- Cable Selection — quiz previewPlus · Installation
- Inspection & Testing — quiz previewPlus · Testing & Inspection
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.
CPC stands for circuit protective conductor: the protective connection between a circuit’s exposed metal equipment parts and the main earthing terminal. It provides a path for earth-fault current so the protective arrangement can disconnect the supply.
Compare related terms
Open a meaning to compare its role with Adiabatic equation, or follow its full entry.
CPC — Circuit Protective Conductor
CPC stands for circuit protective conductor: the protective connection between a circuit’s exposed metal equipment parts and the main earthing terminal. It provides a path for earth-fault current so the protective arrangement can disconnect the supply.
Read the CPC entryk factor
The number in the adiabatic equation that allows for a conductor's material, its insulation and its starting and final temperatures. A higher k means the conductor can take more fault energy for its size. Protective conductor values come from the tables in BS 7671's Chapter 54, so read the table that matches the conductor rather than reusing a remembered figure.
Read the k factor entryPFC — Prospective Fault Current
The current that could flow at a point if a fault of negligible impedance occurred there. Assessment considers the relevant short-circuit and earth-fault conditions. Protective devices must have adequate breaking capacity, or a verified coordinated arrangement with upstream protection, for the fault current at their location.
Read the PFC entryShort circuit
A short circuit is an unintended connection of very low impedance between points that should have a voltage difference, such as line and neutral. It bypasses the intended load and can allow a large current to flow, limited by the supply and circuit impedance. It differs from an overload caused by too many loads. PSCC describes the current that could flow in such a fault, rather than a current flowing in normal use.
Read the Short circuit entryA definition is a start. Using it in a real question is where it sticks.