What a proper assessment needs
Before this topic can be assessed for a real installation, the person responsible for the design or verification needs to establish the relevant facts:
- Exact protective device standard, type and rating
- Earthing arrangement and required disconnection time
- Current manufacturer time-current information
- Design versus measured impedance basis
- Conductor temperature and the measurement method
Factors to consider
A number copied from a table for one device cannot safely be applied to a different fuse or breaker. Design values and values used for comparison with measurements need their respective temperature assumptions. A result below one number is not a complete installation assessment.
Keep the evidence with the decision
Record the equipment, installation conditions and the source of the data used. Where a manufacturer supplies product-specific instructions or coordination data, the exact product matters. A generic example should not silently replace that information.
For UK installation work, use the applicable edition of BS 7671 together with relevant guidance and manufacturer instructions. The IET’s publication hub is the starting point for current editions and supporting material. This brief does not reproduce the standard or establish compliance with it.
IET · BS 7671 and supporting guidance ↗Related calculations
Our arithmetic tools show their scope, inputs and formulas explicitly. They can help you understand a relationship between quantities. They cannot replace the installation assessment described above.