Use the actual device limit
MCB/RCBO type, rating and manufacturer data matter. Enter the limit you are working to, then decide whether to apply 80% headroom for site conditions.
Electrical calculator · max Zs / MCB / RCBO / BS 7671
Max Zs Checker for MCBs and RCBOs
Check a measured Zs value against an entered device limit, with optional 80% rule headroom and clear pass/check wording.
For reviewing Zs results on MCB/RCBO circuits before recording or investigating a circuit.
Field notes
Field notes for Max Zs checker
Practical checks to run before this calculator result turns into a site decision.
Amber means investigate, not ignore
Borderline Zs can point to lead length, poor terminations, wrong device assumptions or changing temperature. Treat the warning as a prompt to verify.
Record what was compared
A clean note should state measured Zs, limit used, whether 80% headroom was applied and whether further testing is needed.
FAQ
Max Zs checker FAQ
Short answers written for UK temporary electrical and HVAC planning work.
What is the Max Zs checker used for?
Check a measured Zs value against an entered device limit, with optional 80% rule headroom and clear pass/check wording. It is mainly for UK electrical review work, especially where a quick pre-check is needed before selecting equipment or changing a temporary setup.
Can this replace BS 7671 design, inspection or testing?
No. It is a competent-person planning aid only. Final decisions still need current BS 7671 requirements, manufacturer data, inspection, testing, risk assessment and the actual site conditions.
What should I verify before acting on the result?
Check current BS 7671 values, manufacturer device data, measured results, earthing arrangement, correction factors and site installation conditions. If any assumption is uncertain, use the result as a prompt to investigate rather than as permission to energise.
What does an amber or red result usually mean?
It normally means the margin is weak, an assumption is missing, or the load should be split, staged, moved closer to the supply, reduced or reviewed by a competent electrician before use.
Next tools in this workflow
See super flows
Calculator · BS 7671 / voltage drop
HVAC voltage drop envelope
Estimate voltage drop over long leads for heaters, air movers, dehumidifiers and portable AC, with compressor start warnings.
Calculator · PFC / PSCC
PFC / fault current
Estimate PSCC or PEFC from Ze and voltage, compare against device breaking capacity, and draft a quick test-sheet note.
Calculator · CPC / adiabatic
CPC adiabatic
Use the adiabatic equation shape to compare fault current, disconnection time, k factor and CPC size as a planning aid.
Calculator · cable correction factor / Iz
Cable correction factors
Stack ambient temperature, grouping, insulation and installation correction factors to see whether corrected Iz still covers load current.