Measuring Zs on Site — What the BS 7671 Calc Means in Practice
The designer hands you a Zs limit. Your loop tester gives you a reading. The 80% rule says one is alive and the other is calculated for a hot conductor. Here's how to take the measurement properly, what to do when your reading sits between 80% and 100% of the limit, and why the inspector marks you down for the very common mistake of measuring at the wrong terminal.
Marcus Webb
NICEIC Approved + 18th Edition + 2391 · 22 yrs · Senior Approved Electrician + Inspector, Independent (ex-Atkins-Réalis M&E)
The design has its first principles — the physics. So does delivery. These are the three a good practitioner carries onto every job; get them in your head and the rest becomes easy. The read from Marcus Webb on this topic:
the basics that make everything else easy
Disconnection time is the whole game. Every test — continuity, insulation, Zs — exists to prove the circuit trips fast enough to not kill someone. If you don't feel that in your gut, you'll cut the corner that matters.
the order of operations — what comes before what
Dead tests before live, always: continuity, insulation resistance, polarity, earth electrode, then Zs live — in that order. People jump straight to the live loop test because it's quick; do it out of order and your certificate is worthless.
what to look ahead for — anticipate, don't react
Know your max Zs for every circuit before you lift a probe — write the table value next to each. Then a borderline reading at the socket isn't a debate on the day, it's a number you already expected. Spot the long runs and high-Zs circuits before you test them.
On site — the practice points
- 1
Measure at the furthest point
Zs at the DB is irrelevant. Take it at the last accessible socket on each circuit. That's where the fault current matters for disconnection time.
- 2
Apply the 80% rule yourself
Your meter reads ambient. BS 7671 Table 41.x is the hot-conductor max. Your reading must be ≤ 80% of the table value. If it's 81%, the circuit fails — even though it 'passes' your meter.
- 3
Test before energisation, not after
Continuity + insulation + earth electrode resistance + Zs. In that order. Before energising. Otherwise you're commissioning blind and the certificate is worth nothing.
Site walkthrough video
Measuring Zs on Site — What the BS 7671 Calc Means in Practice
Mock — live video activates when content arrives
The single thing new starters struggle with most: turning a drawing into work on site. This is how Marcus Webb reads the schedule of circuits and the BS 7671 design — max Zs per circuit:
On the drawing
Max Zs = 1.09 Ω (Type B, 32 A)
What it means on site
that's the cold-conductor maximum — your live reading has to come in at or under 80% of it (0.87 Ω) to allow for the conductor running hot. 0.95 Ω passes your meter and fails the standard.
On the drawing
measured at the DB
What it means on site
irrelevant on its own — take it at the furthest socket on the circuit, because that's where the disconnection time is worst.
On the drawing
RCD shown on the circuit
What it means on site
the designer may be relying on the RCD for disconnection where Zs alone won't trip the breaker in time — so test the RCD, don't just read Zs and move on.
This is the delivery half.
Same topic, two credential chains. The chartered engineer sets the intent; here's how it's made real on site.
Delivery knowledge is verified by a Master-Trade or chartered validator and passes the AI pre-publication check. Codes are the floor; judgement is the value.