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NEXUS

M&E DESIGN

⚡ Understanding Fault‑Loop Impedance (Zs) and Why It Must Be Measured

  • Writer: MTS DNC ENERGY CONSULTANTS LIMITED
    MTS DNC ENERGY CONSULTANTS LIMITED
  • Jan 3
  • 3 min read
Maximum permitted fault‑loop impedance (Zs) values for 230 V circuits up to 35 A, listing allowable Zs limits for different fuse types and MCB types (B, C, D).

When you install or upgrade a fuse, MCB, or RCBO, one of the most important safety checks is ensuring that the circuit’s fault‑loop impedance (Zs) is low enough for the protective device to disconnect quickly during a fault. This is a core requirement in I.S. 10101 and BS 7671, and it’s essential for preventing electric shock and fire.

To help with this, the wiring rules include a table that lists the maximum permitted Zs values for different fuses and circuit breakers at 230 V. These values ensure that each device will trip within the required 0.4 seconds for most final circuits up to 35 A.


🔍 What the Zs Table Actually Tells You

The table provides the maximum Zs allowed for each device rating and type. For example:

  • A 16 A Type B MCB must have Zs ≤ 2.87 Ω

  • A 16 A Type C MCB must have Zs ≤ 1.44 Ω

  • A 10 A fuse may allow Zs up to 4.5 Ω, depending on the fuse type

These values are based on the standard supply voltage:

U₀ = 230 V

(the nominal line‑to‑earth voltage used in Ireland and the UK)

The table has already done the maths for you — you simply compare your measured Zs to the maximum allowed. Extract from the table included below


Example Extract from Table 41.2 (for illustration)

This small extract shows how different protective devices have different maximum Zₛ limits at 230 V for a required disconnection time of 0.4 seconds.


Fuses (gG / General Purpose)

Rating (A)

6

10

16

20

25

32

Max Zₛ (Ω)

8.9

5.3

2.8

1.85

1.5

1.1

MCB Type B (I.S. EN 60898)

Rating (A)

6

10

16

20

25

32

Max Zₛ (Ω)

7.67

4.60

2.87

2.30

1.84

1.44

MCB Type C (I.S. EN 60898)

Rating (A)

6

10

16

20

25

32

Max Zₛ (Ω)

3.83

2.30

1.44

1.15

0.92

0.72

MCB Type D (I.S. EN 60898)

Rating (A)

6

10

16

20

25

32

Max Zₛ (Ω)

1.92

1.15

0.72

0.57

0.46

0.36


🛠️ Do You Really Need to Measure Zs?

Yes — absolutely. Measuring Zs is not optional; it’s a required part of:

  • New installations

  • Modifications

  • Consumer unit upgrades

  • Periodic inspections

  • Adding or changing protective devices

Why?Because real‑world conditions vary. Cable lengths, loose terminations, corrosion, and installation quality all affect Zs. A design calculation alone cannot guarantee safety — you must verify it on-site using a loop impedance tester.


⚡ Why Measuring Zs Matters

If Zs is too high:

  • Fault current may be too low

  • The breaker or fuse may not trip fast enough

  • Exposed metalwork can become dangerous

  • Fire risk increases

  • The installation becomes non‑compliant

Measuring Zs is one of the simplest and most important ways to confirm that your protective devices will actually protect.


🧠 Final Takeaway

When installing or modifying a circuit, always check the fault‑loop impedance. The wiring rules provide the maximum Zs values for each device, and your measured Zs must be equal to or lower than those limits.

It’s practical.It’s required.And it’s one of the most critical safety checks in any electrical installation.


📍 Disclaimer

The content shared in these posts is intended for informational purposes only and should not be interpreted as design advice, specifications, or a calculation template. For professional guidance or design services, please contact us through our contact form.


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