When a fault occurs in an electrical installation, the overcurrent protective device (fuse or circuit-breaker) must disconnect the supply fast enough to prevent danger. But how fast is fast enough? And how much fault current is needed to make a device operate that quickly?
The time/current characteristic curves answer these critical questions. They show us how long a given protective device takes to operate for any level of prospective fault current.
Regulation 411.3.2 specifies maximum disconnection times: 0.4 seconds for final circuits not exceeding 32 A, and 5 seconds for distribution circuits and final circuits exceeding 32 A (in TN systems).
The Core Safety Chain
The curves sit at the heart of a chain linking fault current, disconnection time, and earth fault loop impedance (Zs):
The safety verification chain — from loop impedance to disconnection time
In simple terms: lower Zs → higher fault current → faster disconnection. The curves let you verify that the device will operate quickly enough under the worst-case conditions at a given installation.
Key regulation: The maximum Zs values in Tables 41.3 and 41.6 of BS 7671 were derived directly from these curves. Understanding the curves means you understand where those Zs limits come from.
Anatomy of a Time/Current Curve
Every curve in Appendix 3 is a log-log graph — both axes use logarithmic scales. This is necessary because the values span enormous ranges: from 0.01 seconds to 10,000 seconds in time, and from 1 A to 10,000 A in current.
Anatomy of a log-log time/current curve — higher fault current = faster disconnection
How to Read the Logarithmic Scales
On a logarithmic scale, each major division represents a tenfold increase (a "decade"). The spacing between 1 and 10 is the same as between 10 and 100, or 100 and 1,000. Intermediate values are not evenly spaced — the gaps get smaller as you approach the next decade.
Logarithmic vs linear scales — each decade occupies equal space on a log scale
Key principle: Every curve slopes downward from left to right. This means the higher the fault current, the faster the device operates. At very low overcurrents (just above the rating), a fuse may take hours or even thousands of seconds. At high fault currents, it operates in fractions of a second.
The Protective Devices Covered
Appendix 3 provides curves for two families of overcurrent protective device: fuses and circuit-breakers.
Figure
Device
Standard
Typical Use
3A1
BS 88-3 fuse system C
BS 88-3
Consumer units (older type)
3A2(a/b)
Semi-enclosed (rewirable) fuses
BS 3036
Older domestic installations
3A3(a/b/c)
BS 88-2 fuse systems E & G
BS 88-2
Industrial / commercial distribution
3A4
Type B MCBs / RCBOs
BS EN 60898 / 61009-1
Resistive loads (lighting, socket outlets)
3A5
Type C MCBs / RCBOs
BS EN 60898 / 61009-1
Small inductive loads (small motors, fluorescent)
3A6
Type D MCBs / RCBOs
BS EN 60898 / 61009-1
High inrush loads (transformers, X-ray, welding)
Fuses vs MCBs — Different Curve Shapes
Fuse curves are smooth, continuous lines — as current increases, the fuse element melts progressively faster. MCB curves have two distinct regions: a thermal (overload) region that behaves like a curve, and a magnetic (instantaneous) trip region where the MCB snaps open almost instantly above a threshold.
Fuses have a smooth inverse curve; MCBs have a thermal region that drops into instantaneous magnetic tripping
MCB Instantaneous Trip Ranges
The defining difference between Type B, C, and D MCBs is the current at which the magnetic trip engages:
MCB Type
Magnetic Trip Range
Example: 32 A MCB
Type B
3 to 5 × In
96 A to 160 A
Type C
5 to 10 × In
160 A to 320 A
Type D
10 to 20 × In
320 A to 640 A
Why does this matter? A Type D MCB needs much higher fault current to trip instantly — so it requires a much lower Zs (more conductive fault path) to achieve the same disconnection time. This is why Type B MCBs are preferred for most domestic and commercial circuits.
How to Read the Curves — Step by Step
Finding Disconnection Time from Fault Current
This is the most common use: you know (or have measured) the prospective fault current at a point in a circuit, and you need to check the device will disconnect quickly enough.
Reading disconnection time from fault current — three-step process on the graph
Finding Required Fault Current (Ia) from Disconnection Time
The reverse process: you need to know what minimum fault current (Ia) is required to achieve disconnection within the regulatory time limit. This is the value used to calculate maximum Zs.
This is exactly how Table 41.3 values were calculated. The IET looked up Ia for each device rating at 0.1 s (and 5 s), then used the formula Zs = (U₀ × Cmin) / Ia to produce the tabulated maximum Zs values.
Zs = (U₀ × Cmin) / Ia
Zs = maximum earth fault loop impedance (Ω) U₀ = nominal line voltage to Earth (typically 230 V) Cmin = minimum voltage factor (0.95 for UK DNO supplies) Ia = current causing operation within the required time (from the curves)
Interactive Curve Explorer
Use this tool to look up Ia values and calculate maximum Zs for common protective devices. Select a device type, rating, and required disconnection time.
Select a device and press Calculate to see results.
Worked Examples
📝 Example 1 — Verifying Zs for a 32 A Type B MCB
A 32 A Type B MCB protects a final ring circuit in a domestic installation. The measured Zs is 1.05 Ω. Does this circuit comply with the 0.4 s disconnection requirement?
1Identify Ia: From Figure 3A4 (or the table), a 32 A Type B MCB requires 160 A to operate between 0.1 s and 5 s.
2Calculate maximum Zs:
Zs(max) = (U₀ × Cmin) / Ia = (230 × 0.95) / 160 = 1.37 Ω
3Apply the 0.8 correction: When comparing a measured value (at ambient temperature), multiply the tabulated max by 0.8:
Zs(measured max) = 0.8 × 1.37 = 1.09 Ω
4Compare: Measured Zs = 1.05 Ω, which is ≤ 1.09 Ω → COMPLIANT ✔
📝 Example 2 — Using the Fuse Curve for a BS 88-2 63 A Fuse
A distribution circuit is protected by a 63 A BS 88-2 fuse. What is the maximum Zs for 5-second disconnection?
1Identify Ia from Figure 3A3: At 5 seconds, a 63 A BS 88-2 fuse requires 280 A.
Your measured Zs must be at or below 0.62 Ω for compliance.
📝 Example 3 — Why Type C Needs Lower Zs Than Type B
Compare the maximum Zs for a 20 A Type B MCB vs a 20 A Type C MCB, both at 0.4 s disconnection.
1Type B (Fig 3A4): Ia for 20 A Type B = 100 A
Zs(max) = (230 × 0.95) / 100 = 2.19 Ω
2Type C (Fig 3A5): Ia for 20 A Type C = 200 A
Zs(max) = (230 × 0.95) / 200 = 1.09 Ω
3Conclusion: The Type C requires double the fault current and therefore half the maximum Zs. This is why Type B is preferred where inrush currents are not a problem — it allows longer cable runs with higher impedance while still meeting disconnection times.
Real-World Applications
1. Design Stage — Selecting Protective Devices
During design, you estimate the earth fault loop impedance using R1+R2 values and the external impedance Ze. You then check whether the chosen protective device will disconnect within the required time for the expected Zs. If not, you can select a different device type, a lower rating, or increase the cpc size to reduce impedance.
2. Inspection and Testing — Verifying Compliance
During initial verification or periodic inspection, the measured Zs at the furthest point of each circuit is compared against the maximum permissible value. This maximum comes directly from the time/current curves via the Zs formula. The 0.8 correction factor accounts for conductor temperature rise under load.
3. Discrimination (Selectivity)
When multiple protective devices are installed in series (e.g. a main fuse and a final circuit MCB), the curves help you verify that the device nearest to the fault operates first, while upstream devices remain closed. This is called discrimination — and it prevents unnecessary disconnection of healthy circuits.
Discrimination: the downstream device curve sits below (faster) than the upstream device at the fault current level
4. Adiabatic Equation — Cable Protection
The curves also feed into the adiabatic equation (Regulation 434.5.2) for verifying that cables can withstand fault current energy:
t = (k × S / I)2
t = maximum disconnection time the cable can withstand (s) k = factor from Table 43.1 (depends on conductor and insulation type) S = cross-sectional area of the conductor (mm²) I = prospective fault current (A)
You use the curve to check that the protective device disconnects before the cable's thermal limit is reached.
Key Concepts — Flip Cards
Tap each card to reveal the answer.
What does the X-axis represent on the curves?
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Prospective fault current in rms Amperes — plotted on a logarithmic scale from 1 A to 10,000 A.
What does the Y-axis represent?
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Operating time in seconds — also on a logarithmic scale from 0.01 s to 10,000 s.
Why are both axes logarithmic?
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Because the values span huge ranges — from hundredths of a second to thousands, and from a few amps to tens of thousands. A linear scale could not show this on one readable graph.
What magnetic trip range defines a Type B MCB?
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3 to 5 times the rated current (In). For example, a 32 A Type B trips magnetically between 96 A and 160 A.
What value of Cmin is used for UK DNO supplies?
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Cmin = 0.95 — this accounts for voltage variations due to transformer taps, time of day, and other factors.
What is the 0.8 correction factor for?
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It accounts for conductor temperature rise under load. Measurements are taken at ambient temperature, but under normal load the conductor is hotter and resistance is higher — so the measured Zs must be ≤ 0.8 × tabulated maximum.
What is "discrimination" in overcurrent protection?
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Discrimination (selectivity) means ensuring that the protective device closest to the fault operates first, so upstream devices stay closed and healthy circuits are not disrupted.
Which figure number is the Type B MCB curve?
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Figure 3A4 — Type B circuit-breakers to BS EN 60898 and overcurrent characteristics of RCBOs to BS EN 61009-1.
Test Your Knowledge
Try these questions to check your understanding of the time/current curves.
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Created for EAL Level 3 Electrical Installation students