Protection selectivity exercises
Classic coordination mistakes: circuit MCB > main MCB, RCD with the wrong sensitivity, RCBO with no neutral connected. You simulate a fault and watch which protective device trips — and why it’s wrong.
Fault simulator — which protective device trips?
Set up the cascade, choose the fault and watch an animation of which device trips
Main (upstream)
Circuit (downstream)
Set up the protective devices and press “Simulate the fault” to see which one trips.
Thermal-magnetic circuit breaker (MCB) calculator
Operating current → standard In, thermal range, magnetic threshold (curve B/C/D)
0.85 typical residential, 1.0 purely resistive
C: 5–10×In — sockets, appliances
Operating current Ib
15.2 A
Circuit breaker In
16 A · C
Thermal range
18–23 A
Magnetic threshold
80–160 A
Ib = P / (230 × 1) = 15.22 A
Standard In ≥ Ib: first rating ≥ 15.22 A → 16 A
Curve C: magnetic threshold = 5–10 × 16 = 80–160 A
The thermal range 1.13–1.45×In is the conventional overload tripping (IEC 60898-1); the magnetic release is instantaneous, in the band above.
RCD selectivity calculator (cascaded RCDs)
Compare downstream and upstream RCD → IΔn ratio, steps, V29 verdict
Downstream (on the circuit)
General type (instantaneous)
Upstream (main)
IΔn ratio
10.0×
IEC steps
2
✓ Selective
Selective: upstream > downstream, ratio ≥ 3 and ≥ 2 steps on the IEC scale — a fault on the circuit trips only the downstream RCD. (Time-delayed type “S” upstream = recommended for time selectivity.)
Standard IΔn scale: 10 · 30 · 100 · 300 · 500 · 1000 mA. Downstream is usually 30 mA (sockets, bathrooms — Art. 4.1.5.2.1).
Short-circuit & breaking capacity calculator
Prospective Icc (Ik) from the loop impedance Zs vs the device’s Icn (V54)
Sum of impedances: source + cables up to the fault point
Typical residential MCB: 6 kA
Prospective Icc
575 A
Icc
0.57 kA
✓ Icn ≥ Icc
Ik = c × Un / (√3 × Zs) three-phase · Ik = c × U0 / Zs single-phase · c = 1.0
Ik = 230 / (0.4) = 575 A
Icc decreases with distance (Zs increases). Art. 4.3.5.1: Icn ≥ Icc; if Icn < Icc it is only allowed with a suitable upstream device (backup/cascading). Here c = 1.0 (maximum Icc at the origin, for the breaking capacity); the app’s validator (rule V44) estimates the MINIMUM Icc at the end of the line with the factor 0.8 (EN 60909): Icc_min = 0.8 × 230 / Zs, with Zs = 0.35 Ω (source) + Σ(2·ρ·L/S).
Loop impedance (Zs) & minimum Icc calculator
Zs from the cable → minimum Icc at the end of the line (validator formula V44)
network + transformer; typical residential ≈ 0.35 Ω
the one-way length of the circuit (×2 = phase + neutral loop)
Cable loop resistance
0.432 Ω
Total impedance Zs
0.782 Ω
Minimum Icc at the end
235 A
Zs = Z_source + 2 × ρ × L / S · Icc_min = 0.8 × 230 / Zs
Zs = 0.35 + 2 × 0.027 × 20 / 2.5 = 0.782 Ω → Icc_min = 235 A
ρ = 0.027 (Cu) / 0.043 (Al) Ω·mm²/m at 70 °C; the factor 0.8 = EN 60909 (minimum short-circuit current). The minimum Icc must exceed the magnetic threshold of the MCB so that it trips instantaneously — exactly the check of rule V44.
Calculation assumptions and scope
- B/C/D thresholds per IEC 60898-1 (B = 3–5×In, C = 5–10×In, D = 10–20×In); conventional thermal range 1.13–1.45×In
- RCD selectivity (V29): upstream > downstream, IΔn ratio ≥ 3:1 and at least 2 steps on the 10/30/100/300/500/1000 mA scale; a time-delayed type “S” device upstream is recommended for time selectivity, but does NOT count toward the V29 verdict (Art. 4.1.5.2.7, 4.1.5.2.8)
- Short-circuit current Ik = c·Un/(√3·Zs) three-phase / c·U0/Zs single-phase, with voltage factor c = 1.0 (estimate; the exact value is determined per NTE 006/06/00, Art. 3.1.4.1)
- The breaking capacity is checked as Icn ≥ prospective Icc (Art. 4.3.5.1, V54); for real projects validate with a full short-circuit calculation
Protection selectivity — step by step
From the two releases of a circuit breaker to MCB/RCD coordination and breaking capacity.
A. Two releases in one circuit breaker
The thermal release — overload
A bimetallic strip heats up proportionally to the current and bends, opening the contact. It acts slowly, on small, sustained overloads. Conventional tripping range: 1.13–1.45 × In (IEC 60898-1).
The magnetic release — short circuit
An electromagnet opens the contact instantaneously when the current exceeds a threshold (curve B/C/D). It protects against short circuits, where the current rises sharply by tens of times.
| Aspect | Thermal (overload) | Magnetic (short circuit) |
|---|---|---|
| Cause | Overload — too high a consumption, sustained | Short circuit — fault, huge sudden current |
| Element | Bimetal (heats up and bends) | Electromagnet (coil) |
| Speed | Slow (seconds–minutes) | Instantaneous (< 10 ms) |
| Threshold | 1.13–1.45 × In | B 3–5 · C 5–10 · D 10–20 × In |
| Example | Too many appliances on the same circuit | The phase touches the neutral or PE |
Art. 4.3.1.2 — I7-2011
“... devices that protect against both overload currents and short-circuit currents (circuit breakers fitted with overcurrent protection relays and with fast releases for short circuits ...).”
B. The B / C / D curves (IEC 60898-1)
| Curve | Magnetic threshold | Typical use |
|---|---|---|
| B | 3–5×In | LED lighting, long circuits (low Icc at the far end) |
| C | 5–10×In | Sockets, appliances (residential standard) |
| D | 10–20×In | Motors, transformers (high inrush currents) |
Time–current characteristic (schematic)
The sloping part = thermal tripping (the higher the current, the shorter the time); the vertical drop = instantaneous magnetic tripping at the curve threshold. A “higher” curve (D) has its magnetic threshold further to the right → it tolerates inrush peaks.
The magnetic threshold = the instantaneous tripping threshold, as a multiple of the rated current In. A “higher” curve (D) tolerates inrush peaks without tripping.
C. The protection sizing condition
Overload protection
Ib ≤ In ≤ Iz
Operating current Ib ≤ rated current of the protection In ≤ permissible current of the cable Iz. The protection must “fit” between the load and the cable.
The conventional tripping current
I2 ≤ 1.45 × Iz
I2 (the current that surely trips the protection within the conventional time) must not exceed 1.45 × Iz, so as not to damage the cable insulation.
Art. 4.3.2.1.3 — I7-2011
“Ic ≤ IN ≤ Iadm” — the design current ≤ the rated current of the device ≤ the permissible current in the conductor.
D. Selectivity between cascaded circuit breakers
The magnetic bands must not overlap
On a short circuit on one circuit, only the downstream circuit breaker must trip, not the main one too. The condition: the minimum magnetic threshold of the upstream protection > the maximum magnetic threshold of the downstream one.
A difference of at least two steps
In practice, there must be at least two standard steps between the rated currents of consecutive protections (e.g. 16 A downstream → 40 A upstream). If the Icc at the circuit exceeds the upstream instantaneous threshold, both trip — loss of selectivity.
Art. 4.3.7.1 — I7-2011
“In cases where several protective devices are placed in series within a distribution, their characteristics are chosen so as to ensure protection selectivity. In the event of a fault, the protection nearest to it must operate, isolating only that section, without taking the whole installation out of service ...”
E. Selectivity between RCDs
General type vs type “S”
A general type RCD trips with no time delay (40–300 ms); a type “S” one is time-delayed (150–500 ms). For selectivity, the upstream is set to type “S” (time-delayed), the downstream to general type (instantaneous).
A ratio of at least 3:1
IΔn_upstream / IΔn_downstream ≥ 3
The upstream rated residual current must be ≥ 3 × the downstream one (e.g. 300 mA upstream, 30 mA downstream → ratio 10). This way a small fault trips only the downstream RCD.
Art. 4.1.5.2.8 — I7-2011
“... selectivity between a type “S” device and a general-type one in series may be considered as achieved if the ratio between their respective rated residual operating currents is at least 3.”
F. Short-circuit current & breaking capacity
The prospective short-circuit current (Icc)
Ik = c × Un / (√3 × Zs) · Ik_mono = c × U0 / Zs
Zs = the fault loop impedance (source + cables). The farther the fault point, the greater Zs and the smaller the Icc. At the end of the line, the minimum Icc must still exceed the magnetic threshold of the MCB, otherwise tripping is no longer instantaneous.
Breaking capacity: Icn ≥ Icc
The protective device must be able to interrupt the maximum short-circuit current at its point (Icn ≥ Icc). In residential, the Icc at the main board is usually 3–6 kA → an MCB with Icn ≥ 6 kA.
Art. 4.3.5.1 — I7-2011
“The breaking capacity must be at least equal to that of the prospective short-circuit current at the place of installation ... a lower breaking capacity is allowed if another protective device having the required breaking capacity is installed upstream ...”
Exercises — pick the correct option
You go through each step by choosing the correct answer. Wrong? Try again.
Thermal-magnetic — socket circuit
A socket circuit is protected by a 16 A circuit breaker, curve C.
Step 1: What does the thermal release of the circuit breaker do?
Selectivity — two cascaded circuit breakers
Main: 40 A curve C (magnetic threshold 200–400 A). Circuit: 16 A curve C (magnetic threshold 80–160 A).
Step 1: Do the magnetic bands of the two circuit breakers overlap?
Selectivity — cascaded RCDs
Downstream: 30 mA general-type RCD. Upstream: 300 mA type “S” RCD (time-delayed).
Step 1: What is the upstream / downstream IΔn ratio?
What you put on the main — 3 circuits
A board with 3 circuits, each with a 30 mA RCD: sockets 16 A/C, lighting 10 A/B, bathroom 16 A/C. You choose the main (upstream) protection.
Step 1: Which main RCD do you fit so that it is selective with the 30 mA ones downstream?
Exercises — you calculate each step
Now you enter the value for each step yourself. You get hints if you go wrong.
Circuit breaker — choose In and the magnetic threshold
4400 W consumer (single-phase, cos φ = 1), protected by an MCB curve C.
Step 1: Operating current Ib = P / (230 × cos φ) = ?
RCD selectivity — the ratio
Downstream 30 mA RCD and upstream 300 mA RCD.
Step 1: Upstream / downstream IΔn ratio = ?
Short circuit — Icc decreases with distance
Single-phase circuit (U0 = 230 V). At the board Zs = 0.1 Ω; at the end of the line Zs = 0.46 Ω.
Step 1: Icc at the board: Ik = U0 / Zs = ?
Minimum In on the main MCB
Downstream circuit 16 A curve C; the main is also curve C. Maximum downstream magnetic threshold = 10 × 16 = 160 A.
Step 1: Minimum In of the main (5 × In > 160), chosen from the standard series = ?
Exercises — direct answer
You solve it on your own and give the final answer. The step-by-step solution is available if needed.
Magnetic threshold — curve B
Circuit breaker In = 25 A, curve B (3–5×In). What is the MINIMUM magnetic threshold (A)?
Steps on the IEC scale
Downstream RCD 30 mA, upstream RCD 300 mA. How many steps are there between them on the standard IΔn scale?
Short circuit & breaking capacity
Single-phase circuit, U0 = 230 V, Zs = 0.115 Ω. What is the prospective Icc (A)?
Discussion
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