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, type, verdict as in Figs. 4.1/4.2 (V29)
Downstream (on the circuit)
General type (instantaneous)
Upstream (main)
IΔn ratio
10.0×
IEC steps
2
✓ Selective
Selective: a time-delayed type “S” upstream over a general-type downstream, with ratio ≥ 3 — as in Figs. 4.1/4.2; a fault on the circuit trips only the downstream RCD.
Scale of rated residual currents IΔn (Art. 4.1.5.2.4): 6 · 10 · 30 · 100 · 300 · 500 mA and 1 A; the calculator uses the common steps 10–1000 mA. Downstream is usually 30 mA (socket outlets and current-using equipment ≤ 32 A, lighting circuits in single-family dwellings — Art. 4.1.5.2.1 as amended in 2023; bathrooms — Art. 7.1.3.5).
Short-circuit & breaking capacity calculator
Prospective Icc (Ik) from the short-circuit impedance vs the device’s Icn (V54)
Sum of the impedances along the phase-neutral loop: source + cables (out and back) up to the fault point. They add as vectors, not by adding the magnitudes: Z = √((ΣR)² + (ΣX)²), resistances with resistances and reactances with reactances — the source is mainly inductive, a small-section cable mainly resistive.
Typical residential MCB: 6 kA
Prospective Icc
575 A
Icc
0.58 kA
✓ Icn ≥ Icc
Ik3 = c × Un / (√3 × Zk), Zk per phase · Ik1 = c × U0 / Zs, Zs the phase-neutral loop · c = 1.0
Ik = 230 / (0.4) = 575 A
Icc decreases with distance (the impedance 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, that is Icc at nominal voltage — not the covering value: for the breaking capacity you compute with c_max (1.1 on networks with ±10 %), so the real Icc can be about 10 % higher. The app’s validator (rule V44) estimates the MINIMUM Icc at the end of the line with the 0.8 factor of the conventional method: 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; 0.35 Ω is deliberately chosen high, on the safe side for the MINIMUM Icc (long service line, end of line). Do not use it for the breaking capacity, where the maximum Icc matters: at 3–6 kA, which this page gives as typical at the main board, the corresponding impedance is of the order of 0.04–0.08 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: design values for the minimum Icc, about 1.5 × ρ at 20 °C (the conductor heated towards the end of the fault); the 0.8 factor = the conventional method (the voltage at the origin of the faulty circuit is taken as 0.8·U0). The minimum Icc must exceed the UPPER limit of the magnetic band — 10 × In on curve C — the only one at which instantaneous tripping is guaranteed (the condition Zs × Ia ≤ U0, Art. 4.1.4.1.10); the validator uses the same formula in rule V44. The two assumptions are combined on purpose: in the pure conventional method the 0.8 factor stands in for the upstream impedance (Z_source = 0), while the impedance method starts from the real source impedance. Here both are used, so the minimum Icc comes out lower than under either method on its own — on the safe side for checking tripping, but not comparable with tabulated values.
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: upstream > downstream, IΔn ratio ≥ 3 (the condition from the 2011 wording of Art. 4.1.5.2.8, removed from the text in 2023) and pairs as in Figs. 4.1/4.2, to which the article in force refers: type S 500 mA or general type 300 mA upstream over general type 100 mA and 30 mA (Fig. 4.1), definite time delay 1 A → type S 300 mA → general type 30 mA (Fig. 4.2); the validator (V29) gives an error for upstream ≤ downstream, a warning below 3 and information for pairs outside the figures. Tripping times: general type 40…300 ms, type S 150…500 ms, depending on the fault current compared with IΔn (Art. 4.1.5.2.7)
- Short-circuit current Ik3 = c·Un/(√3·Zk), with Zk the per-phase impedance, and Ik1 = c·U0/Zs, with Zs the phase-neutral loop, 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 (≤ 0.1 s) — above the upper end of the band |
| 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 = the magnetic tripping threshold. The thermal branch is practically the same for B, C and D — in the drawing the curves are slightly offset only so they can be told apart. What differs between the types is the magnetic band: selectivity is judged by its lower limit, guaranteed instantaneous tripping by its upper limit.
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
For circuit breakers the practical criterion is the separation of the magnetic bands: for C over C, 5 × In_upstream > 10 × In_downstream, that is In_upstream > 2 × In_downstream (16 A downstream → 40 A upstream). Even then selectivity holds only below the minimum magnetic threshold of the upstream device — above it both trip. The two-step rule of Art. 4.3.7.1 is written for fuses, not for circuit breakers.
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 time selectivity, the upstream is set to type “S” (time-delayed), the downstream to general type (instantaneous). Fig. 4.1, however, also shows a general-type 300 mA RCD over general-type RCDs of 100 mA and 30 mA.
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 (original text, replaced in 2023)
“... 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)
Ik3 = c × Un / (√3 × Zk) · Ik1 = c × U0 / Zs
Zk = the short-circuit impedance per phase (source + the line conductor), for the three-phase short circuit; Zs = the phase-neutral loop, for the single-phase one — two different quantities, not the same figure. The farther the fault point, the greater the impedance and the smaller the Icc. At the end of the line, the minimum Icc must exceed the UPPER limit of the magnetic band (Ia = 5 × In on curve B, 10 × In on C, 20 × In on D), the only one at which instantaneous tripping is guaranteed — the condition Zs × Ia ≤ U0 of Art. 4.1.4.1.10.
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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