Protection, Earthing and RCDs
Fuses and breakers, trip curves, fault levels, earth-fault loop impedance, RCDs and selectivity.
What protection must do
A circuit needs three kinds of protection: against overload (a current a little above rating for a long time, which overheats the cable), against short circuit (a very large current between conductors, which must be cleared in milliseconds) and against earth fault (current to earth through an exposed metal part or a person, which must be cleared fast enough to be safe). Each device is chosen so that it clears the fault before the cable or the person is harmed, and so that only the device nearest to the fault operates (selectivity).
Protective devices
| Device | Operates on | Typical use |
|---|---|---|
| Fuse (gG, gL, aM, class J, RK5) | melting of an element: fast on short circuit, slow on overload | cheap, high breaking capacity (to 100 kA); gG general, aM for motors (no overload protection) |
| Miniature circuit breaker (MCB), types B, C, D | thermal (bimetal) for overload, magnetic for short circuit at 3 to 5, 5 to 10, or 10 to 20 × In | final circuits up to 125 A: B for resistive loads, C for mixed and small motors, D for transformers and heavy starts |
| Molded-case circuit breaker (MCCB) | thermal-magnetic or electronic with adjustable settings | feeders 100 to 1 600 A, with selective time delays |
| Motor protection circuit breaker (MPCB) | adjustable thermal band and a fixed magnetic trip at about 13 × In | one device for short circuit and overload of a motor |
| Overload relay | thermal or electronic, class 10, 20, 30 | with a contactor, behind a short-circuit device |
| RCD / RCBO | imbalance between line and neutral currents (leakage to earth) | 30 mA personal protection, 300 mA fire, with or without overcurrent |
| Surge protective device (SPD) | diverts voltage spikes | at the origin and at sensitive loads |
Trip curves
The manufacturer’s time-current curve shows how long a device takes to trip at each multiple of its rated current, as a band between the slowest and the fastest operation. Between about 1.13 and 1.45 times the rating the thermal element takes up to an hour; above 5 to 10 times (for type C) the magnetic element trips in a few milliseconds. The cable’s heating curve (I² t) must stay above the breaker curve at every current.
Fault levels and breaking capacity
Every device must be able to break the highest prospective short-circuit current at its location (its rated breaking capacity, such as 6 kA for a domestic MCB, 10 kA or more in industry) and must do so within the cable’s withstand time (see Cable Sizing and Voltage Drop). The fault current is the supply voltage divided by the impedance back to the source: Isc = (V ÷ √3) ÷ Ztotal. It is highest at the transformer terminals and falls along every cable.
Earthing and the earth-fault loop
The metal frames of equipment are connected to earth so that a fault to the frame draws enough current to trip the protection quickly. In a TN-S system the transformer star point is earthed and a separate protective earth (PE) conductor runs back to it; in TN-C-S (PME) the neutral and earth are combined to the building; in TT the installation has its own earth electrode and always needs an RCD; IT systems are isolated from earth (hospitals, some plants) and tolerate a first fault. The path of a fault current out through the line conductor and back through the PE is the earth-fault loop, with impedance Zs; the protective device must trip within 0.4 s for a 230 V final circuit (5 s for distribution circuits): If = U0 ÷ Zs must exceed the trip current of the device.
Worked example: the press motor circuit
Worked example: breaker, fault levels and earth loop for the 11 kW motor
- Fault level. At the transformer terminals Isc = 100 000 ÷ (√3 × 400 × 0.04) = 3 608 A (Transformers and Distribution). At the motor, after 30 m of 6 mm² cable (0.088 Ω), Isc = 231 ÷ (0.064 + 0.088) = 1 524 A. The main board needs a breaking capacity of at least 3.6 kA (a 10 kA board is the standard choice); the motor breaker, 6 kA, is adequate at 1.5 kA.
- The breaker: a 25 A device (for Ib = 20.5 A). Type C trips magnetically at 5 to 10 × 25 = 125 to 250 A. The motor’s starting current of 7 × 20.5 = 144 A can reach a peak of about 305 A (RMS × √2 × about 1.5 for the DC offset), which is over the 250 A limit: a type C breaker may trip on every start. Use a type D breaker (10 to 20 × In = 250 to 500 A) or, better, a motor protection circuit breaker.
- Earth-fault loop: Zs = Ztransformer + Rline + RPE = 0.064 + 0.107 + 0.107 = 0.277 Ω (copper at 70 °C, 6 mm² PE). If = 230 ÷ 0.277 = 830 A, over 3 times the 250 A that a type D (or 10 × 25 for a type C) needs, so it trips magnetically in under 0.1 s.
- The maximum allowed Zs for 0.4 s disconnection with a 250 A magnetic trip is 0.8 × 230 ÷ 250 = 0.74 Ω (the 0.8 allows for the cable heating up); the circuit, at 0.28 Ω, passes. During the fault the metal frame sits at a touch voltage of about If × RPE = 830 × 0.107 = 88 V, for under 0.1 s: this is why the clearing time matters more than the touch voltage.
RCDs and body current
A residual current device compares the current in the line conductors with the current returning: a difference means current is flowing to earth, perhaps through a person. A 30 mA RCD trips in under 40 ms at rated residual current, which is enough to prevent ventricular fibrillation. The danger depends on current and time: the threshold of perception is about 1 mA, of “let-go” 10 mA, and fibrillation becomes likely above approximately I = 116 ÷ √t mA (Dalziel). A body resistance of about 1 000 Ω across 230 V drives 230 mA. The limit for a shock lasting 0.4 s (a breaker) is 183 mA, so 230 mA is over it; the 30 mA RCD cuts it in 40 ms, where the limit is 580 mA: safe by a wide margin.
- Type AC / A / F / B: type A also detects pulsating DC leakage (most electronics); type B detects smooth DC leakage from three-phase drives and PV inverters. A type AC RCD can be blinded by DC and fail to trip.
- Nuisance tripping: filters on drives and computers leak a few mA to earth. A single drive filter of 20 mA leaks two thirds of a 30 mA RCD’s rating, so total the leakage and use a 300 mA type B unit upstream and 30 mA only for sockets.
- Test monthly with the test button, and annually with a tester that measures the trip time and current.
Motor circuit protection
The coordination of a motor starter has three jobs: a short-circuit device (breaker or fuse) that tolerates the starting current but clears a fault, a contactor to switch, and an overload relay to protect the winding. For the example: overload relay set to 1.05 × the nameplate current (21.6 A), class 10, with phase-failure sensitivity. If one phase is lost (single-phasing), the motor keeps running on two lines at about √3 times the normal current: 35.6 A here, 73 percent over rating, which the relay trips in minutes. Type 2 coordination (IEC 60947-4-1) means the starter is undamaged after a short circuit and can be put back in service.
Selectivity
Selectivity (discrimination) means only the device nearest the fault opens. For fuses, a ratio of 1.6 between the ratings usually suffices; for breakers, the upstream device is given a time delay or a higher magnetic setting. With a 125 A upstream breaker feeding the 25 A motor breaker (ratio 5: 1), the manufacturer’s selectivity tables give the current up to which the downstream device opens alone (often the full fault level when the upstream is a delayed MCCB). RCDs discriminate by time (an S-type delayed device upstream of a 30 mA instantaneous one). Without selectivity a fault in one machine blacks out the whole building.
Related
- Previous: Cable Sizing and Voltage Drop. Next: Power Factor, Harmonics and Energy.
- System Integration for panel layouts and safety systems.
- Grounding, Shielding and EMC.
- Electrical Power: the whole guide.
FAQ
What is the difference between a fuse and a breaker?
A fuse is destroyed by the fault and replaced; a breaker is reset. A fuse limits the let-through energy better and breaks higher currents; a breaker is more convenient and gives repeatable trip curves.
Why is my RCD tripping?
Leakage current to earth: damp heaters, filters on drives and computers, a neutral-earth fault or an insulation fault. Isolate circuits one by one and measure the leakage with a clamp meter.
Do I need an earth electrode?
In TT systems always. In TN systems a local electrode supplements the earth, but the protection depends on the fault loop through the supply.
Can I use a bigger breaker to stop nuisance tripping?
No: find the cause. A larger breaker may not protect the cable. Choose a curve (type D or an MPCB) instead.
References
- IEC 60364-4-41 (protection against electric shock) and -4-43 (overcurrent), IEC 60898-1 (MCBs), IEC 60947-2 (circuit breakers) and -4-1 (contactors and motor starters), IEC 61008/61009 (RCDs).
- BS 7671 (IET Wiring Regulations), NFPA 70 (NEC) and NFPA 70E; Dalziel, C. F., effects of electric current on man (IEC 60479-1).
- Cadick, J., Electrical Safety Handbook; Schneider Electric, ABB and Siemens protection coordination guides.
Last reviewed: October 2026. Trip bands and limits are typical; use the manufacturer’s curves and the code that applies.