Overcurrent protection, RCDs and devices

Overload and fault current

An overcurrent is any current above the rated value, and it takes two forms.

• Overload: excess current in an electrically sound circuit, such as demand from too many appliances. It can begin suddenly, while the resulting heating builds with time.
• Short-circuit current flows through an unintended low-impedance connection between live conductors and can be very large. Earth-fault current follows a fault path to earth or protective metalwork; its size depends on that path’s impedance. A high-impedance earth fault may not operate an overcurrent device, which is one reason RCD fault protection is used.

Coordinating cable and device

For normal overload selection, first check Ib ≤ In ≤ Iz: intended load, protective-device rating and then the cable’s capacity after the relevant installation factors. The device’s operating current also matters. Check I2 ≤ 1.45 × Iz, applying the provisions for the device type, as well as the separate fault-protection requirements.

MCB tripping curves (BS EN 60898)

The curve sets the instantaneous magnetic trip band:

• Type B: 3 to 5 times In, for resistive and lighting loads with little inrush.
• Type C: 5 to 10 times In, for moderate inrush such as fluorescent lighting and small motors.
• Type D: 10 to 20 times In, for high inrush such as transformers and welding sets.

Breaking capacity

Compare the prospective fault current at the device with its breaking capacity. If the device cannot interrupt that current on its own, it needs a verified backup arrangement with suitable upstream protection. Use the manufacturer’s coordination data; an upstream fuse or breaker is not automatic proof that a lower-rated device is adequately protected.

RCDs and related devices

An RCD responds to the residual imbalance in the live conductors passing through it. On a single-phase circuit, current returning outside the monitored neutral creates a difference. Normal equipment leakage can also contribute. The residual-current rating describes the protective response, not the load current the device can carry. Common waveform designations are:

• Type AC: sinusoidal AC residual current only.
• Type A: sinusoidal AC and pulsating DC residual currents.
• Type F: the Type A capabilities plus specified composite residual currents associated with some frequency-controlled equipment.
• Type B: broader residual-current coverage including smooth DC on an AC circuit. This does not make an ordinary Type B RCD a device for a DC supply system.

Select by the actual equipment, expected waveforms and instructions. Type AC is restricted to fixed equipment whose load is known to contain no DC components; Type A is not automatically sufficient for every electronic load.

An RCBO provides both residual-current and overcurrent protection; an RCCB has no integral overcurrent protection. An SPD deals with transient voltage, while an AFDD recognises dangerous arcing that ordinary overcurrent or residual-current protection may not clear. Manufacturers may combine functions, for example in an AFDD with an RCBO.

Choosing and coordinating devices

• Overcurrent protection limits damaging overload and fault effects. A device with an excessive rating can leave the cable inadequately protected. The design load, cable capacity and device characteristic all matter; choosing by load alone or by the number on an old fuse is insufficient.
• The overload operating-current condition is I2 ≤ 1.45 × Iz. For the listed devices in the UK rule, including general-purpose BS 88-2 gG and BS 88-3 fuses, BS EN 60898 circuit-breakers and BS EN 61009 RCBOs, the first two rating conditions also establish the third. A BS 3036 semi-enclosed fuse needs In ≤ 0.725 × Iz when it provides overload protection. Keep this specific cable factor separate from a circuit-breaker’s time-current characteristic.
• After a fault is cleared, a circuit-breaker can be reset; a fuse has to be replaced. The figure in a rectangle on a BS EN 60898 breaker, such as 6000, is its rated short-circuit capacity in amperes: 6 kA.
• Selectivity confines a fault interruption to the appropriate part of the installation. Individual RCBOs reduce the circuits sharing one residual trip, but upstream coordination still needs checking. For RCD selectivity, use a suitable upstream delay and residual-current rating; the usual S-type arrangement needs at least a 3:1 upstream-to-downstream ratio. At rated residual current, the AC test window for a Type S device is 130–500 ms. That test window alone does not establish selectivity between any two devices.
• Electronic equipment can produce residual-current waveforms that an unsuitable RCD may miss. For an EV point, permitted arrangements include Type B or Type A/F with suitable 6 mA DC detection and disconnection. Apply the requirements for the actual charger; do not assume the same arrangement automatically suits a PV inverter or drive.
• A series arc may draw too little current for a fuse or MCB to clear promptly; an AFDD looks for the arc’s electrical signature. The BS 7671 requirement covers single-phase AC final circuits supplying socket-outlets up to 32 A in higher-risk residential buildings, HMOs, purpose-built student accommodation and care homes, with recommendations for other premises.
• An SPD conducts surge current to limit transient voltage between its protected conductors; the path is not always to earth. Mains SPDs have internal overload protection, typically a thermal disconnector. Their connections and any required backup short-circuit protection must also meet the manufacturer’s coordination requirements.
• Battery-circuit devices must suit the DC duty assigned to them. Isolation and fault interruption are different duties. Inverter output can be electronically current-limited, while a generator has its own fault-current profile; verify disconnection and breaking capacity for every source and operating mode.
• For Power over Ethernet, check both the total source budget and each port’s available power against device demand. Assess cabling, connectors and installation conditions separately; an insufficient source budget can cause loss of service without proving that a cable is overheating.
• Mobile equipment up to 32 A used outdoors needs 30 mA RCD protection.