Overload versus fault current, the Ib <= In <= Iz rule, MCB tripping curves, breaking capacity and RCD types.
Overload and fault current
An overcurrent is any current above the rated value, and it takes two forms.
- Overload: excess current in a circuit that is electrically sound, for example too many appliances on one circuit. It rises gradually and is a thermal effect.
- Fault (short-circuit) current: an overcurrent of negligible impedance between live conductors, or an earth fault to a protective conductor. It is very large and must be cleared quickly.
Coordinating cable and device
The device and cable are matched by the rule Ib <= In <= Iz, where Ib is the design current of the circuit, In is the rated current of the protective device, and Iz is the current-carrying capacity of the cable after any derating. The device protects the cable only when its rating sits between the load and the cable capacity.
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
The rated short-circuit breaking capacity of a device must be at least the prospective fault current at the point where it is installed. A device rated below that value could be destroyed while trying to clear the fault.
RCDs and related devices
An RCD senses the imbalance between line and neutral current. In a healthy circuit the two are equal; current leaking to earth creates a residual current that trips the device. A 30 mA RCD gives additional protection against electric shock. The main types are:
- Type AC: sinusoidal AC residual current only.
- Type A: AC plus pulsating DC, now the general minimum for most circuits.
- Type B: AC, pulsating DC and smooth DC, for equipment such as some EV chargers and inverters.
An RCBO combines an MCB and an RCD in one device, protecting a single circuit. An SPD limits transient overvoltage from lightning or switching, while an AFDD detects dangerous arc faults that an MCB or RCD does not.