Welcome to the Consumer Units technical section. Currently we have information regarding the MCBs & RCDs used in consumer units.
Minature Circuit Breakers (MCBs)

Miniture Circuit Breakers (MCBs) are current limiting Thermal / magnetic type with each part performing a different function:

A) Small Overload Conditions
Overload currents that are light in nature are detected by a bi-metalic strip which bends at a rate dependant upon the size of the overload against a trip mechanism which will release contact after a pre-determined timescale. 

B) High Overload Conditions
When the overload current reaches a high enough level (dependant upon the current rating and tripping characteristics of the MCB) the current induces a magnetic flux within the coil sufficient to propel a plunger into the tripping mechanism with enough force to operate this mechanism and trip out the MCB. 

C) Short Circuit Conditions
If a fault occurs that is high enough to be classed as a 'short circuit' the the current limiting mechanism comes into action. The fault current generates so much flux in the coil that the plunger is forced to move so fast and with so much force that it i) trips the mechanism and ii) forces the contacts to open and therefore restricts the short circuit current. The arc generated between the contacts is quickly quenched by the 'Ark Grid' after it is forced to flow along the 'Arc Runners' into the Grid.

B, C & D Curves for MCBs

ALL MCBs are now rated at 10kA Short Circuit Capacity with Positive Contact Indication.

‘B’ Curve to BSEN 60898 (Magnetic Trip 3 to 5 Times In)
Used in domestic situations where the maximum sensitivity is required and very little equipment with high start up current is required.

‘C’ Curve to BSEN 60898 (Magnetic Trip 5 to 10 Times In)
Used in commercial/light industrial situations where close protection is not required and start up currents of devices can run up to 5 times rated current or a short period. Also if many low voltage lights are used in a domestic situation then, due to the inrush currents of the transformers type ‘C’ MCB’s may be required to avoid nuisance tripping.

‘D’ Curve to BSEN 60898 (Magnetic Trip 10 to 20 Times In)
Used in industrial situations where start-up currents may be 10 times rated current, for example protection for machines with electric motors fitted, large installation with a high volume of low voltage lighting may also require ‘D’ Curve.

Residual Current Devices (RCDs)

An RCD is the term that describes a device that opens the protected circuit when the detected residual current exceeds that of the preset value of the RCD. Under normal ‘healthy’ circuit conditions with all conductors passing through the current
transformer (built into RCD, 2 &4 Pole) the vector sum of the current values will be zero. When a fault occurs between any phase or neutral and earth regardless of load, this will give rise to a residual current which will be detected by the RCD. When this residual current exceeds the ‘Tripping Current’ (aka: sensitivity of the RCD) the RCD will operate and open the circuit. it is most important to connect all
Line and Neutral conductors through the RCD as one of the most common causes of nuisance operation is not connecting all the conductors through and RCD.

Three types of RCD are produced. Each with a specific use:
Type AC - Tripping is ensured for residual sinusodial alternating currents, whether the fault is suddenly applied or is a slowly rising fault.

Type A - Tripping is ensured for residual sinusodial alternating currents and residual pulsating direct currents whether the fault is suddenly applied or is a slowly rising fault.

Type S - Tripping Devices with built in ‘Time Delay’ for selectivity within circuits having more than one RCD in line.

Uses of RCDs

RCDs offer very good protection against Earth Fault Currents, the main areas of application are as follows:

Protection against indirect contact or where the Zs value is too high. In Places where overload protection cannot react within the specified timescale due to the earth lop impedance being too high, adding an RCD may solve the problem without having to make any other changes to the system. The high sensitivity of the RCD will ensure disconnection within the specified time and thus without detriment to the overload discrimination (MCB/RCD systems only). In particular situations such as bathrooms and building sites, use of an RCD will help achieve the more stringent tripping times required by regulation. At all times to avoid the danger of exposed conductive parts reaching unaceptably high voltage, the rated residual operating current multiplied by the Zs value must not exceed 50V.

Protection against Direct Contact.
In this case RCD’s offer a high level of protection but must not be the only method of protection. To offer this degree of protection, the RCD must have a sensitivity of 30mA or less and must trip-out with a residual current of 150mA in less than 40mS.

Protection against shock outside the equipotential bonding zone.
Earth bonding is used in an installation to try and ensure that, as near as possible, all
metalic parts are maintained to the same potential as earth. Working with portable
equipment outside the equipotential bonding zone introduces additional shock hazards. Any 32A or lower socket outlet that may reasonably be expected to supply portable equipment should be protected by a 30mA RCD, unless using reduced voltage supplied via an isolating transformer.

Protection for portable equipment.
The provisions of the wiring Regulations demand the use of RCD protection to protect the users of any such equipment with the exception of where a socket is specifically designed to be used outside of equipotential bonding zone. The use of RCDs to control such portable equipment is widely recognised as having greatly increased safety in the home and the workplace.

Protection in Special Locations.
The use of RCDs is either obligatory or strongly recommended in the following locations:

Caravans, Shed’s, Garages or Port-acabins. Near Swimming pools, Construction
Sites, TT wiring systems and on Farms. - at each location a 30mA RCD should be used.