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CIRCUIT PROTECTION DEVICES EXPLAINED

MCBs are designed to protect people, cables and equipment from two main types of fault:

Overloads: the gradual build-up of heat due to increasing current flowing through a

circuit. A thermal device in the MCB detects this build-up and trips the MCB. The speedof this depends upon the severity of the overload and the tripping curve of the MCB asindicated below.

Short circuit: sudden increases in current flowing through a circuit cause a magneticdevice in the MCB to operate in a fraction of a second, protecting the installation and anypersons in contact with the supply.

MCBs have ratings from 1A up to 63A (with some high current devices at 80A and 100A).Selection of the correct version depends upon the circuit it is protecting. MCBs come in threedifferent trip characteristics:

B Curve: trip at 3 - 5 times the rated current e.g. a 6A MCB will trip at 1830A C Curve: trip at 5 - 10 times the rated current e.g. a 6A MCB will trip at 3060A D Curve: trip at 10 - 20 times the rated current e.g. a 6A MCB will trip at 60120A

The correct tripping curve depends upon the type of equipment being protected. For example,florescent lighting and motors can cause an initial surge on start-up which may cause nuisancetripping with a B Curve MCB therefore a C Curve MCB should be installed. If this is stillinadequate then a D Curve should be selected.

MCBs also have a maximum short circuit capacity, expressed as its kA rating. CPN offer rangesof MCBs that have 6KA and 10KA rating.

RESIDUAL CURRENT BREAKER WITH OVERLOAD - RCBO'S

RCBOs are a combination of an RCD and MCB in one unit. This enables both over-currentprotection and earth fault current protection to be provided by a single unit, which allows earthfault protection to be restricted to a single circuit, ensuring only the circuit with the fault isinterrupted, limiting inconvenience tripping of many circuits.

B and C type tripping curves (MCB) in Cudis. 30mA tripping current (RCD). 10kA versions.

RCBOs tend to beselected by the MCB rating e.g. 6A B curve and the RCD side is usually30mA.

Two versions are available:

1 module 1 pole version (17.5mm wide). Although the same width as a standard MCBit is twice the height. Ideal for use in consumer units where more control is required as towhich circuits are protected by RCDs and which by RCBOs, as individual circuits can beprotected as opposed to groups of circuits in a standard split-load consumer unit.

Standard 2 Module RCBO (35mm wide). These usually switch both the live and theneutral connections.

RESIDUAL CURRENT DEVICES - RCD'S

RCDs work on the principle of a load being in balance that is, the current on the Phase

conductor is equal to that flowing out of the Neutral conductor. The Phase and Neutral cables inan RCD pass through a magnetic ring which detects any imbalance. If equal the RCD remains

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operational, if imbalanced it causes the mechanism to trip.Modular RCDs come in 2-pole and 4-pole versions. 2-pole versions are usually mounted inconsumer units to protect a number of circuits at once. However, both 2-pole and 4-pole versionscan be used separately as protection devices in enclosures.

There are two usual ways in which an imbalance of the load can occur:

Indirect Contact: defined in the IEE Wiring Regulations as contact of persons andlivestock with exposed conductive part made live by a fault which may result in an electricshock. E.g. the casing of an appliance or motor.

Direct Contact: defined in the IEE Wiring Regulations as contact of persons andlivestock with live parts which may result in an electric shock. E.g. exposed cable.

In both cases RCDs should not be the only method of protection used - see BS7671 wiringregulations for further information. To provide all-round protection RCDs must be used inconjunction with suitable MCBs.

The dangerous aspect is the magnitude and duration of the current flowing through the human

body and in particular near the heart.

The Cudis RCDs are available in the following sensitivities:

30mA the most popular sensitivity in the UK. In a shock, current flowing through bodyat 240V could be 80 to 240mA, depending on the resistance of the body in question. Toensure no harmful effects the RCD operates within 300mS at 30mA and 40mS at 150mA.

100mA may provide protection against electrocution. However, there is a likelihood thatthe earth fault current may be below the sensitivity of the RCD - increasingly likely ifadditional resistances to that of the human body are in the current path.300mA - providesprotection against risk of fire only, not against electrocution in shock situations. A typicalapplication is lighting circuits where risk of electric shock is small. Note, a current of