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About Miniature circuit breakers

Introduction

Miniature Circuit Breakers (MCBs) are designed to protect the cable downstream of the device

against overloads and short circuits, preventing damage to cables and equipment.

MCBs are installed in modern consumer units, and are a convenient alternative to fuses as they

can be reset without having to replace wire with a simple flick of a switch or button. There are 3MCB types, Type B, Type C and Type D, and the speed at which they trip depends upon the

level of overload, and is usually determined by a thermal device within the MCB.

An MCB works by tripping when a circuit is overloaded or when a short circuit has occurred inthe system, and has a current rating such as 6A, or 10A depending upon its intended use, i.e. for 

residential, commercial, industrial or public buildings.

MCB ratings relate to continuous service under specified installation conditions, although cablescan carry higher currents for short periods without causing permanent damage.

There are 3 MCB types, Type B, Type C and Type D, and the speed at which they trip dependsupon the level of overload, and is usually determined by a thermal device within the MCB.

Selecting the right one should be done in accordance with BS 7671:Requirements for Electrical

Installations (The IEE Regulations, Sixteenth Edition) and will ensure that the device will notgive unwanted tripping.

All 3 MCB types use a magnetic fault protection, which trips the MCB within one tenth of a

second when the overload reaches a set level.

  Type B trips between 3 and 5 time full load current;

  Type C trips between 5 and 10 times full load current; and  Type D trips between 10 and 20 times full load current.

An MCB's circuit rating is given in Kiloamps (KA), and this indicates the level of its ability to

work. For example a domestic MCB would normally have a 6KA fault level, whereas one usedin an industrial application may need a unit with a 10KA fault capability.

Type B devices are commonly used in domestic systems and light commercial applicationswhere surges are low, for instance where inrush currents may come from a small number of 

fluorescent fittings.

However, unwanted tripping can occur due to high arcing currents, often due to poor qualitylamps, and in this case either a higher rating B type MRC should be used or a type C device may

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 be more suitable instead. Again, the MCB type used should be selected in accordance with BS7671.

Type C MCBs are most suitable for commercial and industrial use, where there are motors and

 perhaps a high number of fluorescent fittings which, when switched off together may cause a

high inrush current. Whether an upgrade to a Type D is suitable should be decided according tooperating times and consideration of Regulation 413-02-08.

Type D units are for more specialist industrial use, where current inrushes can be high, for example with X-ray machines and transformers. They may require lower earth loop impedance

(Zs) to achieve the operating times required.

Whichever MCB type is used, it is extremely dangerous to cut corners by using inferior quality

devices, therefore they should only be bought from a reputable manufacturer.

Basic applications

The essential distinction between Type B, C or D devices is based on their ability to handle surge

currents without tripping. These are, typically, inrush currents associated with fluorescent andother forms of discharge lighting, induction motors, battery charging equipment etc. BS 7671

specifically refers to Types B and C, and the choice will normally be between these two types.

y  Type B devices are generally suitable for domestic applications. They may also be usedin light commercial applications where switching surges are low or non-existent.

y  Type C devices are the normal choice for commercial and industrial applications where

fluorescent lighting, motors etc. are in use.y  Type D devices have more limited applications, normally in industrial use where high

inrush currents may be expected. Examples include large battery charging systems,winding motors, transformers, X-ray machines and some types of discharge lighting.

The classification of Types B, C or D is based on the fault current rating at which magnetic

operation occurs to provide short time protection (typically less than 100ms) against short-circuits. It is important that equipment having high inrush currents should not cause the circuit-

 breaker to trip unnecessarily, and yet the device should trip in the event of a short-circuit currentthat could damage the circuit cables.

Detail of MCB

1.  Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the

status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can

still trip even if the lever is held or locked in the "on" position. This is sometimes referred

to as "free trip" or "positive trip" operation.

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2.  Actuator mechanism - forces the contacts together or apart.

3.  Contacts - Allow current when touching and break the current when moved apart.

4.  Terminals

5.  Bimetallic strip.

6.  Calibration screw - allows the manufacturer to precisely adjust the trip current of the

device after assembly.

7.  Solenoid

8.  Arc divider/extinguisher