The decision to use Type B, C or D miniature circuit-breakers for final circuit protection in residential, commercial, industrial or public buildings can be based on a few simple rules. However, an understanding of the differences between these Types of device can help the installer overcome problems of unwanted tripping or make a suitable selection where lines of demarcation are less clearly defined.
It should be stressed that the primary purpose of circuit protection devices such as miniature circuit breakers and fuses is to protect the cable downstream of the device. The first requirement, therefore, is to select a device in accordance with BS 7671:Requirements for Electrical Installations (The IEE Regulations, Sixteenth Edition).
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 and other 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.
- Type B devices are generally suitable for domestic applications. They may also be used in light commercial applications where switching surges are low or non-existent.
- Type C devices are the normal choice for commercial and industrial applications where fluorescent lighting, motors etc. are in use.
- 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 current that could damage the circuit cables.
The tripping characteristics are illustrated in Fig 1.
- Type B devices are designed to trip at fault currents of 3-5 times rated current (In). For example a 10A device will trip at 30-50A.
- Type C devices are designed to trip at 5-10 times In (50-100A for a 10A device).
- Type D devices are designed to trip at 10-20 times In (100-200A for a 10A device).
Normal cable ratings relate to continuous service under specified installation conditions. Cables will, of course, carry higher currents for a short time without suffering permanent damage. Type B and C circuit breakers can generally be selected to achieve tripping times that will protect the circuit conductors against normal surge currents in accordance with BS 7671. This is more difficult to achieve with Type D devices, which may require a lower earth loop impedance (Zs) to achieve the operating times required by Regulation 413-02-08.
Sources of surge currents:
Surge currents in domestic installations are generally low, so that a Type B device is adequate. For example inrush currents associated with one or two fluorescent fittings, or the compressor motor in a refrigerator/freezer are unlikely to cause unwanted tripping.
Fluorescent and other discharge lamps produce surge currents and while one or two fluorescent lamps are unlikely to cause a problem, the block switching of a number of fluorescent lamps in a shop, office or factory can produce substantial inrush currents. For this reason Type C devices are recommended for these applications.
The magnitude of the surge current will depend on the lamp rating, starting system and type of controlgear used in the luminaires. Reputable MCB manufacturers produce tables listing the number of fittings of a particular make and type that can be used with their devices.
Overcoming unwanted tripping:
Sometimes failure of tungsten filament lamps can trip Type B circuit-breakers in domestic and retail environments. This is caused by high arcing currents occurring at the time of failure and is generally associated with inferior quality lamps. If possible the user should be encouraged to use better quality lamps. If the problem persists then one of the measures listed below should be considered.
A Type C device may be substituted for a Type B device where unwanted tripping persists, especially in commercial applications. Alternatively it may be possible to use a higher rated Type B MCB, say 10A rather than 6A. Whichever solution is adopted, the installation must be in accordance with BS 7671.
A change from Type C to Type D devices should only be taken after careful consideration of the installation conditions, in particular the operating times required by Regulation 413-02-08.
The importance of selecting circuit-breakers from reputable manufacturers cannot be over-emphasised. Some imported products, claiming to have a 6kA short-circuit capacity, have failed dramatically under test. In contrast the test procedures followed in British ASTA (Association of Short Circuit Testing Authorities) laboratories are among the most stringent in the world.
Combined overcurrent and residual current circuit breakers (RCBOs) are available as integrated units or, in one case, as a modular device with a field-fittable clip-on RCD 'pod'. It should be borne in mind that if an RCBO trips it is not always clear whether tripping has been caused by an overcurrent or a residual current. Type B devices should only be used in domestic situations where high inrush currents are unlikely and Type C devices should be used in all other situations.
Contact: Richard Hunt
Eaton MEM (Eaton Electric Ltd.,)
Reddings Lane, Birmingham B11 3EZ
Tel: 0121 685 2100
Fax: 0121 706 2012