James Hunt takes a look at the latest in low-voltage circuit protection devices - fuses, RCDs, MCBs and MCCBs - for domestic, commercial and light industrial environments, and the ways in which they have been affected by recent legislation:
Device selection and installation will depend on a number of factors, one being standards changes. The harmonised standard for circuit breakers, BS EN 60947-2 has recently been further revised; the most significant changes affect requirements for earth leakage protection, electromagnetic compatibility (EMC) and test procedures. The latest Edition has been required because of technological change and new legislation. The good news for electrical contractors is that these changes should have little impact on the way they select and install circuit breakers. Overall, the changes affect the contractor little but should give even greater confidence in the quality and suitability of the product.
Choosing MCBs and other devices:
Choosing Type B, C or D MCBs for final circuit protection is simple but requires an understanding of the differences between them to avoid problems. The primary purpose of circuit protection devices is to protect the cable downstream of the device. All such devices must, therefore, be selected in accordance with BS 7671 (16th Edition).
The distinction between devices is based on their ability to handle surge currents (inrush currents associated with fluorescent and other forms of discharge lighting, induction motors and battery charging equipment) without tripping. The choice will usually be between Types B and C. Tripping characteristics are illustrated in Fig 1.
* Type B - Domestic and light commercial applications - switching surges low or non-existent.
* Type C - Normal choice for commercial / industrial applications - fluorescent lighting, motors etc.
* Type D - Limited to industrial use (high inrush currents).
MCBs typically have short-circuit breaking capacities of 6-10kA. Above this, it is customary to use MCCBs.
Combined overcurrent and residual current circuit breakers (RCBOs) are available as integrated units. However, if an RCBO trips, it may be unclear whether tripping has been caused by an overcurrent or a residual current.
Supply reliability can be improved by ensuring that MCCBs are suited to the application environment. For example, in a data or communications centre where harmonic distortion is high, thermal protection units provide more accurate overload protection than electronic sensing. That is because a thermal trip element responds directly to the heating effect of current. Electronic systems, while classed by manufacturers as 'true rms', can only provide an approximation of the current heating effect. The accuracy is limited by the microprocessor's sampling period (short period = high accuracy). Simply, an electronic MCCB doesn't know how hot an overload makes a cable.
However, in 'zero downtime-tolerant' systems, where an MCCB feeds many branch circuits, a Category B electronic MCCB can provide excellent selectivity, and reduced downtime. However, many electronic MCCBs are only Category A, therefore not specifically designed for selectivity. IEC 60 947-2 (2001) gives clear guidance on the classification of Category A and Category B circuit breakers.
Some specific breakers:
Terasaki offers TemBreak MCCBs with thermal overload protection up to 800A. TemBreak electronic MCCBs up to 2500A are all Category B as standard and have a short 0.5 ms sampling period.
ABB's System proM S280 MCBs are claimed to be the only ones in the UK to provide over-current protection for equipment and cables up to 100A within a standard width (17.5 mm) single-pole module. These 80 / 100A 6kA Type B or C single-, double-, triple- or four-pole devices allow compact distribution boards to be built without the need for a separate panel board. A universal current version combines all the operating properties of a standard AC breaker with exceptional DC performance. ABB also has available its space-saving DS271 RCBO.
Memshield 2 15kA MCBs by Eaton MEM meet the needs of modern high-rise commercial buildings where use of large transformers and low-impedance busbar riser systems may result in high prospective fault current levels at the final distribution board. These devices offer a compact alternative to the MCCBs normally used and achieve short-circuit discrimination with downstream devices. A unique feature is conversion to RCBO operation by fitting an RCD 'pod'.
The company's new AD range of eight single-pole MCBs offers a cost-effective solution to common everyday needs for MCB protection. These 6A - 63A Type B DIN-rail mounted devices should satisfy the vast majority of electrical contractors' needs in domestic and commercial installations.
Eaton MEM also has RCBOs available, centre tapped to earth for environments - such as construction sites - where users demand the highest standards of safety with equipment operating through 110V power supplies. They can also be used where the conditions for automatic disconnection of the supply cannot be assured because of uncertain earth fault loop impedances.
Siemens' BETA circuit-protection devices cover all aspects of safety with components that can be combined quickly and easily. The range includes MCBs, RCDs, lightning current and overvoltage protection devices, all complemented by a range of accessories.
Schneider Electric provides a wide range of Merlin Gerin MCBs with current ratings from 1 to 125A for use in distribution boards and loose enclosures. The core of the range, the 15kA C60 MCB, is available up to 63A, B, C or D curves. Accepting cables to 35mm2, the C60 provides a single- or three-phase compact solution for a wide range of applications. The new NG125 MCB provides current ratings to 125A, with breaking capacities of 25 and 36kA (plus 50kA for motor applications). This device is suitable as an outgoer to final circuits fed from panels.
Where protection of machinery and plant is essential, use of E-T-A Circuit Breakers's plug-in type thermal circuit breakers for equipment (CBEs) instead of blade fuses significantly reduces downtime of machinery or plant.
Fuses:
The simple and highly reliable fuse still provides a primary means of protecting electrical circuits against overcurrent. For many applications, these devices are the best. Fuses limit overloads to a single circuit and minimise the danger of fire in the equipment and the circuit leading to it. They are low-cost items and are highly compact; the space saving alone can save money. Current limitation is a strong feature of the fuse and cartridge fuses limit the short circuit fault current and energy let-through, so reducing damage to sensitive circuit components at the point of fault. High breaking capacity, safety and good power quality are other major advantages.
Global fuse manufacturer Cooper Bussmann offers fuses for every application. Its innovative dual-indication NH fuse link is probably the first to feature a highly reliable combined top and centre fuse status indication device and is claimed to be the most advanced NH fuse link available. The company's novel and tiny CUBEFuse, the world's first finger-safe industrial power fuse, is now available in a 100A version, giving it a still greater diversity of applications. Use of CUBEFuse greatly lessens risk of electrocution and arc-fault initiation when working with energised circuits. Cooper Bussmann's ASTA approved fuse links to BS 1363 - for 13A fused plugs - are available in a wide range of current ratings.
Eaton MEM also makes a wide range of LV fuselinks as does Siemens with its BETA range fuses.
This article was originally published in Electrical Times magazine by Highbury Business Communications - a Voltimum UK Media Partner.
Electrical Times
Highbury Business Communications
Nexus House, Azalea Drive, Swanley, Kent BR8 8HY
Tel: +44 (0) 20 8722 6072
Email: [email protected]
Web: www.hhc.co.uk
And: www.electricaltimes.co.uk