Low-voltage circuit protection devices for domestic, commercial and light industrial environments are critical for the safety of people and equipment. James Hunt reports in this article originally published in Electrical Times magazine:
Such devices protect cables and other electrical equipment, but they also reduce the risk of fire occurring because of an electrical fault. Residual current devices (RCDs) protect people from the hazards of electric shock.
Fuses:
Old technology they may be, but simple and highly reliable fuses provide a primary means of protecting electrical circuits against over-current, and, for some applications, they are still the best.
Low-cost and very compact, fuses limit overloads to a single circuit and minimise fire danger in equipment and the circuit leading to it.
Current limitation is a strong benefit, and cartridge fuses limit the short circuit fault current and energy let-through. High breaking capacity and good power quality are other advantages.
Circuit breakers:
Electromagnetic circuit breakers contain a small electromagnet device, which automatically disconnects on over-current.
Purely magnetic breakers are resilient to small overloads and current surges but react very quickly to fault conditions. Although circuit breakers cost more than fuses, and take up more space, they have their own advantages: they are re-settable, and are often much easier to troubleshoot than fuses. Downtime is reduced.
RCDs - Current-operated RCDs protect against electrocution by detecting current flowing to earth. Normally, any earth current should be very low, so RCDs can be made very sensitive - typically 30mA. They must also trip very fast, typically within 0.04s, though the time-delayed type will take longer. Their sensitivity allows detection of other faults such as partial live/earth breakdown, which may cause a fire. RCDs, however, cannot protect against live-neutral shock or overloads, so they are always used in conjunction with MCBs or fuses.
To comply with the IEE Wiring Regulations, all sockets which might, at some stage, have outdoor electrical equipment plugged into them must be RCD protected.
An MCB is normally required for each circuit (currents vary) but, in most cases, only one RCD is needed for a given home. RCDs can be combined with MCBs to form RCBOs, to perform earth leakage and over current protection functions in a single, compact unit. However, if an RCBO trips, it may be unclear whether tripping has been caused by an over-current or a residual current, so fault-finding may not be simple, though some can detect live/neutral reversal or earth disconnection.
The cause of an RCD tripping through earth leakage can usually be found by opening all MCBs on protected circuits, then reactivating them one by one until the RCD trips again. Modern test instruments make the job a lot easier.
MCBs - These generally contain two automatic disconnect devices, and a bimetallic strip which curls in response to current flowing. If the current is large enough, disconnection occurs.
This thermal device will detect small overloads, but the response is quite slow, so that overloads of (say 150%) - which last only a brief period - will not disconnect. Longer-term overloads (at, say 125% for several minutes) will disconnect.
MCBs typically have short-circuit breaking capacities of 6 -10kA. Above this, it is customary to use MCCBs. Choosing MCBs for final circuit protection is simple, but it is important to understand the ability to handle surge currents without tripping.
Such currents are associated with fluorescent and/or discharge lighting, induction motors and battery chargers.
The choice will usually be between the following: 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); and Type D (limited to industrial use with high inrush currents.
MCCBs - These devices include both time-delayed and instantaneous tripping functions, and must be sized large enough to prevent the instantaneous element from tripping on starting current.
Standard MCCBs can take up to two full cycles to interrupt current, making current-limiting devices the better option for high-fault-current applications.
MCCBs are available with current-limiting characteristics, interrupting high currents within the first quarter cycle, so greatly limiting adverse thermal and mechanical effects on electrical equipment - but they are considerably more expensive.
Supply reliability can be improved by ensuring that MCCBs are match the application.
For example, in a data or communications centre where harmonic distortion is high, thermal protection units provide more accurate overload protection than electronic sensing.
Current limiting MCCBs and MCBs can be used to allow an upstream device to provide short-circuit back-up protection to downstream devices.
Known as cascading, this can provide substantial space and cost savings on switchgear and enclosures. MCCBs can achieve full short circuit discrimination on devices of comparatively close current rating.
What is available:
ABB's System proM S280 MCBs provide over-current protection to 100A within a 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 needing a separate panel board.
For higher end industrial or commercial applications, air circuit breakers (ACBs) are often used. ABB's new 'next generation' EMAX ACBs provide protection, network analysis and communications for virtually any electrical application with rated uninterrupted currents from 800 up to 6300A.
The range comes in E1,E2,E3,E4 and E6 frame sizes, with breaking capacities to 150kA at 415V in fixed and withdrawable versions.
Eaton MEM's Memshield 2 15kA MCBs are ideal for use in high-rise commercial buildings where there are often high prospective fault current levels at the final distribution board.
They provide a compact alternative to MCCBs and achieve short-circuit discrimination with downstream devices. Moreover, the device can be converted to an RCBO by fitting an RCD 'pod'. The RCBOs are ideal for 110 V construction sites.
Schneider Electric's Merlin Gerin 1 to 125A current rated MCBs are ideal for distribution boards and loose enclosures. The 15kA C60 device is available up to 63A, B, C or D curves.
The C60 accepts cables to 35mm2 and provides a single- or three-phase compact solution for many applications.
The 125A NG125 MCB has breaking capacities of 25, 36 and 50kA, and is suitable as an outgoer to final circuits fed from panels.
Eaton's adaptable new.Series G MCCBs, independently certified by KEMA to IEC60947-2, are typically 30% smaller than existing units. The range covers 16A - 2,500A in five frame sizes.
The design, plus many accessories, means that the distinction between 'standard' and 'special' devices disappears as all units are assembled, tested and despatched within hours of order receipt. Accessories plug-in, field-fitted easily and safely from the front.
For more information, go to:
- www.abb.com
- www.memonline.com
- www.schneider.co.uk
The picture shows Eaton’s adaptable new Series G MCCBs – independently certified by KEMA to IEC 60947-2. The larger downloadable picture shows a cross-section of an Eaton MCB (186kB file size)
This article was originally published in Electrical Times magazine (June 2005) by Nexas Media Communications - a Voltimum UK Media Partner.
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