Low-voltage (LV) circuit protection devices are critical for the safety of people and equipment, but significant design and performance improvements in recent years have been relatively few. One clear trend, though, has been towards adding intelligence and connectivity to such devices, as James Hunt reports.
Circuit protection devices protect cables and other electrical equipment from damage through overvoltage or other faults. They also reduce the danger of fire occurring through an electrical fault and help to protect people too.
Fuses still have significant advantages over circuit breakers for some applications, and are relatively low cost and thoroughly reliable. However, this article examines the latest technological developments, so we concentrate here upon circuit breakers. There are three basic types of circuit breaker for domestic, commercial and light industrial use – Miniature Circuit Breakers (MCBs), Moulded Case Circuit Breakers (MCCBs) and Residual Current Devices (RCDs).
Circuit breakers are designed to automatically disconnect on over-current, using either electromagnetic or thermo-magnetic techniques, or sometimes both. Circuit breakers are re-settable and are normally quite easy to troubleshoot.
Circuit breaker characteristics
RCDs – Many people are badly injured or die every year through electric shock. RCDs help prevent this by detecting current flowing to earth. Earth currents are low, so RCDs are very sensitive, most reacting to 30mA earth currents and tripping within milliseconds. The time-delayed type takes longer and is suitable for other applications where such characteristics are required.
RCDs can’t protect against live-neutral shock or overloads, so may be used in conjunction with MCBs or fuses. However, the combined RCD/MCB (RCBO) gives earth leakage and overcurrent protection in one compact unit.
Electrium’s Wylex NHX range of 6kA devices – launched last year – includes updated MCBs and matching RCBOs. The range includes B and C curve MCBs, plus B and C curve single-module 30mA RCBOs with ratings from 6 – 50A. All incorporate the latest Siemens breaker technologies.
MCBs – These, usually with thermal-magnetic operation, are typically rated to 100A.
Small over currents that last only a short time will not disconnect; bigger or longer lasting overloads will disconnect. Short-circuit breaking capacities used to be usually around 10kA, but one trend noticeable immediately is that MCB ratings are now being considerably raised. ABB, for example, provides 15, 26, 36 and 50kA examples. However, for higher ratings, MCCBs are normally be used.
MCCBs – These, with thermal or thermal-magnetic operation, are available up to around 1000A and the trip current may be adjustable in larger ratings. MCCBs can interrupt the current immediately (‘instantaneous trip’) with almost no delay, or – alternatively – after a short time to suit the application.
MCCBs must be chosen carefully so that the instantaneous part does not trip with starting current, which would be at least a nuisance. Standard MCCBs can take a little time to interrupt current, so current-limiting devices may be better for high fault current applications. However, some more costly MCCBs can interrupt high currents much faster to protect electrical equipment. Depending upon the voltage, MCCB ratings have also risen and are now up to 200kA.
To take an example, Hager’s MCCB h3 range is rated from 16 – 1600A in six frame sizes – x160, x250, h250, h630, h1000, h1600, with breaking capacities of 18, 25, 40, 50, 65, and 70kA.Installation is fast using simple-to-fit auxiliaries, and there is a choice of terminals, connections and spreaders. Certain devices feature electronic settings for fault current level and time tripping. A second button selects the protection type, and all units comply with IEC 60947-2.
Zone discrimination
Market evolution and changes in legislation or standards affect circuit protection device design, as with nearly all electrotechnical devices and products. Circuit breakers are no different. It turns out that that there haven’t been any big step changes, but there has been constant evolution.
Zone discrimination techniques – or zone selectivity (ZSI) – are being increasingly applied to MCCBs. Discrimination is the ability of circuit protective devices to operate selectively. If all devices work at the same threshold current, they may trip simultaneously upon short circuit. Discrimination helps prevent this inconvenient and potentially expensive problem. ZSI techniques help achieve a more accurate and increased discrimination range.
For MCCB to MCCB selectivity, ABB uses its Early Fault Detection and Prevention (EFDP) algorithm, a technique that can detect any short-circuit at onset and allows the selectivity limit between two MCCBs to be increased. The advantages are a possible MCCB size reduction, reduced trip times and lower thermal/dynamic stresses. In addition, many hierarchical levels are possible between same size breakers. These are significant advantages, but installation cost and complexity is greater, and extra wiring and components are needed.
ABB says that the EFDP zone selectivity function provided by its PR223EF electronic protection trip unit provides ‘a more complete range of discrimination and coordination possibilities. This contributes greatly to achieving power continuity in a supply network’.
Greater intelligence
One noticeable trend has been increasing use of intelligence in circuit protection devices, provided by better and lower cost microprocessors to add functionality. A good example is the incorporation of smart energy meters into devices. Another is modern communication techniques that monitor circuit protection devices, such as Eaton’s Moeller brand, whose NZM-XSWD-704 SmartWire-DT interface for NZM circuit breakers allows interrogation of switch currents and state, load warnings and tripping causes. Switching can be initiated remotely. The same technique is also applied to the company’s electronic motor protection breakers, providing a range of networked motor starters for control panels.
This trend towards greater intelligence will certainly continue, as will a drive towards industrial networking connectivity for some circuit breakers (and many other LV devices). This is, in part because of advantages such as those mentioned, but also because it will make new generations of devices much easier to integrate into the coming smart grids, in which interconnectivity and real-time data sharing will be crucial aspects.
In another example, Schneider Electric’s Micrologic control units for Compact NS and Masterpact NT MCCBs allow information storage and recording of measurement, monitoring and energy quality events. This helps provide greater productivity and better energy distribution management.
Though now three years old, and with the first amendments having just been published, the introduction of BS7671:2008 (17th Edition Wiring Regulations) had a significant effect on circuit breaker installation right from the outset. The main change was significantly greater use of RCDs and RCBOs.
Under Rule 411.3.3, all 13A socket-outlets, plus those up to 20A, for use by ‘ordinary persons’, must be protected by a 30mA RCD. In bathrooms, RCD protection is also required on all circuits (Rule 710.411.3.3.), including lighting. The only exceptions are for socket outlets under the supervision of ‘skilled’ or ‘instructed persons’. Also, the latest consumer units are generally designed to have more ‘ways’ to accommodate greater RCD use.
The choices are, by now, well known for domestic installations, but for commercial properties there is more to consider. Again RCD protection for socket outlets is almost certainly needed. Even in a small office, there will be many PCs, printers and other equipment likely to have a small earth leakage, which may lead to nuisance tripping. For this reason, there is a need for more circuits supplying socket outlets to avoid nuisance tripping, and the increased use of RCDs and/or RCBOs also means that designers and installers should consider discrimination between RCDs throughout the system.
Wiring accessories containing circuit protection devices are also being introduced to help comply with 17th Edition requirements. Legrand’s Electrak brand Rotasoc desk module system is a good example. This under-desk unit with 360-degree rotatable sockets allows rapid configuration or workstation relocation. The RCD and RCBO protection modules quickly lock into place, providing immediate 17th Edition compliance, says Legrand.
For new applications
Although no completely new circuit protection device technologies are evident, there are new models for evolving markets, such as solar PV and wind turbines. These require DC circuit breakers, such as ABB’s S800 PV series MCBs and disconnectors for networks up to 1200V DC (4-pole), and Eaton’s DC string circuit breakers from 12 – 63A that protect PV modules from fault currents.
This is a fast growing sector, which will benefit switched on electrical contractors and installers who take the trouble to train with an approved training organisation and who can then go on to address this highly important and potentially very lucrative new market sector.
