Industry Control & Protection
At the time that he worked on the magamp, Rubin also considered using transistors in this application, but concluded that the early transistors then available were too unreliable. However, with the advent of improved transistors, the company that he worked for and other companies later produced transistorized versions of earth leakage protection.
In , Dalziel, working with Rucker Manufacturing Co. This name for high-sensitivity earth leakage protection is still in common use in the U. GFCIs built into the outlet receptacle became commonplace beginning in the s. The circuit breaker type, installed into a distribution panel , suffered from accidental trips mainly caused by poor or inconsistent insulation on the wiring.
False trips were frequent when insulation problems were compounded by long circuit lengths. So much current leaked along the length of the conductors' insulation that the breaker might trip with the slightest increase of current imbalance. The migration to outlet receptacle based protection in North American installations reduced the accidental trips and provided obvious verification that wet areas were under electrical code -required protection. European installations continue to use primarily RCDs installed at the distribution board, which provides protection in case of damage to fixed wiring; In Europe socket-based RCDs are primarily used for retro-fitting.
Regulations differ widely from country to country.
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In most countries, not all circuits in a home are protected by RCDs. If a single RCD is installed for an entire electrical installation, any fault may cut all power to the premises. In Australia, residual current devices have been mandatory on power circuits since and on light circuits since All socket outlets and lighting circuits are to be distributed over circuit RCDs.
A maximum of three subcircuits only, may be connected to a single RCD. It has been required in private housing since The maximum activation time must not exceed 0.
It needs to be installed on all circuits with power plugs with a maximum leakage current of 30 mA and a maximum rated current of 16 A. Additional requirements are placed on circuits in wet areas, construction sites and commercial buildings. Belgian domestic installations are required to be equipped with a mA residual current device that protects all circuits.
Furthermore, at least one 30 mA residual current device is required that protects all circuits in "wet rooms" e. Electrical underfloor heating is required to be protected by a mA RCD. These RCDs must be of type A. Since NBR residual current devices and grounding are required for new construction or repair in wet areas, outdoor areas, interior outlets used for external appliances, or in areas where water is more probable like bathrooms and kitchens.
Denmark requires 30 mA RCDs on all circuits that are rated for less than 20 A circuits at greater rating are mostly used for distribution.
RCDs became mandatory in for new buildings, and then for all buildings in Since 1 May , RCDs are mandatory for all rooms with a bath tub or a shower. Since June Germany requires the use of RCDs with a trip current of no more than 30 mA on sockets rated up to 32 A which are for general use. It isn't allowed to use type "AC" RCD's since , to be used to protect humans against electrical shocks. It must be Type "A" or type "B". The Italian law n.
The law was recently updated to mandate at least two separate RCDs for separate domestic circuits. Short-circuit and overload protection has been compulsory since In the latest guidelines for electrical wiring in residential buildings handbook  , the overall residential wiring need to be protected by a residual current device of sensitivity not exceeding mA. Additionally, all power sockets need to be protected by a residual current device of sensitivity not exceeding 30mA and all equipment in wet places water heater, water pump need to be protected by a residual current device of sensitivity not exceeding 10mA.
From January , all new circuits originating at the switchboard supplying lighting or socket outlets power points in domestic buildings must have RCD protection. Residential facilities such as boarding houses, hospitals, hotels and motels will also require RCD protection for all new circuits originating at the switchboard supplying socket outlets.
These RCDs will normally be located at the switchboard. They will provide protection for all electrical wiring and appliances plugged into the new circuits. In North America socket-outlets located in places where an easy path to ground exists—such as wet areas and rooms with uncovered concrete floors—must be protected by a GFCI.
Beginning with underwater swimming pool lights successive editions of the code have expanded the areas where GFCIs are required to include: construction sites , bathrooms and outdoor areas , garages , areas near hot tubs or spas , hotel bathrooms , kitchen counter sockets , crawl spaces and unfinished basements , near wet bar sinks , near laundry sinks  and in laundry rooms GFCIs are commonly available as an integral part of a socket or a circuit breaker installed in the distribution panelboard.
GFCI sockets invariably have rectangular faces and accept so-called Decora face plates, and can be mixed with regular outlets or switches in a multi-gang box with standard cover plates. In such cases the sockets must be labeled "no equipment ground" and "GFCI protected"; GFCI manufacturers typically provide tags for the appropriate installation description. RCDs with trip currents as high as mA are sometimes deployed in environments such as computing centers where a lower threshold would carry an unacceptable risk of accidental trips.
These high-current RCDs serve for equipment and fire protection instead of protection against the risks of electrical shocks.
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The greater values are intended to provide protection while minimizing nuisance trips. In Norway, it has been required in all new homes since , and on all new sockets since The RCD must trigger after a maximum 0. South Africa mandated the use of Earth Leakage Protection devices in residential environments e. Protection is required for power outlets and lighting, with the exception of emergency lighting that should not be interrupted. Normal practice in domestic installations was to use a single RCD to cover all the circuits requiring RCD protection typically sockets and showers but to have some circuits typically lighting not RCD protected.
Protection arrangements for other circuits varied.
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To implement this arrangement it was common to install a consumer unit incorporating an RCD in what is known as a split load configuration, where one group of circuit breakers is supplied direct from the main switch or time delay RCD in the case of a TT earth and a second group of circuits is supplied via the RCD. This arrangement had the recognised problems that cumulative earth leakage currents from the normal operation of many items of equipment could cause spurious tripping of the RCD, and that tripping of the RCD would disconnect power from all the protected circuits.
The current edition 18th of the regulations requires that all socket outlets in most installations have RCD protection, though there are exemptions. Non armoured cables buried in walls must also be RCD protected again with some specific exemptions. Provision of RCD protection for circuits present in bathrooms and shower rooms reduces the requirement for supplementary bonding in those locations.
Two RCDs may be used to cover the installation, with upstairs and downstairs lighting and power circuits spread across both RCDs. When one RCD trips, power is maintained to at least one lighting and power circuit. Other arrangements, such as the use of RCBOs, may be employed to meet the regulations. The new requirements for RCDs do not affect most existing installations unless they are rewired, the distribution board is changed, a new circuit is installed, or alterations are made such as additional socket outlets or new cables buried in walls. RCDs used for shock protection must be of the 'immediate' operation type not time-delayed and must have a residual current sensitivity of no greater than 30 mA.
If spurious tripping would cause a greater problem than the risk of the electrical accident the RCD is supposed to prevent examples might be a supply to a critical factory process, or to life support equipment , RCDs may be omitted, providing affected circuits are clearly labelled and the balance of risks considered, this may include the provision of alternative safety measures. From Wikipedia, the free encyclopedia. For the financial center ranking, see Global Financial Centres Index.
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This section possibly contains original research. Register Your Account. Login or Register to comment and create posts. Forgot your username or password. By logging in, you automatically agree to the Terms and Conditions of this website. Log in. Register My Account. Breaking Capacity of Circuit Breaker. Hydraulic-Magnetic Circuit Breakers and Operation. Timber normally comes in running lengths, that is, not sorted for length. Building materials of any type which are loaded are said to be subjected to a fibre stress. The safe fibre stress for a material is the load which the material will safely resist.
Table 3. There are dozens of additional species of trees found in East Africa many of which are used only in very local areas. In order to obtain approximate working stress data for these indigenous species, their densities may be used to place them in the proper group for Table 3. If the density is not known, a good approximation can be found quite easily.
A bucket, a graduated cylinder millilitres and an accurate scale for weighing a sample of the wood will be needed. The procedure is:. For design purposes these should be adjusted for a number of different variables including: grade, moisture content, duration of load, exposure, and use of the structure. Table values need to be reduced when timber is installed green and will remain wet and uncured continuously.
Use Figure 3. The main structural softwood timbers of East and Southeast Africa are not naturally durable. If used in conditions subject to fungal, insect or termite attack, they will fail after some time. To avoid this, the timber used in permanent structures should be treated with a preservative.
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Effective preservation depends on the preservative and how it is applied. An effective preservative should be poisonous to fungi and insects, permanent, able to penetrate sufficiently, cheap and readily available. It should not corrode metal fastenings, nor should the timber be rendered more flammable by its use.
It is sometimes desirable to have a preservative-treated surface which can be painted. Where the above conditions are not present, however, there will be a risk of fungal decay, and proper preservation is recommended. Creosote is an effective general purpose preservative, cheap and widely used for exterior work and to a lesser degree inside. It is a black to brownish oil produced by the distillation of coal-tar, and has many of the properties required of a preservative, but it increases flammability, is subject to evaporation, and creosoted wood cannot be painted.
It should not be used on interiors if the characteristic smell would be objectionable. Unfortunately creosote has been found to be a carcinogen and must be used with caution. Coal-tar as a preservative is not as effective as the creosote produced from it. Tar is less poisonous, it does not penetrate the timber because of its viscosity, it is blacker than creosote and it is unsuitable for interior wood work. Unleachable metallic salts are mostly based on copper salts.
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The copper and arsenical salt are the toxic preservatives which are rendered nonleaching cannot be washed out by the chrome salt acting as a fixing agent. The timber is impregnated by a "vacuum-pressure" process. Preservation by metallic salt is being increasingly used since the treated surfaces are odourless and can be painted or glued. Water-soluable preservarives are not satisfactory for exterior use, as they are liable to be removed from the timber by rain. They are, however, very suitable for interior work, as they are comparatively odourless and colourless, and the timber can be painted.
Used engine oil can often, at least in small quantities, be obtained free of charge. The oil contains many residual products from combustion and some of them will act as preservatives, but it is not nearly as effective as commercial preservatives.
It can be thinned with diesel fuel for better penetration. The combination of of used engine oil and 11 of Dieldrin is a viable alternative in rural construction. Creosote is the main preservative used, but unreachable metallic salts are also commonly applied by this method. Water-borne preservatives must be applied with the pressure treatment if the timber will be exposed to rain or ground moisture. Surface-applied water-borne preservatives quickly leach away leaving the timber unprotected.
The tank with the preservative and timber is heated to nearly boiling, held for one to two hours and then allowed to cool. During the heating period the cells and the air in the cells expand and some of the air is expelled. As the timber and preservative cool the timber contracts and the partial vacuum created causes the liquid to be gradually absorbed into the timber. Creosote or metallic salts can be applied by these methods.
None of these surface treatments are as effective as the pressure and open-tank systems, as the preservative only penetrates the timber slightly. The wood must be seasoned and the surface should be dry and clean before application. Greater penetration generally results if the preservative is applied hot, especially if creosote is used. The timber should have two coats at least; the first coating allowed to dry before the next is applied.
Creosote is the most common preservative used for this method. Superficial treatment with clear liquids is not recommended since the proper application is difficult to control. There are a number of building boards made from wood veneers or the waste products of the timber industry that are convenient and economical materials to use in building construction. In general, they offer excellent bracing for the building frame and a saving in labor because they are available in large sizes requiring a minimum of fitting.
Some manufactured boards are designed with rather specific characteristics such as fire resistance, ease of cleaning, high insulating value or resistance to weathering. Plywood is produced by gluing together three to seven veneers that have been peeled from logs.