Lesson 4 - Mechanical Protection and wiring systems PDF

Authorisation A Part 1 Mechanical Protection and wiring systems of Electrical installations Eng. Albert Zerafa Introduction All electrical installation must be protected against mechanical damage. Sometimes this protection...

Authorisation A Part 1 Mechanical Protection and wiring systems of Electrical installations Eng. Albert Zerafa Introduction All electrical installation must be protected against mechanical damage. Sometimes this protection is part of the cable itself, like in the case of armoured cable, sometimes it is additional to the cable, as in the case of single cores cables run in conduit. The type and method of protection will depend on the location of the installation and the risks of damage there may be onsite. Typical mechanical protection for installations can be: Plastic conduit Steel conduit Flexible conduit Plastic trunking Steel trunking Cable tray/Cable ladder Underground in ducts or directly laid Eng. Albert Zerafa 2024 - 2025 2 Common Wiring Systems and Cable Types Eng. Albert Zerafa 2024 - 2025 3 Common Wiring Systems and Cable Types Eng. Albert Zerafa 2024 - 2025 4 Common Wiring Systems and Cable Types Eng. Albert Zerafa 2024 - 2025 5 Wiring Systems Selecting a method of installation In considering the use of any particular method, it A first major consideration of any electrician should be realised that no method can be the ideal upon being on site is the method of installation one under all conditions. Therefore, all the used. This refers to the type of cable enclosure advantages and disadvantages of the systems that is required to hold wires. should be taken into consideration before a decision is Following are some factors that are considered taken. before selecting a method of installation. 1. Type of premises i.e. whether household commercial or industrial. 2. Type of building construction. 3. Cost. 4. Expected life of the installation. 5. Appearance. 6. Future extensions. 7. Adverse conditions e.g. presence of water, inflammable vapours, dangerous gases, mechanical damage, heat, dust, moisture, etc. Eng. Albert Zerafa 2024 - 2025 6 PVC Sheathed System 1. This is a reasonably inexpensive of method of installation. It consists of 2-core + earth, 3-core + earth, 4-core + earth insulated and sheathed cables fixed to the wall without the need of further mechanical protection, unless in special situations. 2. This system is not suitable for the looping-in method of wiring. 3. Where PVC sheathed cables are to pass through floors, walls, partitions or structural steel-work, the hole must be bushed as to prevent abrasion of the sheath on edges. 4. In the sheathed method of installation, quite a good number of joints are to be made at intersections. These joints are to be properly made in special joint boxes. Great care is to be taken to ensure earth continuity at such joint boxes, particularly with the provision of an effective earth terminal. Eng. Albert Zerafa 2024 - 2025 7 PVC Sheathed System 5. PVC sheathed cables are not to be used where the ambient temperature is constantly below 0oC or above 60oC. 6. Cores of PVC sheathed cables from which the sheath has been removed for termination into an accessory, are to be enclosed inside the accessory itself. Advantage: it is relatively cheap. No special tools are needed and easily installed. Disadvantages: Eyesore if clipped directly to wall. Eng. Albert Zerafa 2024 - 2025 8 Plastic and Metal Conduit 2. Plastic conduits must not be used where the A system of conduits into which unsheathed cables ambient temperature or the temperature of the can be drawn has long been a standard method for enclosed cables will exceed 60°C. Cables with electrical installations. The Regulations applying to thermo-setting insulation are permitted to run very conduit systems may be summarised as follows: hot, and must be suitably down-rated when installed in plastic conduit. 1. All conduits and fittings must comply with the relevant British Standards. 3. Conduit systems must be designed and erected so as to exclude moisture, dust and dirt. This means that they must be completely closed, with box lids fitted. To ensure that condensed moisture does not accumulate, small drainage holes must be provided at the lowest parts of the system. 4. The conduit must be installed so that fire cannot spread through it, or through holes cut in floors or walls to allow it to pass. Eng. Albert Zerafa 2024 - 2025 9 Plastic and Metal Conduit 5. Proper precautions must be taken against the 8. Allowance must be made, in the form of effects of corrosion, as well as against the effects of expansion loops, for the thermal expansion of flora (plant growths) and fauna (animals). Protection long runs of metal or plastic conduit. Remember from rusting of steel conduit involves the use of that plastic expands and contracts more than galvanised (zinc coated) tubing, and against steel. electrolytic corrosion the prevention of contact 9. Use flexible joints when crossing building between dissimilar metals e.g. steel and aluminium. expansion joints. 6. A conduit system must be completely erected before cables are drawn in. It must be free of burrs or other defects which could damage cables whilst being inserted. 7. The bends in the system must be such that the cables drawn in will comply with the minimum bending radius requirements. Eng. Albert Zerafa 2024 - 2025 10 Non-metallic conduit – (PVC conduit) 1. Non-metallic conduit is usually made of PVC, either rigid or flexible and is found in the same sizes as steel conduit. The most common sizes are however, the 20 mm and 25mm external diameter. 2. The rigid conduit is normally bent by the use of a special bending spring of the correct size. When bending, it is required to give the bend a considerable overset to allow for the tendency of PVC conduit to straighten out after bending. As in the case of metal conduit, the internal radius of every bend in non-metallic conduit shall be not less than 2.5 times the external diameter of the conduit. 3. When fixing this type of conduit on the surface, it is important that if frequent variations in temperature are likely to occur, the saddles shall allow for the longitudinal expansion of the pipe. Eng. Albert Zerafa 2024 - 2025 11 Non-metallic conduit 4. Many of the fittings used in PVC conduit are of Advantages: the plain-slip type i.e. they not threading. A PVC conduit is not required to be earthed i.e it is sealing compound to give either a semi- shock free. It is cheaper than metallic conduit. permanent or permanent seal is normally used. No special tools are required its installation. (Glue such as Tanget). No burrs are left after cutting. No threads are needed. 5. Flexible PVC conduit is ideal to pass round Light to handle. obstacles so as to save time when working in Condensation inside the conduit is minimal. awkward positions. This should not however be PVC resists most industrial liquids. used for undue length. Fast to assemble. Disadvantages of the system: Not as robust as steel conduit although extra heavy gauge is also manufactures. (high impact). Not suitable for temperature below -25oC or above 60oC. Eng. Albert Zerafa 2024 - 2025 12 Heavy Gauge Steel Conduit System 1. This method consists of steel tubing through which During threading a suitable type of thread cutting PVC insulated cables are normally drawn. lubricant should be used. It is of the utmost This type of conduit is made from thick material importance that after cutting and threading all sharp and can therefore resist great mechanical burs left inside the conduit are properly reamed off damage. by means of a reamer. Lengths of conduit should be tightened up by 2. The heavy gauge steel conduit is available either in means of pipe wrenches. welded seam tube or in a solid drawn type. Common outside diameters conduits are 20mm, 4. Neat bends and sets are made by the use of a 25mm and 32mm. One may even find larger sizes. bending machine. Sharp bends should always Metal conduit is delivered in either black enamel or be avoided as these will always complicate the galvanised to protect against drawing in of cables. The use of inspection boxes rusting. will make it easier to draw in cables. 3. Conduit should be cut with a fine toothed 5. The cables should always be drawn in hacksaw blade (24 or 32 TPI), while holding the simultaneously, if not, damage to the existing pipe in a pipe vice. Threading is made by means of cables will result due to the friction between the two. conduit type stock and dies of the proper size. A special steel or nylon draw wire is usually used for drawing in cables Eng. Albert Zerafa 2024 - 2025 13 Heavy Gauge Steel Conduit System Disadvantages: 6. When buried, conduit is fixed to the wall by means of crampets (grampuni), but when fixed It is liable to corrosion. to the surface, any one of different types of It is liable to condensation. saddles shall be used. In the latter case, the It is more expensive than the PVC conduit or space should be such that sagging of the pipe is sheathed cable system. avoided. When the conduit is buried, care should be It requires the use of more tools than in taken that plaster mixtures shall not contain other systems. corrosive substances e.g. lime. Time consuming. Advantages: Provides great protection against mechanical damage. Durable. Has a low fire risk. It can be buried safely. Eng. Albert Zerafa 2024 - 2025 14 Metallic Conduit Accessories Zinc: Zinc is a non-ferrous metal which is used mainly to protect steel against corrosion and in making the alloy brass. Mild steel coated with zinc is called galvanized steel, and this coating considerably improves steel’s resistance to corrosion. Conduit, trunking, cable tray, SWA, outside luminaires and electricity pylons are made of galvanized steel. Eng. Albert Zerafa 2024 - 2025 15 Steel conduit Accessories Saddles, bushes, sockets, crampets and locknuts Terminal, and tee boxes and socket outlet boxes Eng. Albert Zerafa 2024 - 2025 16 Specialist tools for steel conduit Eng. Albert Zerafa 2024 - 2025 17 Other Metal Conduit tools and Equipment Eng. Albert Zerafa 2024 - 2025 18  Flexible Steel conduit 4. Separate c.p.c. (earth) is required, as the As its name implies, this type of conduit is designed to be conduit itself does not provide efficient earth bent into any shape without the use of a machine. continuity. 2. Flexible steel conduit consists of a spirally wound, 5. Special brass, aluminium, or stainless steel partially interlocked light gauge galvanized steel strip. It adaptors are used to connect this type of conduit can also be obtained with an overall PVC sheath. This to rigid conduit fittings, switchgear, and machines. conduit provides a useful means of protecting the final connections to motors and other machines, where the transmission of vibration is to be minimised. 3. Flexible metal conduit should be used only for short runs, where mechanical damage is unlikely to occur. Where necessary, it should also be supported. Eng. Albert Zerafa 2024 - 2025 19 Plastic Trunking 1. An installation system used in commercial offices 4. The cables to space factor must not exceed and domestic houses where it is not possible to 45%. install buried conduit while cables are to be aesthetically enclosed. Advantages: The system consists of a high impact PVC single or multi compartment channels, supplied with Easy to handle (light in weight). a continuous clip-on lid. No special tools needed. Corrosion resistant. 2. Fittings such as tees, elbows, corners, etc, are Nonconductive, therefore cannot become live. provided. Different versions of trunking are available. Certain Material does not support combustion. These include skirting, cornice, and architrave and under-floor trunking. Disadvantages: 3. When supporting PVC trunking, care shall be taken Expensive. to allow for the longitudinal expansion in high Cannot be buried. temperatures (max 60oC). A 1.5mm per meter run A separate c.p.c. is needed. will normally suffice. Great care shall be taken when Restricted to a low temperature limit. using solvent adhesives. Contact with the skin is to be avoided and enough ventilation has to be provided. Eng. Albert Zerafa 2024 - 2025 20 Plastic trunking Installations Eng. Albert Zerafa 2024 - 2025 21 Underground installation – Armoured cable Eng. Albert Zerafa 2024 - 2025 22 Support spacing for cables - Ref : OSG Appendix D, p141 onwards Eng. Albert Zerafa 2024 - 2025 23 Spacings of supports for conduits Eng. Albert Zerafa 2024 - 2025 24 Spacings of support for cable trunking Eng. Albert Zerafa 2024 - 2025 25 Provisions for conduit and trunking against fire 1. Where trunking or conduit passes through walls or floors, the hole cut must be made good after the first fix on the construction site to give the partition the same degree of fire protection it had before the hole was cut. 2. Since it is possible for fire to spread through the interior of the trunking or conduit, fire barriers must be inserted as shown in {Fig 4.18}. An exception is conduit or trunking with a cross-sectional area of Fig 4.18 Provision of fire barriers in ducts and trunking less than 710 mm², so that conduits up to 32mm in 3. During installation, temporary fire barriers diameter and trunking must be provided so that the integrity of the fire up to 25 mm x 25 mm need not be provided with prevention system is always maintained. All fire barriers. trunking must be non-flame propagating to BS EN 50085. Eng. Albert Zerafa 2024 - 2025 26 Provisions for conduit and trunking against fire 4. Since trunking will not be solidly packed with cables there will be room for air movement. A very long vertical trunking run may thus become extremely hot at the top as air heated by the cables rises; this must be prevented by barriers as shown in figure 4.19. In many cases the trunking will pass through floors as it rises, and the fire stop barriers needed, will also act as barriers to rising hot air. Fig 4.19 Eng. Albert Zerafa 2024 - 2025 27 Cable Tray/Cable Ladder Eng. Albert Zerafa 2024 - 2025 28 Mineral Insulated Metal Sheathed Cables Mineral Provided that it is kept dry, a mineral insulation such as magnesium oxide is an excellent insulator. Since it is hygroscopic (it absorbs moisture from the air) this insulation is kept sealed within a copper sheath. The resulting cable is totally fireproof and will operate at temperatures of up to 250°C. It is also entirely inorganic and thus non-ageing. These cables have small diameters compared with alternatives, great mechanical strength, are waterproof, resistant to radiation and electromagnetic pulses, are flexible and corrosion resistant. In cases where the copper sheath may corrode, the cable is used with an overall LSF covering, which reduces the temperature at which the cable may be allowed to operate. Since it is necessary to prevent the ingress of moisture, special seals are used to terminate cables. Eng. Albert Zerafa 2024 - 2025 29 MIMS cable Eng. Albert Zerafa 2024 - 2025 30 Cable capacities of conduit ref: OSG p161 Single-core thermoplastic (PVC)insulated cables in straight runs of conduit not exceeding 3 m in length Eng. Albert Zerafa 2024 - 2025 31 Cable capacities of conduit ref: OSG p161 Single-core thermoplastic (PVC)insulated cables in straight runs of conduit exceeding 3 m in length, or in runs of any length incorporating bends or sets Eng. Albert Zerafa 2024 - 2025 32 Single core Cable capacities of trunking ref: OSG P161 Eng. Albert Zerafa 2024 - 2025 33