Earth Electrodes

Earth Electrodes Points : Earth Electrodes, Earth Electrodes, Sizes and Grades of Cables, Size of Main and Sub-main Cables, Size of Final Sub-Circuit Cables, Single or Multi-Core Cables, Which Voltage Grade to Use? The following types of earth electrode are recognized for the purpose of these Regulations:
• Earth rods or pipes,
• Earth tapes or wires,
• Plates,
• Electrodes embedded in foundations.
• Metallic reinforcement of concrete.
• Metallic pipe systems, where not precluded by Regulations 542—14 and 542 — 15.
• Lead sheaths and other metallic coverings of cables, where not precluded by Regulation 542-15,
• Other suitable underground structures.

The type and embedded depth of earth electrodes shall be such that soil drying and freezing will not increase the resistance of the earth electrode above the required value. The materials used and the construction of the earth electrode shall be such as to withstand damage due to corrosion any thing the maximum fault likely to flow through it on the earthed within our installation only and insulated from the earthing arrangements of art:
other installation. In the latter circumstances, if the protective conductor forms part of a cable the protective conductor shall be earthed only in the installation containing the associated protective device. Earth Electrodes The following types of earth electrode are recognized these Regulations: Sizes and Grades of Cables What Size of Cable to Use?
This question is generally solved by choosing the cable which has the nearest “rating above that taken by the load, haying already taken account of temperature, bunching, volt drop etc. However, it is often possible to reduce the size of main, submain and even sometimes final sub-circuit cables by use of a diversity factor. This is based on the assumption that in many installations, not all the connected load would be switched on any one time. A table is given connected load would be switched on any one time. This can be used to expect in different types of installations. This can be used to calculate the rating of fuses and switchgear, as well as cables. It must be emphasized that this is only a guide in the absence of more specific details, and whenever possible the electrical engineer should obtain as many details of the probable loading as he can. Size of Main and Sub-main Cables An example is given below estimating the current demand of one flat in a large block of flats, and the rating of the sub-main cable needed to supply this flat.

Adding say 10% for future additions, this makes a total expected maximum current demand of 76 + 7.6 = 84 amps approximately. Thus the sub-main cable need only have a rating of 84 ampfor t1iis particular installation instead of 121 amps. If however, the same apparatus was installed in an office, the loading would be different and the cable size would probably have to be increased. Size of Final Sub-Circuit Cables For final sub-circuits of 15 amp and over, a diversity factor cannot be applied since they can only supply one point. Thus the cable must have a rating not less than the full load current rating of the connected load. A diversity factor cannot normally be applied to final sub-circuits of under 15 amp. The following table gives the assumed current demand of points normally supplied by a final sub-circuit.

Thus, any cable supplying a final sub-circuit as listed above must have a current rating at least equal to the assumed current demand shown. However, in certain exceptional circumstances, only in non-domestic premises, the cables supplying a final sub-circuit can be reduced in size to allow for certain diversity. This must only be done where the installation has been 4esigned by a competent electrical engineer, and the full loading and requirements are known electrical engineer, and the full loading and requirements are known accurately.

It must be noted that are exceptional circumstances and it. must be made sure before installation that no additional loading will be made at any time, and the consumer must be well, aware that the sockets are not to be for general loading. Single or Multi-Core Cables (The general term “multi-core cable” means a cable. with any number of cores over one i.e. twin three, four core etc.) At some time, the question arises whether it is better to use two single core cables instead of a twin core cable, or three single core cables instead of a three core etc. The choice really depends on: method of installation, degree f protection to cable, ease of installation and cost.

If insulated cables are to be run in conduit for protection and the conduit extends for the whole length of circuit run, then it is not really necessary to install and multi-core cable. A circular multi-core cable costs more than the equivalent number of single core cables. For the same conductor size, 3 or 4 single core cables will carry more current than 3 or 4 multi-core cables when used for three phase a.c., systems. If cables are to be run in closed ducts in the floor or similar enclosures which afford protection against mechanical damage then circular multi-core cables are not necessary.

If a cable run is to be on the surface of a wall or on battens it is always best to run a multi-core cable. Twin and 3 core flat sheathed cables are most suitable for this, The installation is much neater with these flat cables than with a number of single core sheathed cables.

Less work is involved installing them and so installation costs are reduced. The cost of a 3 core flat sheathed cable is approximately the same as for three single core sheathed cables and so there are no cost advantages in installing single core cables.

When burying P.V.C. sheathed cables direct in plaster it is much more convenient and sate to use a twin or 3 core flat cable. If single phase power circuits arc being wired in domestic or commercial buildings by this method, it is best to install a twin core flat P.V.C. sheathed cable with earth continuity conductor enclosed within its sheath. This prevents the wrong size earth continuity conductor being run, as could happen if single core cables were used. Which Voltage Grade to Use? As stated at the beginning of this section, general wiring cables are manufactured in two voltage grades. A summary of the conditions in which each should be used is listed below.
250/440 V grade cable:
Use with single phase a.c. supplied of 250 volts and below.
Use with two and three wire d.c., system where the voltage to earth does not exceed 250 volts or the voltage between conductors does not exceed 440 volts.
Use with a 440 volt a.c. 3 phase 4 wire system having the neutral paint solidly earthed either on or very near the premises, and where the load supplied is not large.
660/1100 V grade cable:
Always use with a 440 volt ac., 3 phase 3 wire system. (i.e., having no neutral).
Use with a 440 volt a.c., 3 phase 4 wire system if the sub station is not on the premises or is some distance away, or the load supplied is large.