Sparks Electrical News April 2020

CONTRACTORS’ CORNER

5

WORKING KNOWLEDGE WITH TERRY MACKENZIE HOY

E very so often somebody (normally a farmer) has a need to get an electrical power supply to a distant point. It is normally for the sup- ply for a pump or worker’s cottage. The supply can be single-phase or three-phase. Depending on the current and the length of the cable, the volt drop from the sending end of the cable to the receiving end can be such that the voltage at the receiving end is too low to supply the load. A volt drop of 10% is borderline: more than this will cause problems. If you want the minimum voltage drop it is best to use a three-phase system (which requires a four core cable) since the current in any conductor for a three-phase system is less than a single-phase system (which requires a three core cable, live, neutral and earth). For example, for a 5 kW load in a single-phase system the current is about 5000 W/220 A = 22,7 A. For a three-phase system the current is about 5000 W/380 V/1.732 = 7,59 A. However, four core cables are more expensive than three core cables Getting power to distant places

is best to increase the voltage of the power line. To do this a single-phase 220 V/1000 V step-up transformer is used at one end and a 1000 V/ 220 V step-down at the other end (for three phase it would be 380 V/1000 V three-phase). Note well that in doing all this you must get a compliance certificate from a registered electrician. A final way of supplying very, very far distant loads is to use a single wire earth return system (SWER). This is, by definition, a single-phase system with one conductor strung between poles and the neutral created by using the ground as a conductor. Here the power line voltage has to be at least 6600 V. SWER lines in southern Africa can be very long (~ 10 km). It hardly has to be said that electricity kills and in RSA deaths and fires result from people trying to steal copper or aluminium cables. Cables in the ground are protected to some degree. On power poles they are not. The possibility of injury, death or theft must always be considered…

is a trap here – the power line has to be constructed (it can’t be bought off the shelf) and it is very easy to mess up the construction of the power line. It would be thought that power line construction is not difficult, but this is deceptive: between power line pole cross arms the conductor hangs in a parabolic shape called a catenery. From one cross arm of one pole of the power line to the other the catenery will exert a certain weight. This weight must be balanced by the weight of the next catenery. If the ground is level and the poles evenly spaced there will be no problem. If not, the poles will pull each other towards each other. You can see this everywhere in this country. The simplest power line has a live and a neutral and the poles are earthed with grounding wires. The power line conductors are aluminium stranded reinforced conductor (ASCR), commonly available from suppliers. A three-phase power line has three conductors and a neutral. If it is desired to supply a load which is really, really far (say 2 km) then it

so a balance has to be struck between costs and volt drops. When the distance between the load and the re- ceiver is significant, a cable can’t be used – for example a 35 mm 2 three core cable supplying 80 A at 200 m would have a 10% voltage drop. At a greater distance the 35 mm 2 cable won’t do the job and a bigger cable has to be used. Cable volt drop is caused by the resistance and in- ductance of the cable. Thus, if a power line and not a ca- ble is used to supply the load, things get simpler since the inductance of the power line is not that great. There 12 UPS systems provided for local coal mine supplier BECKER MINING SOUTH AFRICA has supplied 12 uninterruptible power supply (UPS) systems to a lo- cal coal mine supplier to enhance safety and prevent unnecessary downtime. The specially-designed UPS systems ensure uninterrupted operation of the mine’s programmable logic controller (PLC), particularly during power outages. “The UPS system was developed by Becker Min- ing specialists, as a dependable power back-up system for the PLC system, which controls critical functions at the mine. These operations include all installed conveyer belt safety systems, such as gas monitoring and speed, slippage and load control in- strumentation,” explains Nico de Lange, Vice Presi- dent: Operations & Systems, Becker Mining South Africa. “In the event of a power outage, the PLC would be unable to function without the back-up of a reliable UPS system. The interruption of belt sys- tem and sensor monitoring would prevent normal operations and lead to costly downtime, and could also become a major safety issue.” During a power outage, the UPS system needs to supply the specified load to ensure the electri- cal system continues efficient operation, for a speci- fied time period. In this project, the 3.5 kVA system, with a two-hour back-up time, has been designed to supply a full load current of 15 A at 220 V. This ensures the PLC system, with multiple outlets, runs efficiently in the event of a power failure. As the PLC system consists of many electronic devices, a pure sign wave is required to reduce in- duced electrical noises generated from the circuit. Without this critical feature, electronic components could malfunction and become a major safety risk, also causing costly downtime. The enclosures of Becker’s UPS systems have an International Protection rating of IP 65, which guards against the ingress of dust and jets of water. This protection feature is particularly important in arduous underground conditions. The inter-connected UPS systems, with Ethernet and RS232 connectivity, can be controlled or re- motely managed and monitored from the control- station. This means in the event of a system-failure, critical decisions can be made ahead of schedule, to prevent a disaster or unnecessary downtime. Becker Mining South Africa offers a technical ad- visory and support service throughout the country to ensure total reliability of its extensive range of equipment.

Enquiries: +27 (0)11 617 6300

SPARKS ELECTRICAL NEWS

APRIL 2020