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Chapter 9: Protection Protection is needed for the equipment and the appliances, the human and animal, and their environment Abnormal conditions • Overload • Short circuits (fault level) • Earth faults • Undervoltage • Single phasing of three phase systems • Over voltages and transient surges • Incorrect synchronisations of frequencies • Incorrect phase succession • Reverse power flow Overload happens when the design values of an undamaged circuit are exceeded. It is normally caused by consumers overloading the equipment, and not by failure of equipment. Overloads can normally be handled for short moments A short circuit happens when in the wiring, equipment or appliance the life wire touches the neutral wire. Such a situation leads to big currents depending on the fault level of the supply system An earth fault happens when the life or neutral conductor touches an earthed component of the circuit, equipment or appliance. This situation can lead to consumers being shocked. Low voltage is when the voltage delivered to an appliance is lower as the specified lower limit of the supply voltage. This is normally caused by a high load or conductors that are too long or too thin. This happens whenever a three phase circuit is only supplied with a single/two phase(s), which can lead to damage of appliances. To prevent this three phase systems should be protected by a three phase circuit breaker. Systems sensitive for this, should be protected by a phase monitoring relay. Over voltage can lead to damage to appliances, and so can transient surges. When two or more generators are parallelled they should be synchronised, otherwise they may work against each other Phase rotation refers to the sequence of the phases, which can be clockwise or anti clockwise. The direction a three phase motor is turning is dependant on the phase rotation. If the phase rotation is in the wrong direction, the motor will turn in the wrong direction If a consumer also has a power generator, this power may not be feeded back into the power network – unless permission from the supplier has been obtained. Protective devices: Fuses A fuse is in principle a thin piece of wire that melts when carrying too high a current. It can consist of just the piece of wire, but the wire can also be fixed inside some sort of holder. Sometimes they are put in a substance to increase the time to melt. Fuses are still used as they react very quickly on short circuits, and by doing that reduce the effective fault level A fuse protects against overload and short circuit conditions, and has to be replaced after it melted Protection Equipment: Circuit Breakers A circuit breaker is a piece of equipment designed and manufactured to carry a given current continuously and to trip if that value is exceeded. It can be reset after tripping Three types: • Thermal • Hydraulic Magnetic • Electronic Advantages and disadvantages: thermal Advantages: ● Very reliable ● Cheaper and very simple construction ● Can be mounted in any position Disadvantages ● Sensitive to heat ● When tripped, is has to cool down before it can be switched on again Advantages and disadvantages: hydraulic magnetic Advantages: ● Insensitive to environmental heat ● Can be switched on immediately after a trip Disadvantages: ● More expensive Advantages ● Fully adjustable according to needs Disadvantages: ● More expensive ● Less reliable Regulation: ● Adjustment may only be possible with a tool Advantages and disadvantages: electronic Protection devices: Earth leakage units The earth leakage unit sums the currents of the live conductor(s) and the neutral conductor. If this sum exceeds a given value, the unit will trip. This implies that the neutral conductors going through the ELU should be kept separate from the other neutral conductors. In the UK this device is known as a RCD (Residual Current Device) In the USA it is known as a GFI (Ground Fault Interupter Protective equipment: Earthing systems The principle of earthing is to create a path of lower resistance than a persons body for in case any equipment or appliance would become life, should the person touch it. Clause 5.1.3.2 of SANS 10142-1 stipulates: People, animals and property shall be protected against harmful earth fault currents by protective measures such as: a) earthing and bonding b) electrical separation of circuits c) the use of an isolating transformer with an output of 50V or less; d) the use of equipment that is double insulated With earthing and bonding we mean that all conducting components in a building by means of bonding and earth conductors are kept at the same potential. These conductors are also grounded by means of earth pegs (also called earth electrodes) or an earth mat. Earthing (6.12) The following should be earthed: ● Exposed conductive parts of the installation ● Conductive cable armouring ● Conductive wireways ● Earthing terminal of socket outlets ● Secondary winding of transformers ● Earthing terminal of all permanently connected equipment and appliances ● Conductive parts of discharge luminaires Bonding (6.13) A bonding conductor shall have a minimum cross sectional area of 2,5mm2, and may not be tampered with The following should be bonded to earth, if conductive: ● Hot and cold water systems ● Antennae's (satellite dishes) ● Roofs, gutters, down pipes and waste pipes ● Water pumps and their pipes The resistance of the earth conductor shall not exceed 0,2Ω Protection equipment: Surge/lightning arrestors Surge arrestors (also called lightning arrestors) are being used for both voltage peak trimming and lightning peaks arresting. We have mainly two types: a) Those with a fixed gap between live and earth. A too high voltage will jump the gap and so be earthed b) Those made from zinc oxide, a substance with high resistance when experiencing low voltages, and low resistance when experiencing a high voltage The wiring code distinguishes the following special area’s: ● Bath and washing rooms ● Swimming pools, paddling pools, spa's, ornamental pools, fountains and sauna’s ● Caravan, mobile home and marina sites ● Hazardous locations like fuel stations, wine cellars, coal cellars, spray painting booths and flour mills These zones all have special safety regulations Protection: Special zones Protection: people and the environment Electrical equipment should be mounted in such a way that a standard test finger (12mm dia, 80mm long) is not able to touch any live part during normal operation or when a cover is removed without any tools or a key. Electrical equipment should be appropriately safeguarded against the influence of its environment and should also not endanger its environment. Unless otherwise permitted by an applicable standard electrical equipment shall be so designed, positioned and protected that accessable parts under normal operating conditions do not reach a temperature that exceeds: a) 70°C in the case of metallic parts, and b) b) 90°C in the case of nonmetallic parts SANS 10142-1 § 5.1.2.1 If electrical equipment has to be mounted in a fire risk area or adjacent to flammable material, the equipment shall be a) made from, or enclosed by, thermally non-conductive nonflammable material, or B) so designed or positioned (or both) that the flammable material is not subjected to any hazardous heating, or c) so designed or positioned (or both) that any arcs or sparks are contained within the enclosure SANS 10142-1 § 5.1.4 Electrical equipment, which under normal conditions will be a) exposed to flammable or explosive gas, vapour, dust or liquid, or to external influences such as direct sunlight, corrosive vapour or oil, or b) in a hazardous location, Shall be so selected or enclosed that it is protected against harmfull effects or it shall comply with the requirements of an applicable standard (or both) Certificate of Compliance (CoC Each owner, user or tenant of an electrical installation should have a certificate of compliance (to the requirements of SANS 10142-1) regarding said installation. If there is more than one certificate, the sum of them should cover the complete installation TYPE OF INSTALLTIONS INSTALLED BY CERTIFICATE BY SINGLE PHASE OWNER OR CONTRACTOR SINGLE PHASE TESTER DC,2AND 3 PHASE ELECTRICAL CONTRACTOR INSTALLATION ELCTRICIAN SPACIAL INSTALLATION ELECTRICAL CONTRATOR MASTER ELECTRICIAN In each case the “lowest” level allowed to do a job is indicated. Higher levels may do what lower levels can do General test requirements: ● In the case of multiple tests, the worst result shall be indicated on the certificate ● In case of failure, the failing part shall be corrected, and the test repeated ● Test instruments to have an accuracy of 5% or better and must be regularly calibrated Required tests for certificate of compliance ● Continuity of bonding ● Earth continuity conductor resistance ● Continuity of ring circuits ● Earth fault loop impedance at the main switch ● Voltage of neutral at supply point ● Earth resistance ● Insulation resistance ● Main DB voltage: – no load ● Main DB voltage: – available load ● Main DB voltage: - maximum load ● Earth leakage unit operation ● Earth leakage test button ● Polarity at points of consumption: – Circuit breakers and switches in the phase circuit – Phase terminals connected to phase circuits – Edison screw lamp holder centre contact to phase – Phase rotation correct ● Switches working as expected