School of Electrical Engineering and Computer Science - University of Newcastle The University of Newcastle - Australia University > Faculty > School > Safety Manual Home WHS HSMS WHS Committee WHS Changes 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Emergency Procedures Contacts Facilities Summary Defibrulator Operation Building EE Evacuation Buiding EA Level G Building EE Level G Building EE Level 1 Builidng ES Level 1 Builidng ES Level 2 Builidng ES Level 4 Builidng ICT Level 3 Mobile Phones Reporting Accidents Access to Labs General Lab Induction Access Quiz Access Quiz Tutorial Lab Zoning Access Authorisation After Hours Access Check Lab Access Equipment Tagging Lab Info EE Bench Layouts Equipment Machines Lab Computing Lab Hazards General Testing Hardware GPO's 3 Phase Working from Home Soldering Heat Gun Chemical Live Equipment Risk Assessments General info Course related Info Definition of Terms Documents Links Access Quiz Tutorial Tutorial for the General Access Quiz for entry to Electrical Engineering building EE and ES404. Original Feb 2004 The information in this tutorial is aimed at students wishing to complete the General Access Quiz for entry into areas controlled by the Discipline of Electrical and Computer Engineering. This page contains: General Introduction Multiple Earthed Neutral (MEN) Residual Current Devices (RCD) Earthed GPO's Test and Tag Standard Laboratory Equipment - Bench top Power Supplies - Oscilloscopes General Introduction AS/NZS 3000:2000 Australian/New Zealand Standard Electrical installations (known as the Australian/New Zealand Wiring Rules) AS/NZS 3000:2000 standard sets out the minimum requirements for the design, construction and testing of electrical installations including the selection and installation of electrical equipment forming part of such electrical installations. Back To Top Multiple Earthed Neutral (MEN) The electrical distribution system in Australia operates on a Multiple Earthed Neutral (MEN) configuration. An understanding of the MEN system should be gained by searching the internet for Multiple Earthed Neutral, one good reference can be found in Wikipedia. NB. In the United Kingdom, the MEN system is also known as Protective Multiple Earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals - with a similar system in Australia being designated as Multiple Earthed Neutral (MEN). Back To Top Residual Current Devices (RCD) RCD's are often known by other names, e.g., earth leakage circuit breakers (ELCB) or safety switches. An RCD is an electrical safety device specially designed to immediately switch the electricity off when electricity "leaking" to earth is detected at a level harmful to a person using electrical equipment. An RCD offers a high level of personal protection from electric shock. Fuses or overcurrent circuit breakers do not offer the same level of personal protection against faults involving current flow to earth. Circuit breakers and fuses provide equipment and installation protection and operate only in response to an electrical overload or short circuit. Short circuit current flow to earth via an installation's earthing system causes the circuit breaker to trip, or fuse to blow, disconnecting the electricity from the faulty circuit. However, if the electrical resistance in the earth fault current path is too high to allow a circuit breaker to trip (or fuse to blow), electricity can continue to flow to earth for an extended time. RCD's (with or without an overcurrent device) detect a very much lower level of electricity flowing to earth and immediately switch the electricity off. RCD's have another important advantage - they reduce the risk of fire by detecting electrical leakage to earth in electrical wiring and accessories. This is particularly significant in older installations. Residual Current Device (RCD) Fuses Circuit Breakers RCD's work on the principle "What goes in must come out". They operate by continuously comparing the current flow in both the Active (supply) and Neutral (return) conductors of an electrical circuit. If the current flow becomes sufficiently unbalanced, some of the current in the Active conductor is not returning through the Neutral conductor and is leaking to earth. RCD's are designed to operate within 10 to 50 milliseconds and to disconnect the electricity supply when they sense harmful leakage, typically 30 milliamps. The sensitivity and speed of disconnection are such that any earth leakage will be detected and automatically switched off before it can cause injury or damage, an RCD does not protect from phase to phase short circuits. Analyses of electrical accidents show the greatest risk of electric shock results from contact between live parts and earth. Contact with live parts may occur by touching: bare conductors; internal parts of an appliance; or external parts of an appliance that have become "live" because of an internal fault. Contact with earth occurs through normal body contact with the ground or earthed metal parts. An RCD will significantly reduce the risk of electric shock, however, an RCD will not protect against all instances of electric shock. If a person comes into contact with both the Active and Neutral conductors while handling faulty plugs or appliances causing electric current to flow through the person's body, this contact will not be detected by the RCD unless there is also a current flow to earth. On a circuit protected by an RCD, if a fault causes electricity to flow from the Active conductor to earth through a person's body, the RCD will automatically disconnect the electricity supply, avoiding the risk of a potentially fatal shock. Fault current path/s to earth Back To Top Earthed GPO's This MEN information above shows why the Australian electrical distribution system uses a three pin plug. These plugs connect most electrical equipment to a socket-outlet or General Purpose Outlet (GPO) mounted in a wall. Most GPO's will supply 10Amps so when extending a circuit using a power board with multiple sockets, a board with over current protection should be used. The Electrical Laboratories have both Earthed GPO's as discussed in the MEN documentation which are protected by residual current devices and non-earthed, isolated GPO's which are normally not used in our labs. For clarity the bench non-earthed, isolated socket GPO's have been changed to red plastic single socket-outlets and should not be used unless specifically instructed to do so during a laboratory. Below is an example of the correct method of measurement using an oscilloscope powered from a non-earthed, isolated GPO. Using an earth isolated oscilloscope prevents an earth fault occurring by allowing the reference to float when measuring an earth referenced circuit. Note the ref1 connection may cause the chassis of the oscilloscope to rise to a potentially lethal voltage. Touching the chassis could result in electrocution. Do not use these non-earthed, isolated GPO's unless instructed. Only GPO's mounted on benches should be used in the laboratories as these are the only ones protected by residual current devices. No wall mounted GPO's should be used in the Electrical laboratories as these outlets are for use by the cleaners and generally do not have RCD protection. Back To Top Test and Tag All equipment used in the electrical Laboratories must have a valid Test Tag otherwise it should not be used. See example below. Back To Top Standard Laboratory Equipment Bench top Power Supplies The Topward power supply shown above is a standard example of power supplies used in our laboratories. This supply has two floating (with respect to earth ) variable, current limited supplies and one fixed 5V/5A supply. The variable supplies may be configured to operate independently (as two variable supplies), in series or in parallel. The following is a simplified drawing of the Topward power supply above just showing the dual floating supplies. It is plugged into the earthed GPO as shown and is set with both independent supplies to 12V. The earth pin of the mains plug connects to the chassis of the power supply (as a 'Safety Earth') and also to the terminals marked 'B' and 'E'. The terminals 'B' & 'E' are not part of the floating supplies 'A-C ' or "D-F' but may be used to reference them to mains earth if required. If a connection was made between 'CD" it would be possible to obtain the following supplies. Terminal A CD F +12V 0V -12V +24V +12V 0V 0V -12V -24V As you can see the choice of the 0V reference is entirely arbitrary with floating supplies. The same could also be repeated but using the 'B' or 'E' terminal to create an earth referenced supply. e.g. connect 'B-C-D' 0V earth referenced, 'A' +12V wrt earth and 'F' -12V wrt earth. Generally an earth reference is not required in any of the laboratory experiments, but incorrect referencing of your floating supplies can cause in invalid results. Back To Top Oscilloscopes The above sketch shows a two channel oscilloscope (some times referred to as a CRO). As with most lab equipment it should generally be used in an earthed GPO. The probes, 'P1' and 'P2' can be used to take two simultaneous readings and display these graphically. The reference for these readings is the two reference leads which are common (i.e. both probe references are permanently connected together) and also at the chassis potential which in this example is earth. Placing the reference leads of each channel at different points on the circuit will result in a short circuit and probable equipment damage. (see figure below). Also placing the reference lead at a non-earth point in an earthed referenced circuit can also result in damage. (see figure below). Attaching the reference while doing a measurement on a floating circuit may introduce an unintended earth reference and cause other points of the circuit to reach potentially lethal voltages. Back To Top Direct Links Blackboard Info for Staff Info for New Staff Info for Post Grads Info for FYP Students Info for Lab Supervisors Info for Students Check Lab Access Last Updated: 21st February, 2020
