Week 6

Submitted by coleen.yan@edd… on Mon, 11/21/2022 - 15:08

Week 6

Sub Topics
  Day One
Course Content SELV & PELV systems and applications. AS/NZS 3000 installation requirements.
Self-directed Learning Consequences of non-compliance with ELV requirements for electrical installations.

In order for you to gain the most value from your qualification and to prepare you for your assessment and the industry, make sure you complete all of the online and SDL tasks.

What we're covering:

  • What PELV and SELV systems are

The Electricity (Safety) Regulations 2010, define extra-low voltage (ELV) as any voltage normally not exceeding 50 volts AC or 120 volts ripple-free DC. According to AS/NZS 3000:2018, ELV electrical installations shall be one of two systems - Separated Extra-Low Voltage (SELV) and Protective Extra Low Voltage (PELV). 

Find the definition of each system in the Wiring Rules and complete the sentences:

Exercise 29


Watch this video to learn about SELV and PELV transformers.
SELV and PELV what is it?

SELV Systems

In a SELV system, the voltage is limited to a level that is considered safe for humans even under fault conditions, such as when a short circuit occurs. This is achieved by using a transformer or power supply that isolates the circuit from the main power supply and provides a low voltage output that is isolated from the mains supply.

A SELV system diagram

Notice there is an earth connection on the primary winding, but not on the secondary winding. It is separated from earth, hence the name – separated extra low voltage. Protective earth conductors are NOT permitted to be installed on the secondary SELV side of the transformer. 
Because there is no earth reference on the Secondary side it is unlikely that a person will receive an electric shock from a single fault on the secondary side. As there is no earth reference, there is no electrical path for the electrical fault current to flow, hence, no shock.

Some SELV transformers do not have an earth connection to the primary side either. These can be identified by the Double Insulated logo on the outside of the casing and the absence of an earth in the mains cable.

A diagram showing the double insulated logo

SELV systems are designed to minimise the risk of electric shock in high-risk situations by limiting the voltage and current levels of the power supply. One typical application is Zone 0 in a swimming pool zone - which is in the water container itself. A transformer is used to reduce the mains voltage to a low voltage of 12 volts AC. The low voltage electrical circuit is then used to power the pool lights, which are installed underwater. The SELV system ensures that even if there is a fault or a failure in the lighting system, the low voltage levels will not pose a risk of electric shock or fire.

SELV systems are also used in:

  • Bathroom lighting.
  • Outdoor lighting.
  • Cordless hand tools.
  • Medical equipment, especially equipment that patients come in contact with, such as electrocardiographs and defibrillators.
  • Telecommunications equipment particularly equipment that is used in wet or humid environments, such as marine communication equipment.

Exercise 30

Discuss the requirements for SELV circuits in terms of insulation and separation from other circuits. Email your findings to your tutor.  (Refer to AS/NZS 3000:2018 Clause 7.5.5 for guidance.)

PELV Systems

PELV also refers to circuits that operate at a voltage level safe for humans to touch. In contrast to a SELV circuit, a PELV circuit can have a protective earth connection. Otherwise the PELV system meets all SELV requirements. Protection by PELV is used where extra-low voltage is required, but the risk of electric shock is much lower than what would be expected for a SELV wiring situation.

A PELV system diagram

PELV systems typically consist of a safety isolation transformer is commonly employed to provide electrical separation from the low voltage wiring system on the primary side, with the addition of circuit protection in the secondary conductors of the ELV circuit. 
A PELV circuit only requires protective separation from all circuits that might carry higher voltages, but it may have connections to other PELV systems and earth.

These circuits are typically used in applications where there is a risk of electrical shock, such as in medical equipment or in areas where water is present. An example of a PELV system is the use of low voltage electrical circuits in a hospital operating room. In this application, a transformer is used to reduce the mains voltage to a low voltage of 120 volts AC. The low voltage electrical circuit is then further reduced to 25 volts AC by a secondary isolating device. The PELV system ensures that even if there is a fault or a failure in the electrical system, the low voltage levels will not pose a risk of electric shock or fire. Other applications for PELV systems include: 

  • Garden lighting.
  • General ELV lighting.
  • Automation busways.
  • ELV machine control circuits.

Exercise 31

Discuss the requirements for PELV circuits in terms of insulation and separation from other circuits. Email your tutor. (Refer to AS/NZS 3000:2018 clause 7.5.5 for guidance.)

Exercise 32

Refer to AS/NZS 3000:2018 Clause 7.5.7 to 7.5.11, and clause 8.3.3.2 to complete this worksheet:

Self-directed Learning

Discuss the consequences of non-compliance with the requirements for extra-low voltage electrical installations, as per Clause 7.5 of the AS/NZS Wiring Rules.

Complete any outstanding assessment tasks for this module.

A person reading information on a tablet device

Glossary

The following is a list of words taken from this resource. To help improve your understanding, research their meanings, or find the definitions in the workbook.  There is space to write the definitions. You could also look up how to say the word – most online dictionaries will demonstrate this.

Glossary

Reference List

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  • Basic Electronics. (2022, August, 29). What are the Types of Switches? (Electrical & Mechanical Switches). [Video] YouTube. https://youtu.be/GlHMOK1AnsQ
  • bigclivedotcom. (2019, June 30). Inside a 13,500A surge protector. (With schematic.) [Video] YouTube. https://youtu.be/wIQYbs09vGg
  • Core Electronics. (2016, December 18). All About Electrical Switches. [Video] YouTube. https://youtu.be/4neVMgdRNCk
  • Dahl, O. N. (2021, March 17). How Transistors Work – A Simple Explanation. https://www.build-electronic-circuits.com/how-transistors-work/
  • Deco Bliss. (2011, February 3). How To Change A Fuse In A Traditional Fuse Box. [Video] YouTube. https://youtu.be/kzpch3wfmjE
  • Donald Allen. (2012, December, 20). Circuit Breakers and Fuses Explanation. [Video] YouTube. https://youtu.be/0ihkfUbddsk
  • Electrical Electronics Applications. (2021, March 9). What is a Thyristor? How Thyristors Work? (Silicon Controlled Rectifier - SCR). [Video] YouTube. https://youtu.be/jIA3fs9rF50
  • Harris, T., & Homer, T. (2022, January 19). How Surge Protectors Work.   https://electronics.howstuffworks.com/gadgets/home/surge-protector1.htm
  • HowToDoItYourself. (2015, July 17). How to Rewire an Australian Ceramic Style Fuse. [Video] YouTube. https://youtu.be/eSOPIXXQ5_w
  • Joe Robinson Training. (2018, October, 25). How Does an RCD Work? [Video] YouTube. https://youtu.be/MfQAoPOKil8
  • Learn Electrics. (2021, April 5).  How RCDs Work. [Video] YouTube. https://youtu.be/6OgaB0BIj18
  • Learn Electrics. (2021, March 1). SELV and PELV what is it? ? Learn the difference between the two types of transformer. [Video] YouTube. https://youtu.be/2_0fZpcDYGw
  • Learn Electrics. (2023, April 30). 3 Wire 2 Way Lighting Circuits – Why You Will Still Come Across Them Today and How They Work. [Video] YouTube. https://youtu.be/YU70PDmnpw0
  • Pete G. (2014, November 14). How Fuses and Circuit Breakers Work. [Video] YouTube. https://youtu.be/xS2-Q9l9_ew
  • RealPars. (2018, July 17). What are the Advantages of PLCs over Relay Systems? [Video] YouTube. https://youtu.be/EWAe8RSNG1g
  • RealPars. (2018, May 15). What is a Relay System? [Video] YouTube. https://youtu.be/1lbdwPfFegY
  • SmartDraw. (2021, October, 30). How to Draw Circuit and Electrical Diagrams with SmartDraw. [Video] YouTube. https://youtu.be/g49JljLE5M4
  • Sparkydave. (2019, April 14). Basics: Wiring a Lighting Circuit. [Video] YouTube. https://youtu.be/QCuQw-GmUEA
  • Sparkydave. (2019, April 15). How To Wire a 2 Way Switch. [Video] YouTube. https://youtu.be/IerkJYNlEc8
  • TechBench. (2019, June 11). What is inside an HRC fuse? (#15) [Video] YouTube. https://youtu.be/85W5KpbbmFw
  • The Engineering Mindset. (2019, January 21). Two Way Switching Explained - How to wire 2 way light switch. [Video] YouTube. https://youtu.be/opoEswRp_jg
  • The Engineering Mindset. (2020, January 19). Diodes Explained - The basics how diodes work working principle PN junction. [Video] YouTube. https://youtu.be/Fwj_d3uO5g8
  • The Engineering Mindset. (2020, September 19). How Relays Work - Basic working principle electronics engineering electrician amp. [Video] YouTube. https://youtu.be/n594CkrP6xE
  • The Engineering Mindset. (2021, March 7). Transistors Explained - How transistors work. [Video] YouTube. https://youtu.be/J4oO7PT_nzQ
  • University of Moratuwa – JRL. (2008, June). https://engineeringnotesonline.com/wp-content/uploads/2020/03/vvbaf.jpg
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NZMA-M00005A
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An electrician working on underwater lights in a pool
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