Lesson 9

Question 1

What are 9 abnormal conditions in electrical systems?

Answer 1

• Overload: circuits exceed design values, usually due to excessive use by consumers.
• Short circuit: live wire touches a neutral, resulting in high current.
• Earth fault: Live or neutral conductor contacts an earthed part, creating shock risks.
• Undervoltage: Low supply voltage due to high load or inadequate conductor size.
• Overvoltage and surges: Can damage devices; surge protectors are required.
• Single phasing in three-phase systems: Partial supply, potentially damaging equipment, controlled by phase monitoring relays.
• Incorrect synchronisation of frequences: arallel generators must be synchronized to avoid conflict.
• Incorrect Phase Sequence: Phase rotation impacts motor direction, potentially reversing it.
• Reverse flow power: Prevents backfeeding unless approved by the network supplier.

Question 2

What are my protection devices?

Answer 2

• Fuses: Wires that melt at high current, protecting against overloads and short circuits. It is for single use and must be replaced after activation.
• Circuit breakers: Automatically trips if current exceeds the limit.

Question 3

What are the types of circuit breakers?

Answer 3

• Thermal: Heat sensitive and needs to ool down before resetting.
• Hyrdaulic: Heat insensitive, ressetable but expensive
• Electronic: Adjustable but more expensive and less reliable

Question 4

What is the function of an Earth Leakage Unit?

Answer 4

• Detects current leakage by summing live and neutral currents. It also trips if leakage exceeds safe levels.
• Also known as RCD (Residual Current Device) in the UK.
• Also known as GFI (Ground Fault Interrupter) in the US.

Question 5

What are the common types of Earth Leakage Unit?

Answer 5

• Single Phase (230V)
• Three Phase (400V)

Question 6

What is the purpose of Earthing Systems?

Answer 6

• Directs fault current to the ground and protects indiviuals.

Question 7

What are the earthing requirements?

Answer 7

• Exposed conductive parts, cable armouring, and sockets must be earthed.
• Water, antennae, roofs, and water pipes must be bonded.

Question 8

What should the Earth Resistance be?

Answer 8

Less than or equal to 0.2 Ohms.

Question 9

What are the types of surge and lightning arrestors?

Answer 9

• Fixed Gap Arrestors: Discharge when voltage surpasses gap.
• Zinc Oxide Arrestors: Resistance decreases with high voltage

Question 10

What is the purpose of relays?

Answer 10

• It protects against undervoltage, reverse rotation, sychronisation errors and reverse power flow
• Controls circuit breakers to protect equipment.

Question 11

What are special protection zones?

Answer 11

Areas with high safety regulations and harzardous conditions.

Example: Bathrooms, pools, fuel stations

Question 12

How should equipment be designed to protect the environment and people?

Answer 12

• Prevent access to live parts
• Temperature limits for metal parts: 70 degrees Celsius
• Temperature for non-metal parts: 90 degrees Celsius

Question 13

What are the requirements of a Certificate of Compliance?

Answer 13

• Ensures that installations meet SANS 10142-1 standards

Question 14

What should you test for in a CoC?

Answer 14

• Continuity checks
• Earth resistance
• Voltage checks
• Phase rotation

Accuracy: Test equipment must have less than or equal to 5% error