What are 9 abnormal conditions in electrical systems?
- 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.
What are my protection devices?
- 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.
What are the types of circuit breakers?
- Thermal: Heat sensitive and needs to ool down before resetting.
- Hyrdaulic: Heat insensitive, ressetable but expensive
- Electronic: Adjustable but more expensive and less reliable
What is the function of an Earth Leakage Unit?
- 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.
What are the common types of Earth Leakage Unit?
- Single Phase (230V)
- Three Phase (400V)
What is the purpose of Earthing Systems?
- Directs fault current to the ground and protects indiviuals.
What are the earthing requirements?
- Exposed conductive parts, cable armouring, and sockets must be earthed.
- Water, antennae, roofs, and water pipes must be bonded.
What should the Earth Resistance be?
Less than or equal to 0.2 Ohms.
What are the types of surge and lightning arrestors?
- Fixed Gap Arrestors: Discharge when voltage surpasses gap.
- Zinc Oxide Arrestors: Resistance decreases with high voltage
What is the purpose of relays?
- It protects against undervoltage, reverse rotation, sychronisation errors and reverse power flow
- Controls circuit breakers to protect equipment.
What are special protection zones?
Areas with high safety regulations and harzardous conditions.
Example: Bathrooms, pools, fuel stations
How should equipment be designed to protect the environment and people?
- Prevent access to live parts
- Temperature limits for metal parts: 70 degrees Celsius
- Temperature for non-metal parts: 90 degrees Celsius
What are the requirements of a Certificate of Compliance?
- Ensures that installations meet SANS 10142-1 standards
What should you test for in a CoC?
- Continuity checks
- Earth resistance
- Voltage checks
- Phase rotation
Accuracy: Test equipment must have less than or equal to 5% error
