content operating principles of rcds types of rcds circuit arrangement of rcds rcd current rating...
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ContentContent• Operating principles of RCDs• Types of RCDs• Circuit arrangement of RCDs• RCD current rating• Wiring rules requirements concerning
RCDs• RCD selection
Inside a Inside a RCDRCD
Inside a Inside a RCDRCD
Toroidal Transformer
Tripping Relay
Trip RelaysTrip RelaysThree Types
• Magnetically Held
• Electronic
• Electro-mechanical
Magnetically HeldMagnetically Held• Magnet holds trip circuit closed against a spring
• Fault causes a current to flow in a coil around magnet
• Magnetic field is reduced
• Spring is stronger than magnetic field and trips breaker
• Polarised
• Saturation
Trips on +ve cycle only
Trips on both +ve and –ve of the cycle
How a RCD Works With a How a RCD Works With a FaultFault
How a RCD Works With a How a RCD Works With a FaultFault
What Must A RCD What Must A RCD Do?Do?• Detect leakage current up to its rated value
• Switch of rapidly when such leakage current is detected
• Ignore leakage currents 50% below its rated value
• Discriminate between earth leakage current and other line disturbances
Causes of Unwanted Causes of Unwanted TrippingTripping
•Standing Leakage Currents
•Electrical Disturbances
• Installation practices & Faults
Standing Leakage CurrentsStanding Leakage Currents
Is insulation is perfect?AS/NZS 3000:2007
Clause 8.3.6.2Page 321
1MΩ
230V= 0.23 mA
Class I appliances may legally have up to 5mA
What about sheathed heating elements?AS/NZS 3000:2007
Clause 8.3.6.2Page 321
0.01MΩ
230V= 23 mA
Electrical Electrical DisturbancesDisturbances
Caused by:
• Lightning strikes
• Switching transients
Natural
Manmade
Lightning StrikesLightning StrikesGenerally cause transients in the form of:
• High Voltage
• High frequency
Switching Switching TransientsTransientsWhat happens when a
magnetic field changes?Generation of:
• High Voltages
• High frequencies
Reduce the Xc of the circuit
f
IX c
cX
VI
Leakage current increases
Two insulated conductors running together
Switching Switching TransientsTransients• Starting induction motors have a very low PF
• High inrush currents
Relay TypesRelay TypesType I
RCDs and relays with a trip current (IΔn)not exceeding 10 mA.
Found in high risk areas such as:
• Hospitals• Doctors rooms• Dialysis rooms• Dentists rooms• Kindergartens
Relay TypesRelay TypesType II
RCDs and relays with a trip current (IΔn)exceeding 10mA but not 30mA
General purpose:As per AS/NZS 3000:2007
Relay TypesRelay TypesType III
RCDs and relays with a trip current (IΔn)exceeding 30mA but not 300mA
Time Delay
But without “Selective Tripping”
Relay TypesRelay TypesType IV
RCDs and relays with a trip current (IΔn)exceeding 30mA but not 300mA
Time Delay
With “Selective Tripping”
But What Type of But What Type of Current?Current?
Type and shape of current will affect the tripping time of an RCD AS/NZS 3000:2007
Clause 2.6.2.2Page 97
AC Guaranteed for only AC operation
A Will take some pulsating DC current
B AC up to 1000Hz, pulsating DC, and DC
S Selective RCD
RCD HousingsRCD Housings
Does not trip due to over current
2 Pole
Connecting a 2 Pole Connecting a 2 Pole RCDRCD
Neutral link Protected Neutral link
4 Pole RCD4 Pole RCD
Single Phase CircuitX2
OR
3 Phase + Neutral
Note
Some RCDs are Polarised
RCD Power PointRCD Power Point
Combination RCD Combination RCD MCBMCB
RCD + MCB
OR
RCD/MCB
= 3 Modules
= 2 Modules
Toroidal RCDOutput Used to supply Trip Coil on a Circuit breaker