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![Page 1: Content Operating principles of RCDs Types of RCDs Circuit arrangement of RCDs RCD current rating Wiring rules requirements concerning RCDs RCD selection.](https://reader035.fdocuments.us/reader035/viewer/2022062516/56649e365503460f94b26185/html5/thumbnails/1.jpg)
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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
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Inside a Inside a RCDRCD
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Inside a Inside a RCDRCD
Toroidal Transformer
Tripping Relay
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Trip RelaysTrip RelaysThree Types
• Magnetically Held
• Electronic
• Electro-mechanical
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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
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How a RCD Works With a How a RCD Works With a FaultFault
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How a RCD Works With a How a RCD Works With a FaultFault
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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
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Causes of Unwanted Causes of Unwanted TrippingTripping
•Standing Leakage Currents
•Electrical Disturbances
• Installation practices & Faults
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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
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Electrical Electrical DisturbancesDisturbances
Caused by:
• Lightning strikes
• Switching transients
Natural
Manmade
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Lightning StrikesLightning StrikesGenerally cause transients in the form of:
• High Voltage
• High frequency
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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
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Switching Switching TransientsTransients• Starting induction motors have a very low PF
• High inrush currents
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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
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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
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Relay TypesRelay TypesType III
RCDs and relays with a trip current (IΔn)exceeding 30mA but not 300mA
Time Delay
But without “Selective Tripping”
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Relay TypesRelay TypesType IV
RCDs and relays with a trip current (IΔn)exceeding 30mA but not 300mA
Time Delay
With “Selective Tripping”
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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
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RCD HousingsRCD Housings
Does not trip due to over current
2 Pole
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Connecting a 2 Pole Connecting a 2 Pole RCDRCD
Neutral link Protected Neutral link
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4 Pole RCD4 Pole RCD
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Single Phase CircuitX2
OR
3 Phase + Neutral
Note
Some RCDs are Polarised
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RCD Power PointRCD Power Point
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Combination RCD Combination RCD MCBMCB
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RCD + MCB
OR
RCD/MCB
= 3 Modules
= 2 Modules
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Toroidal RCDOutput Used to supply Trip Coil on a Circuit breaker