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Electrical Installation Practice 2
LSEGG304A 9080D
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Protection Devices RCDs
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Content 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 RCD
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Inside a RCD Toroidal Transformer Tripping Relay
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Trip Relays Three Types Magnetically Held Electronic
Electro-mechanical
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Magnetically 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 Fault
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How a RCD Works With a Fault
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What Must A RCD 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 Tripping
Standing Leakage Currents Electrical Disturbances Installation practices & Faults
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Standing Leakage Currents
AS/NZS 3000:2007 Clause Page 321 Is insulation is perfect? 230V = 0.23 mA 1MΩ Class I appliances may legally have up to 5mA What about sheathed heating elements? AS/NZS 3000:2007 Clause Page 321 230V = 23 mA 0.01MΩ
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Electrical Disturbances
Caused by: Lightning strikes Switching transients Natural Manmade
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Lightning Strikes Generally cause transients in the form of:
High Voltage High frequency
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What happens when a magnetic field changes?
Switching Transients What happens when a magnetic field changes? Generation of: High Voltages High frequencies Reduce the Xc of the circuit Leakage current increases Two insulated conductors running together
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Switching Transients Starting induction motors have a very low PF
High inrush currents
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RCDs and relays with a trip current (IΔn)
Relay Types Type 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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RCDs and relays with a trip current (IΔn)
Relay Types Type 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 Types Type III RCDs and relays with a trip current (IΔn)
exceeding 30mA but not 300mA But without “Selective Tripping” Time Delay
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Relay Types Type IV RCDs and relays with a trip current (IΔn)
exceeding 30mA but not 300mA With “Selective Tripping” Time Delay
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But What Type of Current?
Type and shape of current will affect the tripping time of an RCD AS/NZS 3000:2007 Clause Page 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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Does not trip due to over current
RCD Housings 2 Pole Does not trip due to over current
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Connecting a 2 Pole RCD Neutral link Protected Neutral link
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4 Pole RCD
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Some RCDs are Polarised
Single Phase Circuit X2 OR 3 Phase + Neutral Note Some RCDs are Polarised
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RCD Power Point
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Combination RCD MCB
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RCD + MCB = 3 Modules OR RCD/MCB = 2 Modules
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Toroidal RCD Output Used to supply Trip Coil on a Circuit breaker
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