Download Electrical Installation Practice 2

Content
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Operating principles of RCDs
Types of RCDs
Circuit arrangement of RCDs
RCD current rating
Wiring rules requirements concerning RCDs
RCD selection
Inside a RCD
Inside a RCD
Toroidal Transformer
Tripping Relay
Trip Relays
Three Types
• Magnetically Held
• Electronic
• Electro-mechanical
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
Trips on +ve cycle only
• Saturation Trips on both +ve and –ve of the cycle
How a RCD Works With a Fault
How a RCD Works With a Fault
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
Causes of Unwanted Tripping
• Standing Leakage Currents
• Electrical Disturbances
• Installation practices & Faults
Standing Leakage Currents
Is insulation is perfect?
230V
1MΩ
AS/NZS 3000:2007
Clause 8.3.6.2
Page 321
= 0.23 mA
Class I appliances may legally have up to 5mA
What about sheathed heating elements?
230V
0.01MΩ
= 23 mA
AS/NZS 3000:2007
Clause 8.3.6.2
Page 321
Electrical Disturbances
Caused by:
• Lightning strikes
Natural
• Switching transients Manmade
Lightning Strikes
Generally cause transients in the form of:
• High Voltage
• High frequency
Switching Transients
What happens when a
magnetic field changes?
Generation of:
Reduce the Xc of the circuit
• High Voltages
I
• High frequencies
X c
f
V
I
Xc
Leakage current increases
Two insulated conductors running together
Switching Transients
• Starting induction motors have a very low PF
• High inrush currents
Relay Types
Type I
RCDs and relays with a trip current (IΔn)
not exceeding 10 mA.
Found in high risk areas such as:
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Hospitals
Doctors rooms
Dialysis rooms
Dentists rooms
Kindergartens
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
Relay Types
Type III
RCDs and relays with a trip current (IΔn)
exceeding 30mA but not 300mA
But without “Selective Tripping”
Time Delay
Relay Types
Type IV
RCDs and relays with a trip current (IΔn)
exceeding 30mA but not 300mA
With “Selective Tripping”
Time Delay
But What Type of Current?
Type and shape of current will affect the tripping
AS/NZS 3000:2007
time of an RCD
Clause 2.6.2.2
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
RCD Housings
2 Pole
Does not trip due
to over current
Connecting a 2 Pole RCD
Neutral link
Protected Neutral link
4 Pole RCD
Single Phase Circuit
X2
OR
3 Phase + Neutral
Note
Some RCDs are Polarised
RCD Power Point
Combination RCD MCB
RCD + MCB
= 3 Modules
OR
RCD/MCB
= 2 Modules
Toroidal RCD
Output Used to supply Trip
Coil on a Circuit breaker