1. Electrical Installation Practice 2
LSEGG304A
9080D

2. Protection Devices RCDs

3. Content Operating principles of RCDs Types of RCDs
Circuit arrangement of RCDs
RCD current rating
Wiring rules requirements concerning RCDs
RCD selection

4. Inside a RCD

5. Inside a RCD
Toroidal Transformer
Tripping Relay

6. Trip Relays Three Types Magnetically Held Electronic
Electro-mechanical

7. 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

8. How a RCD Works With a Fault

9. How a RCD Works With a Fault

10. 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

11. Causes of Unwanted Tripping
Standing Leakage Currents
Electrical Disturbances
Installation practices & Faults

12. 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

13. Electrical Disturbances
Caused by:
Lightning strikes
Switching transients
Natural
Manmade

14. Lightning Strikes Generally cause transients in the form of:
High Voltage
High frequency

15. 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

16. Switching Transients Starting induction motors have a very low PF
High inrush currents

17. 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

19. 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

21. Relay Types Type III RCDs and relays with a trip current (IΔn)
exceeding 30mA but not 300mA
But without “Selective Tripping”
Time Delay

23. Relay Types Type IV RCDs and relays with a trip current (IΔn)
exceeding 30mA but not 300mA
With “Selective Tripping”
Time Delay

25. 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

26. Does not trip due to over current
RCD Housings
2 Pole
Does not trip due to over current

27. Connecting a 2 Pole RCD
Neutral link
Protected Neutral link

28. 4 Pole RCD

29. Some RCDs are Polarised
Single Phase Circuit X2 OR
3 Phase + Neutral
Note
Some RCDs are Polarised

30. RCD Power Point

31. Combination RCD MCB

32. RCD + MCB
= 3 Modules OR
RCD/MCB
= 2 Modules

33. Toroidal RCD
Output Used to supply Trip Coil on a Circuit breaker