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