Protection And Installation Flashcards
When is the additional protection of a fixed setting RCD with a rated operating residual current not exceeding 30 mA required?
Where excessive Earth leakage current in the event of failure of other measures of protection or carelessness of users could present significant risk of electric shock.
AS/NZS 3000: 2.6.1
Define overvoltage
Overvoltage is caused by lightning discharge or switching operations. A more serious but less common cause of overvoltage is a breakdown in insulation between low voltage and high voltage.
What is the purpose of a residual current protection device?
To rapidly cut off power to the circuit when measured conditions in the circuit indicate that a current is at a level likely to cause harm.
Give an example of nuisance tripping in a domestic installation.
Small appliances such as toasters and heaters are a common source of nuisance tripping.
Why is it important to limit the number of socket outlets on a circuit for the connection of Class 1 appliances?
To avoid nuisance tripping.
Describe what is meant by the term maximum demand?
Maximum demand is the continuous current required to supply an installation or circuit reliably and economically.
Name three factors to consider when selecting cables to meet voltage drop limitations?
Load current, conductor size, route length of Circuit.
What limitations do voltage drop and earth fault loop impedance place on a circuit?
Voltage drop and earth fault loop impedance limit the maximum route length of a circuit for a given conductor size, load current and protective device.
List four advantages of an hrc fuse link?
High rupturing capacity, operating speed, non deteriorating in service, reliable discrimination.
Why must the impedance of the Earth fault loop of a circuit be low?
To limit the rise in touch voltage as required by the wiring rules. (1.5.5.3 and 2.4.2)
List four types of protective device that will provide protection against overload and Short Circuit currents.
Circuit breakers, fuse combinations, fuses having enclosed fuse links, circuit breakers in conjunction with fuses. AS/NZS 3000: 2.5.2 (a to d)
Describe a switched fuse.
A switched fuse is a switch containing a fuse usually mounted in a carrier. The fuse and carrier physically move when switching occurs.
What adjustments can be made to overcurrent protection relays?
Tripping characteristics for coordination of discrimination with upstream and downstream protection devices and motor overload protection relays.
Describe the working of the magnetic element of a modern circuit breaker.
The magnetic element causes an almost instant trip of the circuit breaker when it detects a high inrush current as in the case of a short circuit.
What is the primary function of a circuit breaker?
To open a circuit automatically as a result of predetermined conditions such as a Short Circuit or overload current fault.
