Week 5 Flashcards
Cabling must: (2)
1• carry permanent full load current and short time over current
2- it shouldn’t have voltage drop cuz it results in poor performance like long acceleration period when starting a motor
What is IZ?
It’s current carrying capacity
It is the maximum permissible that the cabling of the circuit can carry without reducing it normal life expectancy
The current of a cross sectional area depends one (4):
1 - constitution of cable and cable way
2- ambient temperature
3- method of installation
4- influence of neighbor circuits
Define the: design current / current carrying capacity of the cable / rating current of protective device-
I B:
The actual current drawn by the load under normal operating conditions
Iz:
The maximum continuous current that a cable can safely carry without overheating
In:
The nominal current rating of a circuit breaker or fuse used for protection
The purple of:design current / current carrying capacity of the cable / rating current of protective device-
Ib:
Ensure the circuit is able to handle the expected load
In:
Prevent overheating the cable or insulation fault
Iz:
Protects the circuit by disconnecting the power if the current exceeds its value
design current / current carrying capacity of the cable / rating current of protective device
Equal
IB ≤ IN ≤ Iz
What is over current? And what must be done?
Over current is when current exceeds the maximum load current (IB)
Must be cut off quickly to prevent damaging the cabling and appliance
What are the two types of over current:
1 - overload:
Happens in healthy circuits due to short duration loads like motor starting
2- short circuit:
Happens due to insulation failure between a live conductor and earth, forms:
Three phase short circuit ( with or without neutral to earth)
Two phase short circuit ( with or without neutral to earth )
Single phase short circuit ( with neutral to earth)
What is I ^2 T?
It is the thermal energy ( heat ) a cable can withstand during a short-circuit or overcurrent event
Represent a curve between current and duration of the event ( time) to blow a fuse or trip a circuit tracker
Why 1^2t is important (3):
1- to ensure that the cable will be able to withstand the thermal during fault or over current
2. To determine the minimum size required to prevent the cable from overheating or catching fire
3- the I^2 t rating should be higher than let-throgh energy of the protective device
What is the formula of I^2t
I^2t = 0.495
Current carrying capacity of cables in air is based on
Average air temperature = 30°C
