Inside CCU Flashcards
What should be the weakest part of any given circuit?
The protective device, be that the MCB or RCD.
If a break were to occur in a final ring circuit, and the two halves of the loop were discontinuous with one another, what part of the circuit would be the weakest part?
(Assuming the final ring circuit is installed with 2.5mm² PVC/PVC).
The weakest part of the circuit would be the cables themselves, because each cable would have a current carrying capacity of 27A, which is lower than the rating of the protective device (usually a 30A or 32A MCB).
What is an AFDD?
An Arc Fault Detection Device.
What are the two most typical kinds of fault?
- Earth fault
- Short circuit between live conductors
What are the four factors determining the choice of protective device used on any given circuit?
- Design current (Ib)
- Disconnection time limitation
- Prospective short circuit current (PSSC/ PFC)
- Rating of the protective device (Iₙ)
What are the two main ways that tripping is achieved in protective devices?
Which of the two is faster?
- Thermal (through buildup of heat)
- Magnetic (through current), this method is faster.
What are the four possible causes for a short circuit in any given circuit?
- Accidental damage
- Contact between two poles of the supply due to incorrect connection
- Equipment failure
- Ingress of moisture
True or false:
Under normal conditions, the neutral cable carries no current.
False.
Neutral is the return conductor for all AC circuits!
It’s the alternating part of Alternating Current.
You muggins.
True or false:
Any exposed line and neutral conductor coming into contact with each other constitutes a short circuit.
True.
This is what this means.
What kind of protective device can be used to reduce the risk posed by a Short Circuit?
An RCD.
This is set to trip at a specific rating of current fluctuation, usually 30mA, and will trip when detecting overcurrent or undercurrent.
Which must be higher than the other?
Design current (Ib) or Protective device rating (In)?
Why is this?
The protective device needs to be higher than or equal to the design current: In ≥ Ib
The reason for this is that you want the circuit to be able work at full current carrying capacity without tripping the protective device.
THIS IS NOT THE SAME AS OPERATING UNDER FAULT CONDITIONS.
Why are BS 3036 reweireable fuses potentially dangerous in the event of overcurrent?
What part of future designs was also dangerous?
The fuse element can melt, splashing metled copper around which could cause fires.
Future models also contained asbestos, which is harmful.
What are the colours used in BS 3036 fuses and what were their corresponding fuse ratings?
5 ratings and colours
5 A - White
15 A - Blue
20 A - Yellow
30 A - Red
45 A - Green
What is the BS name of the two cartridge fuses?
Where are they found?
BS 1361/ 2
BS 1361 fuses are usually found in distribution boards at the main intake.
BS 1362 fuses are usually found in domestic plug tops.
What are the two most common fuse ratings for cartridge fuses?
What are the power limits for such fuses?
The two most common ratings are 3A and 13A.
3A fuses are only to be used in equipment that will use less than 720 watts.
13A fuses are to be used in equipment that will use more than 720 watts.
