MCBs

Question 1

What is arch extinction

Answer 1

Guiding the arch through V splitter plates
V plates split, cool and de-ionises arch
Arch chamber arrangements provide the breaking capacity of MCB

Question 2

Advantages of MCB over fuses

Answer 2

Readily identified
Cannot be reset when fault exists
Tamperproof
Do not age from use
Locking devices can be attached

Question 3

What page of rules has fault loop impedance

Answer 3

Page 125

Question 4

How to find circuit current

Answer 4

P = V I cos0

Question 5

How to find prospective fault current

Answer 5

If = V/ ZL (fault loop impedance)

Question 6

What dangers arise if an earth fault occurs

Answer 6

Touch voltage rises above 50v
Cable suffers extreme thermal stress

Question 7

Method for reducing danger of earth fault

Answer 7

RCD protection

Question 8

Type B MCB

Answer 8

Domestic installations
Little or no switching surge

Question 9

Type C MCB

Answer 9

High inductive circuits
ie. Banks of fluorescent lighting

Question 10

Type D MCB

Answer 10

High inrush currents
ie. Transformers and welding equipment

Question 11

Procedure to verify RCD

Answer 11

1. Connect RCD tester
2. Power off and polarity correct
3. Set test current to 5xI^n
4. Selector switch to 0 degrees
5. Record RCD response time
6. Selector switch to 180 degrees
7. Verify longest trip time doesn’t exceed maximum operation time