Simple concept in electricity

[Shyam]

hmmm, here is a very elementary concept to most of you.
The situation is that a live wire comes into contact with the metal casing of a fan which is earthed. Connected to the live wire is a fuse of rating 5A.
Since it is earthed, the excess charges will be conducted away. This will pose no danger to the handlers. However, the fuse is still used.
Why is there a need to use the fuse?
Is it primarily to detect the fault? Or is it serving another purpose?

[Shyam]

By the way, under normal conditions, the fan draws a current of 4A.

[chroot]

When you say 'live wire,' you first need to consider what 'live' means. What is the potential on this wire? What is that potential measure relative to? In the example of normal three-conductor house wiring, one wire is carrying 110 V AC, the other is the return (neutral), and the third is grounded. The hot wire's 110 V AC is measured with respect to the neutral, not the ground. The relationship between neutral and ground is not specifically known; it is possible that they are at 0V with respect to each other, but they may not be.

Without knowing what potential your "live wire" has with respect to ground, you cannot say how much current will flow.

- Warren

[BoulderHead]

Originally posted by Shyam
Since it is earthed, the excess charges will be conducted away. This will pose no danger to the handlers.
You hope [:)]
The person touching the case would set up a parallel path for the current to flow...

However, the fuse is still used.
Why is there a need to use the fuse?
Is it primarily to detect the fault? Or is it serving another purpose?
To prevent current carrying wires from getting so hot they start a fire.

[Janus]

As Boulderhead pointed out, the fuse is there to protect the wiring and the fan. The ground wire is there to protect against shock, in two ways:

1. To provide a better path to ground than a human body.

2. By drawing enough current to blow the fuse in case of a short, to prevent the possible parallel path mentioned by Chroot.

The fuse by itself offers no shock protection, as it takes only about 1/10 of an amp to kill you.

[Shyam]

Cool. THanks people. I understand it much better. So, its actually a system that works together to prevent accidents rather than a single safety measure.
Thanks a lot!!