Electricity Qn

[jonno]

An electrical cable contains three wires: live, neutral and earth. The cable is correctly wired to a plug which contains a 3 A fuse. The insulation becomes damaged and bare metal wires show.

 

Five possible events can occur.

• A person touches the earth wire.
  
• A person touches the neutral wire.
  
• A person touches the live wire.
  
• The live wire touches the neutral wire.
  
• The live wire touches the earth wire.
  

 

How many of these five events cause the fuse in the plug to blow?

 

A. 1 B. 2 C. 3 D. 4

[swansont]

What do you think, and why?

[jonno]

I think the 3rd, 4th and 5th events will lead to a surge in current which will cause the fuse to blow. So, I think the answer is C (3). Am I right?

[swansont]

If a person touches the live wire, what causes the fuse to blow?

[jonno]

The electricity will bypass the appliance. Presumably the human body is of lower resistance thance the appliance. Hence there is a surge in current exceeding 3 A, and this will cause the fuse to blow. Events 4 and 5 will cause short circuit and the fuse will blow too.

[swansont]

Presumably the human body is of lower resistance thance the appliance.

 

That can be investigated (it's somewhere around 1 to several [math]k\Omega[/math]), but there must also be a path to ground. If I'm standing on a plastic crate, is there any way to complete the circuit?

 

The question doesn't clearly indicate if it means "might cause" or "must cause." A person touching the wire might cause the fuse to blow, but it might not.