caz asked in Science & MathematicsEngineering · 1 year ago

# How can a live wire still be dangerous if the switch is open?

Hi. My exam board s specification says the following which I don t really understand: Know that a live wire can still be dangerous even if the switch is open ... How can this be possible? (Current doesn t flow is it is open). Could someone please explain this to me?

(All I can guess is: would it be the short time between the switch being closed, then opened, where current would still be flowing to the appliance so the live wire will still dangerous!)

Thank you

Relevance
• 1 year ago

Locate ground and firmly grasp with one hand. Grasp live wire with other hand. You'll see just how dangerous it can be.

• M.
Lv 7
1 year ago

Are you playing a word game with us?

A live wire is live. Is that your terminology or the testing terminology?

Also, in any kind of circuit that is more than low voltage, you must assume that it is live unless YOU can determine that it is not live. You must lock it out if possible. You must protect yourself anyway.

What "exam board" do you speak of? What is the exam? What is it for?

Let me tell you a story that happened a few months ago. A primary wire (13.2kv) had broken behind my neighbor's house, and danced around on the ground and caught their garage on fire. The fire department was unable to do anything while the wire was moving around.

Finally the power to the neighborhood was shut off. The garage had burned down to charcoal, unfortunately.

The power company had to replace the primary and secondary (240 volts) wires due to the fire melting everything.

100 feet away, at a junction of the primary wires, they connected jumper wires to the ground wire at

(cont)

• 1 year ago

I would have to see the specifications of the question. If the switch is wired properly and it is open there is no way for it to provide dangerous current. They may have stated somewhere that you must assume that wires are hot until you test them just as you assume a gun is loaded until...

Or It is one of the questions which should be eliminated because it is wrong.

If the switch is open the wire is not live. If it is a live wire it may be dangerous. Trick question.

• 1 year ago

the live side still has an elevated potential, a voltage. It is not providing any electron flow at the moment because it has no access to ground (earth), but current will flow if you provide a route with your body. Absence of electron flow NOW does not mean that it does not contain a potential for such flow. It just needs a route.

• What do you think of the answers? You can sign in to give your opinion on the answer.
• 1 year ago

It really depends on where the switch is in the circuit and if it is a single pole or double pole switch. You really need a circuit diagram to answer this question. Absent such information you have to assume the wire is hot.

• 1 year ago

Live line still carries Hot line voltage that is attached to one terminal from switch. Switch in Off position just cut off the path between lamp to Hot line. It is safe to touch the lamp path wiring while sure the switch is in Off position and sure the switch is working well.

• Dixon
Lv 7
1 year ago

one side of the switch is always at supply voltage

• 1 year ago

There are basically 3 reasons ...

Firstly, you are making an assumption that the live wire is connected in the way you think it is. How do you know that it isn't connected somewhere else or someone has done a bit of DIY and messed it up? So the assumption should always be live is live until you've tested.

Secondly, if you are looking at a live wire you've taken something apart, be it a light fitting or socket or switch. The point is you might dislodge terminal blocks or connections people used and who knows what shorts might be created by getting at the wires. Again, test it to be sure.

And the third reason is some equipment has big capacitors in the power supplies (microwaves are a prime example). These caps can store energy and discharge back through the live or neutral lines.

• 1 year ago

On an exam, the prudent assumption would be that anything is dangerous.

Like you, I don't see how the wire could normally be energized if the switch is open, but maybe we only THINK the switch is open, or the wrong switch is open, or some ding-dong walked by and decided to turn on the lights. Or some moron wired the thing wrong and is switching the neutral wire. My electronics teacher told me that in the Navy, his buddy swore to him twice over the radio that the breaker was open, but he decided to check anyway and found the circuit was energized. His lesson was that when your life is on the line, always check.

• 1 year ago

Does the circuit in question use a capacitor?

Still have questions? Get answers by asking now.