Safety devices like fuses should be connected with the live wire rather than the neutral wire?

Question

So, I was told in my class that Safety devices like fuses should be connected on the live wire. But isn't the conventional current opposite to the direction of the flow of electrons? And what actually happening is that electrons are flowing into the load from the neutral wire? So how does that add up?

Brick · Accepted Answer

You may want to check out this answer on our sister site: https://electronics.stackexchange.com/questions/189867/ac-why-differentiate-between-ground-and-neutral
In summary here:

"Ground" and "neutral" are both grounded in the sense of being attached to the actual earth somewhere in the vicinity of your house.
Despite both being grounded, they are different.  Under nominal conditions, the neutral wire is carrying significant current and the ground is carrying none.
Wires aren't perfect conductors.  So while ground and neutral are at equal potential somewhere - usually outside of your house - they are at different potentials at arbitrary points inside your house.

The ground is there as a safety only.  If something, say your cloths dryer, shorts into its metal housing and the housing is connected to the safety ground, that's supposed to provide a low-resistance path to ground for just the shorted-out current (vs. all of the current your house is pulling).  In particular, it's hoped to be lower resistance to ground than your body when you touch the metal housing.
The live wire, on the other hand, is being driven +/- hundreds of volts relative to the ground and neutral wires.  Under normal conditions the direction of current through live and neutral reverses many times per second through a completed circuit connecting a load to live and neutral.
And now we get to your question. A short between neutral and ground is not great, but those wires will typically be at relatively similar voltages, so it's not as bad as a short between live and ground.  A short between live and ground allows for much greater voltage difference and therefore greater current.  Greater current, in turn, creates a variety of other hazards:  For example, bad for you if you are part of the short, more heat in wires, etc.  Putting the fuse (or circuit breaker) on the live side will cause it to blow in the more dangerous of the two cases.
This is not the only reason, but it's one good one to get you thinking in the right direction.

Bob D · Answer

When an overcurrent protection device like a fuse is placed in the ungrounded (live) circuit conductor it disconnects line voltage from all downstream conductors and electrical components when it opens, making all downstream circuits dead and not a shock hazard.
Hope this helps

trula · Answer

you call it the life wire , so it is an AC current, the second pole than is usually earth. If you put the fuse on the earth part, and you touch the life wire you still are exposed to the full voltage and you are the best connection to earth.

Answered by trula on August 5, 2020

Safety devices like fuses should be connected with the live wire rather than the neutral wire?

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