# AC Voltage and Neutral/Ground

## Main Question or Discussion Point

Hi, did you ever get to understand why a fuse installed in the Hot (Live) conductor of a circuit doesn't rupture when the AC Polarity reverses with the Neutral conductor bonded to Ground ?????? I still don't get it i.e. if current flows in both directions then the polarity is allowing the Hot wire bond to Ground as the frequency changes????? If the source below is an Alternator then the Neural conductor becomes the Hot conductor frequently then why doesn't the fuse blow with a Hot conductor bonded to Ground as below???????????? Hot to Ground is surely a catastrophe???????

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berkeman
Mentor
Hi, did you ever get to understand why a fuse installed in the Hot (Live) conductor of a circuit doesn't rupture when the AC Polarity reverses with the Neutral conductor bonded to Ground ?????? I still don't get it i.e. if current flows in both directions then the polarity is allowing the Hot wire bond to Ground as the frequency changes????? If the source below is an Alternator then the Neural conductor becomes the Hot conductor frequently then why doesn't the fuse blow with a Hot conductor bonded to Ground as below???????????? Hot to Ground is surely a catastrophe???????

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Welcome to the PF.

Calm down a bit, the world will survive an AC source driving a load...

The fuse is traditionally put in the Hot lead so that a short to Earth Ground will blow the fuse (since Neutral is bonded to Earth ground somewhere, at least in the US). The fuse also blows if the differential load current becomes too great.

You did not show the fuse in your diagram. Can you add it in and maybe re-state your question is a bit calmer manner? Thanks.

Asymptotic and sophiecentaur
sophiecentaur
Gold Member
Welcome to PF.
You have drawn a circuit through which current flows. No current can flow to ground because there is only one connection to it. Why should a fuse (wherever you have drawn it in your head) pass a different current on each half cycle?
I could add to Berkeman's comment about fuse position. Once a fuse in the live leg has blown (for whatever reason) the circuit becomes 'safe' because the live voltage never gets to any part of the downstream circuit. If you put a fuse in the neutral leg then, yes - it will blow with an overload but all the originally live parts of the circuit are still live. Not a good idea.
There used to be a dodgy practice of fusing both legs. I never was sure what it was supposed to achieve. But I fully expect someone to tell me, with a few seconds.

Asymptotic
anorlunda
Staff Emeritus
Your source is not a battery. It is an AC source. Think of the ground side of the source always being at 0 volts, and the other side of the source varying plus to minus in a sin wave.

A common misconception is that current will flow into the ground. In a closed circuit, current flowing out of the source at the top of your diagram always matches current flowing into the source from the bottom (ground) side. No current should flow into the ground connection in normal circumstances. Your question makes it sound that you visualize on the plus half of the cycle that current flows from the source through the load to the ground. That's wrong.

In circuit analysis, the ground symbol has no physical meaning. All voltages are measured between two points. The ground symbol just shows which point we arbitrarily call zero volts.

In a house with a physical ground, there should be zero current going to ground in normal circumstances. It is there to protect you in abnormal open circuit and short circuit cases.

Asymptotic and sophiecentaur
Fuses don't really care which way current is flowing. You have a common misconception I see all the time. In an AC system, the ground is NOT a current carrying conductor under normal system operation. The ground in your example exists as a means to give normally non-current-carrying parts a low-resistance path to ground should those parts become energized so that the over current protection device can open up the circuit. The earth actually has a much higher resistance than the neutral, so the current will travel through the neutral and not the ground. If there is enough current on the GEC (grounding electrode conductor) to notice and/or read your neutral is likely open.

Another misconception you have: You are not distinguishing between the line (supply side) of your circuit and the load side of your circuit. The line side supplies a load with power, while the neutral supplies a return path for the current. The OCPD (over current protection device) should be placed on the line side of the circuit, so that in the event it activates and opens the circuit, the circuit is not still energized.

Averagesupernova
Gold Member
I have been thinking about this all day. I fail to why you think that somehow sparks will fly and fuses will blow when the current flows in one direction as compared to the opposite direction. The neutral conductor is called neutral because of the way it is connected to the transformer(s). It is not neutral because it is bonded to a ground rod. There are 3 phase systems in the USA that are identical except different parts of the transformer group are connected to a ground rod. A high leg Delta is one with the ground connected to the center tap of one transformer and is considered the neutral. The corner grounded Delta grounds to one of the legs.
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You use the word bond quite incorrectly. Bonding in electrical work is just what it sounds like. For instance, an equipment grounding conductor is BONDED to a metal enclosure with a screw or bolt.

sophiecentaur
Gold Member
I notice the OP has not replied to any of this. Perhaps we should wait for a reaction? Else we will be just repeating our previous conversations on the subject.

Welcome to the PF.

Calm down a bit, the world will survive an AC source driving a load...

The fuse is traditionally put in the Hot lead so that a short to Earth Ground will blow the fuse (since Neutral is bonded to Earth ground somewhere, at least in the US). The fuse also blows if the differential load current becomes too great.

You did not show the fuse in your diagram. Can you add it in and maybe re-state your question is a bit calmer manner? Thanks.
Thanks for your reply I'm still confused as to why you imply that I'm in some way 'not calm' I have got pleasant and decent replies????

I have been thinking about this all day. I fail to why you think that somehow sparks will fly and fuses will blow when the current flows in one direction as compared to the opposite direction. The neutral conductor is called neutral because of the way it is connected to the transformer(s). It is not neutral because it is bonded to a ground rod. There are 3 phase systems in the USA that are identical except different parts of the transformer group are connected to a ground rod. A high leg Delta is one with the ground connected to the center tap of one transformer and is considered the neutral. The corner grounded Delta grounds to one of the legs.
-
You use the word bond quite incorrectly. Bonding in electrical work is just what it sounds like. For instance, an equipment grounding conductor is BONDED to a metal enclosure with a screw or bolt.
Thanks so much you see what confuses me is that if current flows in both directions then the Neutral has to rise to a higher voltage than the hot wire to drive current in the opposite direction thro the load? So if this IS the case then the Neutral/Earth point should blow the Line fuse?????? I'm stupid maybe?

Thanks for your reply I'm still confused as to why you imply that I'm in some way 'not calm' I have got pleasant and decent replies????
I’m on several forums dealing with several different technical subjects, and PF stands out as the most polite and measured place to be.

Written posts lack the usual verbal cues, and can be interpreted differently from natural speech. Berkeman is, I think, referring in a light-hearted way to your use of loads of question marks - in one place, there are twelve together. You only need one, and a great bunch of them can come across as aggression or exasperation. That may not be what you intended.

A little thing, maybe, but it’s what makes PF what it is: a very good place to discuss sciencey stuff. Pointless arguments are rare here.

berkeman
Thanks so much you see what confuses me is that if current flows in both directions then the Neutral has to rise to a higher voltage than the hot wire to drive current in the opposite direction thro the load? So if this IS the case then the Neutral/Earth point should blow the Line fuse?????? I'm stupid maybe?
Think of neutral staying at 0V and line going negative. Line is the conductor that does the work, either shoving current into the neutral, or sucking current out of it. The line fuse will only blow if current exceeds its rating, regardless of direction.

Averagesupernova
sophiecentaur
Gold Member
Think of neutral staying at 0V and line going negative. Line is the conductor that does the work, either shoving current into the neutral, or sucking current out of it.
Alternatively, think of the different electrical systems in cars. Nowadays they use 'negative Earth' but in the past they used 'positive Earth. All the circuitry was, in principle the same and the only difference was which bit of the circuit the car body was joined to.
With AC, the situation changes twice every cycle.

berkeman
Mentor
Thanks for your reply I'm still confused as to why you imply that I'm in some way 'not calm' I have got pleasant and decent replies????
LOL. @Guineafowl has it correct -- your OP came across quite exasperated with your use of multiple ???? and statements like this:
Hot to Ground is surely a catastrophe???????
As he points out, you really only need one "?" per question sentence, and adding more isn't really appropriate for a science discussion forum.