Shocked by Neutral vs. Ungrounded

[winnie]

Agreed that current cannot change instantaneously through an inductor. And that can boost the voltage when a circuit is opened. I remember a vivid demonstration of this as an undergrad, where a large electromagnet was powered by the building DC system. When the big knife switch was used to open the circuit, you got a fairly fat arc.

The questions is: how large is the inductance and what is the 'time constant' of the system compared to the time the circuit is being evaluated.

For many situations, in the context of 60 Hz AC currents, the time constant of inductive effects is very short compared to the averaging time of the meters we use. But the inductive effects will always be there to some extent.

It _might_ be that the very short transient voltage from opening the circuit makes the 'bite' from opening the neutral seem worse...perhaps you get a very high momentary shock which then dies away, so that you experience the sensation of a high current shock but then survive it because it is short duration. I still think 'myth' but as I said previously there might be details of the experience which make it real.

 

[LarryFine]

Years ago, I had a battery-powered cigarette lighter that ignited a lighter-fluid flame with a spark when you pulled the wand out of the base.

I had to know how it worked (like with everything since I was a kid), so I opened it to see (like with everything since I was a kid).

It had an inductor in series with the batteries, and the tip of the wand bridged, then broke the circuit when you pulled it out, making a spark.

 

[jaggedben]

Unless I learned it wrong in school, current CANNOT change instantaneously thru an inductor, so if it is carrying 10 amps to a motor and you separate the neutrals and have one lead in each hand, you will have 10 amps going thru you no matter your resistance as the coil will up the voltage to make up for resistance. I believe this is the reason that motors spark at the plug end when unplugged under load. Basically the coil bumps the voltage as much as necessary to bridge the air gap and make a spark. Please tell me if I have this wrong.

Mark

Okay you're probably right in this rather strange scenario but I don't think that was the scenario the OP was asking about. More like, you open the neutral, so current stops flowing, and then at a later point in time, probably because you thought the neutral was de-energized, you accidentally make contact between it and ground and get a shock. In which case the load being inductive is no longer very relevant.

 

[mtnelect]

I was changing out a defective light switch in a junction box with 277 volts and got zapped with another 277 volts leg. 480 volts to my right arm, which was numb until the next day. This was an office building and didn't want to turn the power off. That was the last time I worked a circuit live. I thought that you couldn't have two legs of 277 volts in a junction box but could never find the section.

 

[kwired]

Okay you're probably right in this rather strange scenario but I don't think that was the scenario the OP was asking about. More like, you open the neutral, so current stops flowing, and then at a later point in time, probably because you thought the neutral was de-energized, you accidentally make contact between it and ground and get a shock. In which case the load being inductive is no longer very relevant.

Which I mentioned there is no inductance when the circuit is open. For a brief time shortly after opening the circuit yes but once fields have collapsed there is no inductance all you are left with is coil of conductor with voltage to ground but no current if it is a grounded conductor that gets opened. A few milliamps passing in series through you to get to ground is not restoring the inductor to it's normal operating conditions and the resulting current will be limited by resistance (mostly of you) much more so than by any inductance that may be present

 

[busman]

Okay you're probably right in this rather strange scenario but I don't think that was the scenario the OP was asking about. More like, you open the neutral, so current stops flowing, and then at a later point in time, probably because you thought the neutral was de-energized, you accidentally make contact between it and ground and get a shock. In which case the load being inductive is no longer very relevant.

Agreed with what you said above. My first Electrician when I was an apprentice said, "always beware of the noodle."

 

[hillbilly1]

I was changing out a defective light switch in a junction box with 277 volts and got zapped with another 277 volts leg. 480 volts to my right arm, which was numb until the next day. This was an office building and didn't want to turn the power off. That was the last time I worked a circuit live. I thought that you couldn't have two legs of 277 volts in a junction box but could never find the section.

Perfectly legal, and you can all three phases to neutral in a box or fixture. Older Walmarts have three 277 volt lighting circuits in the strip fixtures with each phase alternating to feed each fixture.

 

[busman]

Perfectly legal, and you can all three phases to neutral in a box or fixture. Older Walmarts have three 277 volt lighting circuits in the strip fixtures with each phase alternating to feed each fixture.

Maybe he means the requirement for barriers in a switch box when over 300V between devices. 404.8(B)

Mark