Confused about simple electrical safety concept (pics)

[chson]

Hello, newbie here with newbie question:

Why is the man in the 1st picture shocked while in the 2nd he is not? I'm assuming the current is traveling counter-clockwise so the only difference between the two pictures is that the man is standing either before or after the resistor. Why does his position in the circuit matter in regards to getting shocked?

 

[Nigel Goodwin]

Hello, newbie here with newbie question:

Why is the man in the 1st picture shocked while in the 2nd he is not? I'm assuming the current is traveling counter-clockwise so the only difference between the two pictures is that the man is standing either before or after the resistor. Why does his position in the circuit matter in regards to getting shocked?

Your diagram already explains it perfectly, in the second case the man is connected across the supply - so he gets a shock. In the first case he isn't connected across the supply, so he can't get a shock.

Try getting a battery and a small bulb!.

Connect the leads from the bulb to each end of the battery, the bulb lights up - picture 2.

Now connect both wires from the bulb to the SAME end of the battery, the bulb doesn't light - picture 1.

Exactly the same as your diagram.

 

[MrMikey83]

THis looks right to me. In the first pic, the guy is forming a short from Ground to Ground...no danger there. In the second pic though, he is forming a short from Possitive to Ground bypassing the resistance and creating a human lightbulb. The bird is not grounded, therefor it is ok.
~Mike

Edit: Damn, late again :lol:

 

[Pommie]

The voltage is across the resistor. The bottom of the resistor is connected to ground - the man is connected to ground = no potential (voltage) difference.

The top of the resistor is connected to the high voltage side of the power source. The man is still connected to ground. Voltage across man = lots. = dead man.

The bird on the other hand is not connected to ground and so can happily wander around on the high potential wires.

HTH

Mike.

Edit: damb even later.

 

[chson]

Nigel, MrMikey, and Pommie. Thanks for your explanations.

So basically only the top half of the circuit is not "electrically common" therefore causing the man to be shocked upon contact?

 

[chemelec]

Personally I Dislike that Ground Reference.

You Don't need to be "Grounded" to get a Shock.

What you need is an Electrical Path, consisting of Two (or More) Contact Points at different Potentials to the Persons Body.

And you Won't find a Bird on Really High Voltage Lines, As even though it won't be grounded, It Will Radiate Corona into the Air around it, Frying It.

Gary

 

[samcheetah]

there are two requirements for current to flow

(1)Physical Contact
(2)Potential Difference

in the first pic the man had physical contact with two leads but both were at the same potential (zero volts). so no current flowed in the first case. and there was no shock

but in the second pic there is both a physical contact and a potential difference. so current is established and the person feels a shock

i hope that helps

 

[Styx]

samcheetah:

I dont fully agree with the 1st statement: (1)Physical Contact

I would go for minimum arching distance. else how would lightning flow...
physical contact just means a very low arching distance - ie none

 

[samcheetah]

okay good point but in the case of lightning there is a physical contact between the sky and the ground due to the atmosphere.

 

[Styx]

okay good point but in the case of lightning there is a physical contact between the sky and the ground due to the atmosphere.

no, there is a contact because of the potential that is built up on the clouds, the air is ionised and it is that ion path that makes a contact.

Without the high-voltage the ion "aura" could not have extended down to ground/tera-ferma and thus completing the cirucuit

 

[audioguru]

You can get a nasty shock without physical contact with a capacitive voltage divider. Don't try it!
Stand beneath high voltage lines while holding up a metal pole with a big metal plate on top! If your shoes have rubber soles they will produce arcs to ground. You'll be jolted with each arc.
You can even light a flourescent tube by simply holding it up.

 

[Dean Huster]

Back in the 1960s, Life magazine ran a two-panel photograph. The first panel showed a long row of little birds on an electric wire, all huddled together because of the cold weather, little feathers fluffed out. The next picture showed the end bird that reached over and found a nearby ground, sending the entire line-up to birdie heaven as the entire strip, to a bird, ended up hanging upside-down on the wire, little feet clenched in death.

The birds have no problem as long as there's not a complete circuit. But add that path and ......

Dean

 

[Pommie]

As to whether someone needs to be grounded in order to get electrocuted. This used to be the case (most of the time). Now with RCDs (residual current device), if you are not grounded they will not trip. If you contact live and neutral together without being grounded then the RCD will treat you like any other electrical appliance and fry you. Most people mistakenly believe that RCD give them complete protection. Here where I live in Australia, grounding and load balancing are so bad that touching the neutral wire will trip the RCD. While being grounded of course.

Mike.

 

[Phasor]

Now with RCDs (residual current device), if you are not grounded they will not trip.

Whilst we're on the topic of RCDs (GFCI's for the yanks), can anybody explain to me why bridging the neutral (line and load) terminals on an RCD, causes it to NOT trip, with or without an earth fault.

I have done it a number of times, always works, but no-one can explain it to me!

 

[gerty]

Phasor, next time try shorting hot to ground instead of neutral. I believe
the GFCI is designed to prevent current from going to ground and will allow it to go to neutral (load).

 

[Someone Electro]

They aslo trip if you short the hot to neutral (if the fuse dosent trip first)

some also ptotect fro lightning strikes in the power network.Where i live ligthing stiked in the power netowrk about 7 years ago,luckuly it switced fast enugh so it only fryed a fuse in the domophone.At some houses it fryed TVs,VCR,phones...

It was a storm and sudenly all the lights shut off then we checked the power box then we swithed it back on and luckely evrytihng worked.Then my frends told me that some of there aplyences ware fyed totaly.

 

[Phasor]

Phasor, next time try shorting hot to ground instead of neutral. I believe
the GFCI is designed to prevent current from going to ground and will allow it to go to neutral (load).

No no, I don't think you have the right idea... sometimes, when performing safety tests on an electrical installation, it is desirable to disable the tripping action of the RCD (as it is a nuisance). We do this by bridging the neutral terminals. It works, but we don't know why.

I certainly don't want to short active to ground (KaBoom!!).

 

[spuffock]

Both the line and neutral wires form the primaries of a current transformer. A current that flows from line to neutral flows in both windings, and the combined effect is zero. A current that flows from line to ground obviously does not return through the neutral winding, and the unbalance trips the breaker. Bridging the neutrals will short one of the primary windings, and dramatically reduce the sensitivity.

 

[captainkirksdog]

Not all birds are safe all the time sittting on wires. I have seen a magpie blown to bits while trying to land in the middle of a 60-foot high-tension drop. Never touched a thing. During certain weather conditions, a phenomenon called St. Elmo's Fire can have sufficient punch to make Polly into fried chicken. I have been zotted twice by it--once while INSIDE a Navy sub-hunter plane. St. Elmo was/is the patron saint of sailors and the early swabbies called on him to rid their ships of the fuzzy, ozony-stinky, bluish-purplish cracklies that would sometimes light up their entire sail sheets. We know it today as high-voltage coronal discharge or plasma. If you live in the north country where you can get some freaky winter weather, keep your eyes on the nearby powerlines,--it can happen there, too.

 

[samcheetah]

no, there is a contact because of the potential that is built up on the clouds, the air is ionised and it is that ion path that makes a contact.

Without the high-voltage the ion "aura" could not have extended down to ground/tera-ferma and thus completing the cirucuit

exactly, thats what i am saying! its both the potential difference and the contact. when both are present, current flows.

what happens in lightning is that the clouds get charged and when they have a very high potential (w.r.t ground) the air in between gets ionized and due to the ionization current flows from the sky to the ground. so there was a potential difference and the physical contact. although that physical contact cannot be seen or felt it doesnt mean there isnt any physical contace. had the air not been there, no ionization would have taken place and no current would have established.

for a detailed explanation of how lightning works go to https://science.howstuffworks.com/lightning.htm