1
\$\begingroup\$

image depicting circuit with a ground

(this image taken from http://www.allaboutcircuits.com/vol_1/chpt_3/3.html)

Based on this picture, would a tree across the bottom wire of the circuit (the solid wire, not the dashed wire representing earth) cause the person to be electrocuted? Am I correct in thinking the tree would cause a voltage drop?

EDIT: To clarify, what would happen if a resistor was added to the bottom, solid wire?

\$\endgroup\$
  • \$\begingroup\$ I don't see a tree? What do you mean by that? \$\endgroup\$ – abdullah kahraman Jun 16 '13 at 8:12
  • \$\begingroup\$ @abdullahkahraman Sorry, I meant if a tree fell on the bottom wire - there isn't one currently in the diagram. \$\endgroup\$ – sdasdadas Jun 16 '13 at 8:25
  • \$\begingroup\$ Do you mean adding a resistor from neutral to earth or in circuit with the neutral wire? A Neutral Earthing Resistor (NER) would have the effect of limiting earth fault current \$\endgroup\$ – Raggles Jun 16 '13 at 11:35
3
\$\begingroup\$

I assume the neutral wire is still unbroken after the tree is across it, in this case nothing would happen. The tree is already at ground potential, as is the neutral wire, as is the person. If the neutral conductor is broken then there will likely be an introduced earth fault impedance (either tree or person) and the load would see a voltage drop. If the person becomes the lowest impedance to earth then he will likely be zapped.

\$\endgroup\$
  • \$\begingroup\$ Sorry, I'm a unclear about the term 'neutral wire'. Does that mean the bottom, solid wire or the bottom, dashed wire (the earth)? \$\endgroup\$ – sdasdadas Jun 16 '13 at 8:29
  • \$\begingroup\$ The solid one at the bottom \$\endgroup\$ – Raggles Jun 16 '13 at 8:35
1
\$\begingroup\$

It appears the person in the diagram is touching the bottom wire, which is connected to ground. The person is not shocked because there is no potential from the wire being touched to earth ground underfoot.

I assume you mean if a tree (as in the plant) connected both the top and bottom wires, would the person be shocked then?

The answer is no, the potential difference would be across the portion of the tree that is making the connection.

Here's a video showing a tree branch shorting out high voltage distribution lines:

http://www.youtube.com/watch?v=-6Alur-65_w

If the person continues to make contact with the ground wire during such an event, there would be no noticeable effect.

\$\endgroup\$
  • \$\begingroup\$ If one added a resistor to the bottom, solid wire - would the person be safe? \$\endgroup\$ – sdasdadas Jun 16 '13 at 8:28
  • 1
    \$\begingroup\$ A resistor has two connection points. If one added a resistor connected between the bottom wire and what other point? \$\endgroup\$ – JYelton Jun 16 '13 at 9:23
1
\$\begingroup\$

The situation you illustrate is one of the major reasons why it is important that safety ground connections be kept isolated from neutral connections.

If there were two wires connected to grounds at the left of your picture, one of which was carrying current for the load, and one of which was being touched by the person, then for a harmful condition to occur it would be necessary both that one or more connections failed, and also that one or more erroneous connections was present. Neither any combination of failed connections alone, nor any combination of erroneously-present connections alone, would suffice.

One of the limitations of normal safety ground protocols is that while multiple faults are required to create harmful conditions, it's possible that enough faults may develop that the system is one fault away from electrocuting someone without those faults causing any symptoms. For example, it's important that the safety ground and neutral wire have no connection to each other, except via separate paths to earth, but there's no way short of disconnecting the neutral to ensure that's the case.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.