Electrical Safety

• Electric shock

  when electricity causes our body to experience pain or trauma
• An electric current is conducted through material, any opposition to that flow of electrons (resistance) results in a dissipation of energy, usually in the form of heat
• Current makes living tissue heat up, and if heat generation is sufficient enough, the tissue may be burnt; electricity can burn tissue well beneath the skin, even burning internal organs
• nervous system

  network of special cells in the body called "nerve cells" or neurons which process and conduct the multitude of signals responsible for regulation of many body functions
• transducers

  create electrical signals (very small voltages and currents) in response to the input of certain chemical compounds called neurotransmitters
• When electrical current is conducted through a living creature, it will override the tiny electric impulses generated by neutrons, overloading the nervous system and preventing reflex and volitional signals from being able to actuate muscles
• What happens to muscles when shocked?

  They involuntarily contract
• What happens when both bending and extending muscles contract?

  the bending muscles win because they are more developed; this causes the hand to firmly grasp the wire, securing excellent contact with the wire
• tetanus

  involuntary muscle contraction
• What is the only way shock-induced tetanus can be interrupted?

  stopping current through the victim
• Results of tetanus

  the diaphragm muscle controlling the lungs and heart can be "frozen"

  smaller currents can scramble nerve cell signals enough that the heart is sent into fibrillation
• fibrillating heart

  a heart that flutters rather than beats
• Modern power systems use alternating current (AC)

  • low-frequency: 50 to 60 Hz, more dangerous than DC of the same voltage and same amperage (3 to 5 times more dangerous)
  • low-frequency causes extended tetany which may freeze the hand to the current's source
• DC is more likely to cause a single convulsive contraction

  • defribrillating equipment works by supplying a DC current, which halts fibrillation and allows the heart to recover
• What is required to shock someone?

  two contact points on the body for current to enter and exit, respectively
• Why is a bird on a power line not shocked?

  the bird only contacts a single point in a circuit, and it has no voltage applied across its body to establish a current through it

  even though it rests on two feet, both feet are touching the same wire, making them electrically common
• Why do humans get shocked?

  people are usually standing on the ground when they contact a live wire. Many times, one side of the power system will be intentionally connected to earth ground, so the person touching a single wire is actually making contact between two points in the circuit (the wire and earth ground)
• The presence of an intentional grounding point in an electric circuit is intended to ensure that one side of it is safe to come in contact with
• Because the bottom side of the circuit is firmly connected to ground through the grounding point on the lower-left of the circuit, the lower conductor of the circuit is made electrically common with earth ground

  • since there can be no voltage between electrically common points, there will be no voltage applied across the person contacting the lower wire, and they will not receive a shock
• An accidental ground, like a tree branch touching a power line and providing connection to earth ground, can cause a shock
• ground fault

  an accidental connection between a power system conductor and the earth (ground); caused by dirt buildup, ground water infiltration, birds landing on power lines
• Many ground faults are unpredictable. For example, no one can guarantee which wire a tree's branches might touch
• If there are two people in a circuit:

  • with each person standing on the ground, contacting different points in the circuit, a path for shock current is made through one person, through the earth, and through the other person
  • one person acts as the ground fault which makes it unsafe for the other person
• Normal footwear is not enough to guarantee protection against electric shock because they aren't thick enough, and any moisture, dirt, or conductive salts from body sweat will compromise the small bit of insulation they have
• Rubber is a far better insulating material than leather, but the presence of water in a porous substance like leather greatly reduces electrical resistance
• Although electric current is extremely dangerous, it doesn't just occur on its own: there must be voltage available to motivate electrons to flow through a victim
• Ohm's Law

  I = E/R

  current = voltage/resistance
• Why is high voltage dangerous?

  high voltage means potential for large amounts of current through your body; just how much voltage is dangerous depends on how much total resistance is in the circuit to oppose the flow of electrons
• Is body resistance a fixed quantity?

  No. It varies from person to person and from time to time
• Contact with a wire made by a sweaty hand or open wound will offer much less resistance to current than contact made by clean, dry skin
• frequency

  the measure of how rapidly alternating current alternates; unit of Hertz (Hz)
• Electrical resistance between any contacting objects diminishes with increased contact area, all other factors being equal:

  • with two hands holding a pipe, electrons have two, parallel routes through which to flow from the pipe to the body
  • parallel circuit pathways always result in less overall resistance than any single pathway considered alone
• Electric current itself may be an indirect cause of injury causing you to jump or spasm parts of your body into harm's way

  • also, whatever path current takes makes difference in how dangerous it is (ex: heart and lung muscles)
• What is a way to ensure safety when working on live circuits?

  Only use one hand, keeping the other one tucked into a pocket so as to not accidentally touch anything

  also, work on a circuit when it is unpowered
• How can resistance be added to the body?

  insulated tools, gloves, boots, or other gear
• What are some safe practices?

  Shut off power to the circuit before working on it

  Place a circuit in a Zero Energy State

  A lock-out/tag-out procedure
• Explain the purpose of a disconnect switch

  When in the open position (no continuity), the circuit is broken and no current will exist. There will be zero voltage across the load, and the full voltage of the source will be dropped across the open contacts of the disconnect switch
• Benefit of grounding conductors

  protection against the possibility of the disconnect switch being closed (turned "on" so that circuit continuity is established) while people are still contacting the load
• What is grounding?

  the temporary wire connected across the load would create a short-circuit when the disconnect switch was closed, immediately tripping any overcurrent protection devices in the circuit, which would shut the power off again
• How does a lock-out/tag-out procedure work?

  all individuals working on a secured circuit have their own personal padlock or combination lock which they set on the control lever of a disconnect device prior to working on the system; the system is maintained in a Zero Energy State until every last lock is removed from all the disconnect and shutoff devices