Poster Showing Safety Precautions When Using Electricity

What are five safety devices used in electrical work?

Some examples of electrical protection devices are lightning arresters, surge protectors, fuses, relays, circuit breakers, reclosers, and other devices. Every electrical circuit has a maximum voltage or amperage.

What are the 5 golden rules for electricity safety?

Five rules to prevent electrical risks: Disconnect, prevent any possible feedback, verify the absence of voltage, ground and short-circuit, signal and delimit the working area.

What are 2 electrical safety devices?

Figure 1: A fuse box in a basement is one type of electrical safety device. Many of the energy services around the house use electricity. It is extremely important to have various safety devices to protect from fire and electrocution. Industrial electricity use has similar problems.

What is the one hand rule in electrical safety?

How Much Electric Current is Harmful? – The answer to that question also depends on several factors. Individual body chemistry has a significant impact on how electric current affects an individual. Some people are highly sensitive to current, experiencing involuntary muscle contraction with shocks from static electricity.

  1. Others can draw large sparks from discharging static electricity and hardly feel it, much less experience a muscle spasm.
  2. Despite these differences, approximate guidelines have been developed through tests that indicate very little current being necessary to manifest harmful effects (again, see the end of the chapter for information on the source of this data).

All current figures given in milliamps (a milliamp is equal to 1/1000 of an amp):

Slight sensation felt at hand(s) Men 1.0 mA 0.4 mA 7mA
Women 0.6 mA 0.3 mA 5 mA
Threshold of Pain Men 5.2 mA 1.1 mA 12 mA
Women 3.5 mA 0.7 mA 8 mA
Painful, but voluntary muscles control maintained Men 62 mA 9 mA 55 mA
Women 41 mA 6 mA 37 mA
Painful, unable to let go of wires Men 76 mA 16 mA 75 mA
Women 60 mA 15 mA 63 mA
Severe pain, difficulty breathing Men 90 mA 23 mA 94 mA
Women 60 mA 15 mA 63 mA
Possible heart fibrillation after 3 seconds Men and Women 500 mA 100 mA

Hz” stands for the unit Hertz, It is the measure of how rapidly alternating current alternates, otherwise known as frequency, So, the column of figures labeled “60 Hz AC” refers to a current that alternates at a frequency of 60 cycles (1 cycle = period of time where current flows in one direction, then the other direction) per second.

The last column, labeled “10 kHz AC,” refers to alternating current that completes ten thousand (10,000) back-and-forth cycles each and every second. Keep in mind that these figures are only approximate, as individuals with different body chemistry may react differently. It has been suggested that an across-the-chest current of only 17 milliamps AC is enough to induce fibrillation in a human subject under certain conditions.

Most of our data regarding induced fibrillation come from animal testing. Obviously, it is not practical to perform tests of induced ventricular fibrillation on human subjects, so the available data is sketchy. Oh, and in case you’re wondering, I have no idea why women tend to be more susceptible to electric currents than men! Suppose I were to place my hands across the terminals of an AC voltage source at 60 Hz (60 cycles per second).

How much voltage would be necessary on this clean, dry-skin condition to produce a current of 20 milliamps (enough to cause me to become unable to let go of the voltage source)? We can use Ohm’s Law to determine this: E = IR E = (20 mA)(1 M \Omega) \textbf Bear in mind that this is a “best case” scenario (clean, dry skin) from the standpoint of electrical safety and that this figure for voltage represents the amount necessary to induce tetanus.

Far less would be required to cause a painful shock! Also, keep in mind that the physiological effects of any particular amount of current can vary significantly from person to person and that these calculations are rough estimates only, With water sprinkled on my fingers to simulate sweat, I was able to measure a hand-to-hand resistance of only 17,000 ohms (17 kΩ).

  1. Bear in mind that this is only with one finger of each hand contacting a thin metal wire.
  2. Recalculating the voltage required to cause a current of 20 milliamps, we obtain this figure: E = IR E = (20 mA)(17 k \Omega) \textbf In this realistic condition, it would only take 340 volts of potential from one of my hands to the other to cause 20 milliamps of current.

However, it is still possible to receive a deadly shock from less voltage than this. Provided a much lower body resistance figure augmented by contact with a ring (a band of gold wrapped around the circumference of one’s finger makes an excellent contact point for electrical shock) or full contact with a large metal object such as a pipe or metal handle of a tool, the body resistance figure could drop as low as 1,000 ohms (1 kΩ), allowing an even lower voltage to present a potential hazard.

  • E = IR E = (20 mA)(1 k \Omega) \textbf Notice that in this condition, 20 volts is enough to produce a current of 20 milliamps through a person; enough to induce tetanus.
  • Remember, it has been suggested a current of only 17 milliamps may induce ventricular (heart) fibrillation.
  • With a hand-to-hand resistance of 1000 Ω, it would only take 17 volts to create this dangerous condition.

E = IR E = (17 mA)(1 kW) \textbf Seventeen volts is not very much as far as electrical systems are concerned. Granted, this is a “worst-case” scenario with 60 Hz AC voltage and excellent bodily conductivity, but it does stand to show how little voltage may present a serious threat under certain conditions.

  • The conditions necessary to produce 1,000 Ω of body resistance don’t have to be as extreme as what was presented (sweaty skin with contact made on a gold ring).
  • Body resistance may decrease with the application of voltage (especially if tetanus causes the victim to maintain a tighter grip on a conductor) so that with constant voltage a shock may increase in severity after initial contact.

What begins as a mild shock—just enough to “freeze” a victim so they can’t let go—may escalate into something severe enough to kill them as their body resistance decreases and current correspondingly increases. Research has provided an approximate set of figures for electrical resistance of human contact points under different conditions:

Situation Dry Wet
Wire touched by finger 40,000 Ω – 1,000,000 Ω 4,000 Ω – 15,000 Ω
Wire held by hand 15,000 Ω – 50,000 Ω 3,000 Ω – 5,000 Ω
Metal pliers held by hand 5,000 Ω – 10,000 Ω 1,000 Ω – 3,000 Ω
Contact with the palm of hand 3,000 Ω – 8,000 Ω 1,000 Ω – 2,000 Ω
1.5-inch metal pipe grasped by one hand 1,000 Ω – 3,000 Ω 500 Ω – 1,500 Ω
1.5 inch metal pipe grasped by two hands 500 Ω – 1,500 kΩ 250 Ω – 750 Ω
Hand immersed in conductive liquid 200 Ω – 500 Ω
Foot immersed in conductive liquid 100 Ω – 300 Ω

Note the resistance values of the two conditions involving a 1.5-inch metal pipe. The resistance measured with two hands grasping the pipe is exactly one-half the resistance of one hand grasping the pipe. Figure 1.8 With two hands, the bodily contact area is twice as great as with one hand. This is an important lesson to learn: electrical resistance between any contacting objects diminishes with increased contact area, all other factors being equal. With two hands holding the pipe, the current has two, parallel routes through which to flow from the pipe to the body (or vice-versa). Figure 1.9 As we will see in a later chapter, parallel circuit pathways always result in less overall resistance than any single pathway considered alone. In industry, 30 volts is generally considered to be a conservative threshold value for dangerous voltage.

The cautious person should regard any voltage above 30 volts as threatening, not relying on normal body resistance for protection against shock. That being said, it is still an excellent idea to keep one’s hands clean and dry and remove all metal jewelry when working around electricity. Even around lower voltages, metal jewelry can present a hazard by conducting enough current to burn the skin if brought into contact between two points in a circuit.

Metal rings, especially, have been the cause of more than a few burnt fingers by bridging between points in a low-voltage, high-current circuit. Also, voltages lower than 30 can be dangerous if they are enough to induce an unpleasant sensation, which may cause you to jerk and accidentally come into contact across a higher voltage or some other hazard.

I recall once working on an automobile on a hot summer day. I was wearing shorts, my bare leg contacting the chrome bumper of the vehicle as I tighten battery connections. When I touched my metal wrench to the positive (ungrounded) side of the 12-volt battery, I could feel a tingling sensation at the point where my leg was touching the bumper.

The combination of firm contact with metal and my sweaty skin made it possible to feel a shock with only 12 volts of electrical potential. Thankfully, nothing bad happened but had the engine been running and the shock felt at my hand instead of my leg, I might have reflexively jerked my arm into the path of the rotating fan, or dropped the metal wrench across the battery terminals (producing large amounts of current through the wrench with lots of accompanying sparks).

  1. This illustrates another important lesson regarding electrical safety; that electric current itself may be an indirect cause of injury by causing you to jump or spasm parts of your body into harm’s way.
  2. The path current takes through the human body makes a difference as to how harmful it is.
  3. Current will affect whatever muscles are in its path, and since the heart and lung (diaphragm) muscles are probably the most critical to one’s survival, shock paths traversing the chest are the most dangerous.

This makes the hand-to-hand shock current path a very likely mode of injury and fatality. To guard against such an occurrence, it is advisable to only use one hand to work on live circuits of hazardous voltage, keeping the other hand tucked into a pocket so as to not accidentally touch anything.

Of course, it is always safer to work on a circuit when it is unpowered, but this is not always practical or possible. For one-handed work, the right hand is generally preferred over the left for two reasons: most people are right-handed (thus granting additional coordination when working), and the heart is usually situated to the left of center in the chest cavity.

For those who are left-handed, this advice may not be the best. If such a person is sufficiently uncoordinated with their right hand, they may be placing themselves in greater danger by using the hand they’re least comfortable with, even if shock current through that hand might present more of a hazard to their heart.

The relative hazard between shock through one hand or the other is probably less than the hazard of working with less than optimal coordination, so the choice of which hand to work with is best left to the individual. The best protection against shock from a live circuit is resistance, and resistance can be added to the body through the use of insulated tools, gloves, boots, and other gear.

Current in a circuit is a function of available voltage divided by the total resistance in the path of the flow. As we will investigate in greater detail later in this book, resistances have an additive effect when they’re stacked up so that there’s only one path for current to flow: Figure 1.10 Person in direct contact with voltage source: current limited only by body resistance. I = \frac } Now we’ll see an equivalent circuit for a person wearing insulated gloves and boots: Figure 1.11 Person wearing insulating gloves and boots; Current now limited by circuit resistance: I = \frac +R_ +R_ +} Because electric current must pass through the boot and the body and the glove to complete its circuit back to the battery, the combined total ( sum ) of these resistances opposes the flow of current to a greater degree than any of the resistances considered individually.

  1. Safety is one of the reasons electrical wires are usually covered with plastic or rubber insulation: to vastly increase the amount of resistance between the conductor and whoever or whatever might contact it.
  2. Unfortunately, it would be prohibitively expensive to enclose power line conductors’ insufficient insulation to provide safety in case of accidental contact.

So safety is maintained by keeping those lines far enough out of reach so that no one can accidentally touch them. If at all possible, shut off the power to a circuit before performing any work on it. You must secure all sources of harmful energy before a system may be considered safe to work on.

Harm to the body is a function of the amount of shock current. Higher voltage allows for the production of higher, more dangerous currents. Resistance opposes current, making high resistance a good protective measure against shock. Any voltage above 30 is generally considered to be capable of delivering dangerous shock currents. Metal jewelry is definitely bad to wear when working around electric circuits. Rings, watchbands, necklaces, bracelets, and other such adornments provide excellent electrical contact with your body and can conduct current themselves enough to produce skin burns, even with low voltages. Low voltages can still be dangerous even if they’re too low to directly cause shock injury. They may be enough to startle the victim, causing them to jerk back and contact something more dangerous in the near vicinity. When necessary to work on a “live” circuit, it is best to perform the work with one hand so as to prevent a deadly hand-to-hand (through the chest) shock current path. If at all possible, shut off the power to a circuit before performing any work on it.

When working on equipment, remove all sources of power before performing any work. In industry, removing these sources of power from a circuit, device, or system is commonly known as placing it in a Zero Energy State, The focus of this lesson is, of course, electrical safety.

Dangerous voltage Spring pressure Hydraulic (liquid) pressure Pneumatic (air) pressure Suspended weight Chemical energy (flammable or otherwise reactive substances) Nuclear energy (radioactive or fissile substances)

Voltage by its very nature is a manifestation of potential energy. In the first chapter, I even used the elevated liquid as an analogy for the potential energy of voltage, having the capacity (potential) to produce a current (flow), but not necessarily realizing that potential until a suitable path for flow has been established and resistance to flow is overcome.

A pair of wires with a high voltage between them do not look or sound dangerous even though they harbor enough potential energy between them to push deadly amounts of current through your body. Even though that voltage isn’t presently doing anything, it has the potential to, and that potential must be neutralized before it is safe to physically contact those wires.

All properly designed circuits have “disconnect” switch mechanisms for securing voltage from a circuit. Sometimes these “disconnects” serve a dual purpose of automatically opening under excessive current conditions, in which case we call them “circuit breakers”.

  • Other times, the disconnecting switches are strictly manually-operated devices with no automatic function.
  • In either case, they are there for your protection and must be used properly.
  • Please note that the disconnect device should be separate from the regular switch used to turn the device on and off.

It is a safety switch, to be used only for securing the system in a Zero Energy State: Figure 1.12 With the disconnect switch in the “open” position as shown (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. Figure 1.13 With the temporary ground connection in place, both sides of the load wiring are connected to ground, securing a Zero Energy State at the load. Since a ground connection made on both sides of the load is electrically equivalent to short-circuiting across the load with a wire, that is another way of accomplishing the same goal of maximum safety: Figure 1.14 Either way, both sides of the load will be electrically common to the earth, allowing for no voltage (potential energy) between either side of the load and the ground people stand on. This technique of temporarily grounding conductors in a de-energized power system is very common in maintenance work performed on high voltage power distribution systems.

A further benefit of this precaution is 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. The temporary wire connected across the load would create a short-circuit when the disconnect switch was closed, immediately tripping any overcurrent protection devices (circuit breakers or fuses) in the circuit, which would shut the power off again.

Damage may very well be sustained by the disconnect switch if this were to happen, but the workers at the load are kept safe. It would be good to mention at this point that overcurrent devices are not intended to provide protection against electric shock.

  • Rather, they exist solely to protect conductors from overheating due to excessive currents.
  • The temporary shorting wires just described would indeed cause any overcurrent devices in the circuit to “trip” if the disconnect switch were to be closed, but realize that electric shock protection is not the intended function of those devices.

Their primary function would merely be leveraged for the purpose of worker protection with the shorting wire in place.