7.3: Industrial Electrical Safety
Electrical Safety Considerations
Electrical Shock
Electricity flows along a circuit that consists of a power source, a load, and conductors. The human body can become a conductor and a part of the electric circuit which can result in electrical shock. Exposure to electrical energy may result in no injury at all or may result in physical and/or neurological damage or death. An minor electrical shock may cause muscle pain and may trigger mild muscle contractions or startle people, causing a fall. However, high resistance contact may cause dielectric breakdown at the skin, lowering skin resistance, causing surface damage, but more often tissues deeper underneath the skin have been severely damaged. Electric shocks can paralyze the respiratory system or disrupt heart action, causing instant death.
The outcome of an electrical shock depends on several factors that are determined by the relationship between current, voltage, and resistance, also known as Ohm’s Law.
Ohm’s Law
Voltage or Electrical Force (V)
Amperage or Current Flow (I)
Resistance or Ohms (Ω or R )
Current=Electrical Force/Resistance or I=V/R
Exposure Conditions play an important role in the extent of injuries sustained as a result of electric shock. These factors include:
- Duration- The longer a human body remains part of an electrical circuit, more tissue and neurological damage can occur.
- Pathway- Electricity is always seeking the path of least resistance to ground. If both hands of a person are part of the pathway, the current has more potential to affect the heart resulting in ventricular fibrillation. If the current chooses another path such as hand-to-foot, tissue and internal organ damage still may occur.
- Humidity/Saturation- Electricity can easily flow through water or moisture in the air. Humidity also can effect how much a body sweats, which can lower a persons resistance to electrical current.
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Skin Condition-
The human body’s resistance to current is affected moisture content:
- Dry Skin- 100,000 to 500,000 ohms of resistance
- Perspiring (sweaty hands)- 1000 ohms of resistance
- In Water (completely wet)- 150 ohms of resistance
Amperage Kills
See interactive table Amperage Kills
Example:
A worker is using a faulty 120 volt tool on a hot and humid day and is sweating heavily. The worker’s body resistance is approximately 1,000 ohms.
Using Ohm’s law:
• Current = 120 volts / 1,000 ohms.
• Current = 0.12 amps or 120 mA.
According to the above table, this amount of current will cause a painful shock, the workers’s breathing may stop, there will be severe muscle contractions, and death is possible.
Arc Flash and Arc Blast
Arc-Flash-Arc- Flash burn can occur when an electrical equipment malfunction causes an extremely high temperature area around the arc that can reach as high as 35,000 degrees Fahrenheit. Electrical burn can also occur any time an electrical current flows through bone or tissue.
Arc-Blast- When an arc occurs, a blast causes molten metal to be thrown through the air and onto the skin or into the eyes. The speed of the molten metal in an arc-blast is estimated at approximately 700 mph.
Donnie’s Accident Story
An interactive or media element has been excluded from this version of the text. You can view it online here:
Lockout-Tagout
Lockout-tagout is a safety procedure used in industry settings to ensure that dangerous machines and circuitry are properly shut off and not started up again prior to the completion of maintenance or service work. It requires that hazardous power sources be “isolated and rendered inoperative” before any repair procedure is started. “Lock and tag” works in conjunction with a lock securing the device or the power source with the hasp, and placing it in such a position that no hazardous power sources can be turned on. The procedure requires that a tag be affixed to the locked device indicating that it should not be turned on.
When two or more subcontractors are working on different parts of a larger overall system, the locked-out device is first secured with a folding scissor-like clamp that has many padlock holes to hold it closed. Each subcontractor secures their own padlock to the clamp. The locked-out device cannot be activated until all workers have signed off on their portion of the project and removed their padlock from the clamp. A lock selected by color, shape or size (e.g. red padlock) is used to designate a standard safety device, locking and securing hazardous energy. No two keys or locks should ever be the same. A person’s lock and tag must not be removed by anyone other than the individual who installed the lock and tag unless removal is accomplished under the direction of the employer.
- Identify the energy source(s)
- Isolate the energy source(s)
- Lock and Tag the energy source(s)
- No keys alike
- May only be removed by the installer
- Prove that the equipment isolation is effective
Electrical Systems and Testing Terminology
- Continuity- presence of a complete path for current to flow.
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Resistor-
implements electrical resistance as a circuit element. In electrical circuits, resistors are used to reduce current flow, adjust signal levels, and to divide voltages.
- Fixed value- have a single value of resistance.
- Potentiometer- provides variable resistance by adjustment.
- Open Circuit- has intended interrupted path.
- Closed Circuit- has a complete path.
- Short Circuit- unintended path between two conductors.
- Ground Fault- unintended path to ground.
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Arc Fault-
Normal when motor brushes spark and at receptacles when plugging in appliances and equipment that are in the “on” position.
- Series- conductor is series with load is unintentionally broken.
- Parallel- caused by short circuit or ground fault.
General Safety Rules for Electrical Maintenance Technicians
- Safety glasses, goggles, or face shields must be worn any time a hazard exists that can cause foreign objects to get in your eyes from the front or the sides.
- Head protection must be worn whenever there is a potential for objects to fall from above, for bumps to your head from objects fastened in place, or for accidental contact with electrical hazards.
- Hand protection must be worn any time your hands are exposed to a potential hazard.
- Do not wear clothing with exposed zippers, buttons, or other metal fasteners.
- Remove rings, wristwatches, and any other metal jewelry before beginning work.
- Make sure that tools used on energized electrical equipment are nonconductive and have the proper voltage rating.
- Install all electrical wiring according to the current NEC® codes.
- Work with a buddy. Avoid working alone.
- Always turn power off and lock it out before working on any electrical circuits or equipment.
- Never cut off the grounding prong from a three-prong plug on any power extension cord or from a power cord to any piece of equipment.
- Do not defeat the purpose of any safety devices such as fuses or circuit breakers.
- Do not open and close switches under load unless absolutely necessary.
- Assume all electrical equipment to be “live” and treat it as such.
Non-Energized Testing
Never test an energized circuit when individual components of the circuit can normally be tested by other means. Most electrical components and pathways of electrical systems can be individually tested for continuity and resistance.
Insulated Tools
Insulated tools are designed for safety and are rated for live use up to 1000 VAC or 1500 VDC. They must be tested at 10 times that value (more than 10,000 V). Insulated tools should be used and stored differently from conventional, non-insulated tools. When being used, they should be kept isolated from other tools, including other insulated tools, to prevent them from getting scraped or nicked. They should be inspected prior to each use and discarded or tested by a reliable authority if damage is suspected.
- Tool Rating: 1000 VAC or 1500 VDC
- Glove Rating: Tested: 20000VAC/50,000V DC., Max Use:17000VAC/25500V DC