Electrical
Reference Guide (OSHA)
What is the definition of the following terms?
Voltage – the potential difference (pressure or force) that causes charged particles (electrons) to move through a conductor.
Volts (V) – the unit of measurement of voltage. The potential difference or the amount of work done to move a charged particle through a conductor.
Current – the flow of electric charges (electrons) that move past a specific point in a conductor within a specific amount of time.
Amperes (Amps) – the unit of measurement of the rate at which current flows.
Resistance – the opposition that a conductor offers to the flow of electric current through it. Good conductors offer little resistance and poor conductors offer high resistance. As the size of the conductor increases, the resistance decreases.
Ohms () – the unit of measurement of resistance.
Ohms Law – states that V = I/R; where V = voltage in volts, I = current in amps and R = resistance in ohms.
Ground – a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or some conducting body that serves in place of the earth.
Earth Resistance – the resistance of soil to the passage of current. The earth is a relatively poor conductor of electricity. However, if the path for current is large enough, the resistance can become quite low. In this situation, the earth can become a good conductor.
What type of electrodes are normally designed and installed for grounding?
Driven and buried rods.
Buried ground ring and/or grid.
Buried metal plates.
What factors affect resistance when designing grounding electrodes and/or systems?
Soil moisture and composition.
Soil temperature.
Number and type of electrodes/rods.
Electrode depth.
Electrode diameter.
Electrode(s) spacing distance.
What is the Fall of Potential Method (Three Point Test) for testing grounding conductors?
This testing method measures resistance between an electrode and its surrounding earth. The meter used requires three test leads – one lead to electrode being tested and two leads to test probes.
What is the Two Point Ground Test Method?
This testing method utilizes the series resistance readings of the electrode under test, existing electrode and their surrounding earth. The test is similar to a continuity test and requires the test leads to be “zeroed out” prior to testing.
What safety issues must be taken into consideration when performing ground testing?
A fault could develop in the power system, which could result in a large current flowing into the ground system while the test is being completed. This condition could result in high voltage and current levels being present at the test probes and at the meter. Those employees conducting the test should use appropriate personal protective equipment, use only non-metallic tape measures and check for stray voltages prior to conducting the test.
What causes ground faults?
Ground faults occur when the electrical current in a circuit strays outside of the path of the circuit. Current can stray outside of the path of the circuit due to damaged conductors (wires) or due to water getting into an electrical appliance or tool. If a person’s body provides a path for the stray current to the ground, electric shock can occur.
What are ground-fault-circuit-interrupters (GFCIs)?
GFCIs are electrical devices installed in an electrical circuit that are designed to quickly disconnect the circuit and thus stop the flow of current in the circuit if the flow of electricity strays or leaks out of the conductors (wires) due to bad insulation, nicks, cuts or cracks. GFCIs "trip" or disconnect the circuit if a person’s body becomes a path to ground.
How do ground-fault-circuit-interrupters (GFCIs) work?
GFCIs operate by measuring the current flowing in the two conductors (the white and black wires) leading to the receptacle or equipment in the circuit protected by the GFCI. GFCIs do not depend on or use the ground wire (the bare copper or green wire). If the current in the two conductors differs by more than 6/1000 of an amp, the GFCI disconnects the circuit, thus shutting off the current in the circuit in a fraction of a second.
What are some factors that influence the effects of electric shock on the human body?
There are several factors that can determine the seriousness of the injuries caused by electrical shock.
Path of current as it enters and exits the body. If the current flows through a vital organ, such as the heart, serious injury is likely to occur.
Amount and duration of the current. The amount of physical damage done to the human body increases as the amount and duration of the current increases.
Surface area of the body making contact. The conductivity of a person’s body increases as the surface area making contact with the electrical source increases.
Wet or damp conditions. Since water will conduct electricity, a person’s body resistance will decrease if the person is working in a wet or damp area, or if the person is perspiring. Dry, unbroken skin has a high resistance of approximately 100,000 ohms. Skin that becomes wet may have a resistance as low as 1,000 ohms. Injury is more likely to occur as the body’s resistance decreases.
Metal objects such as jewelry. Gold and silver are excellent conductors and will heat when they are in contact with electricity. Persons wearing these objects when they come in contact with an electrical current are more subject to serious burns. These pieces of jewelry can also become part of a conductive path, thus increasing the likelihood of electric shock.
Contact with ground or grounded objects. Since the flow of electrical current is determined by the resistance it encounters back to the source, a person’s body could complete a low resistance path back to the source if the person comes in contact with a ground or a grounded object. The amount of physical damage done to the human body increases as the flow of current increases.
Personal protective equipment. Workers who use proper personal protective equipment such as rubber insulated/tested electrical gloves, rubber mats, hot sticks, etc. can reduce or eliminate the possibility of electric shock.
Human Body Response to Electric Current
60 Hz AC Current /Response
0.5 - 3 mA / Begin to feel the currentas a tingling sensation.
3 - 10 mA / Experience pain, muscle contractions.
10 - 40 mA / Grip paralysis threshold, where
the body can not physically let go.
30 - 75 mA / Respiratory system shuts down.
100 - 200 mA / Heart fibrillation.
200 - 500mA / Heart contracts and clamps tight.
Over 1,500 mA / Tissue and organs burn.
1