Electrical Shock and its Effects
Anyone working on electrical equipment should have respect for all voltages, have knowledge of the principles of electricity, and follow safe work procedures.
An electrical shock occurs when a body becomes a part of an electrical circuit. The effects of an electrical shock vary from a moderate sensation to paralysis to death. Also, severe burns may occur internally and where the current enters and exits the body. The quantity of electric current flowing through the body in milliamps (mA), the amount of time the body is exposed to the electric current, the route the current takes through the body, and the physical condition of the body through which the current flows; all influence the severity of an electrical shock.
|Approximate Current*||Effect on body**|
|over 20||Causes serious muscle contractions, respiratory paralysis, and cardiac convulsions.|
It may be frozen or locked to the point of electrical contact until the electric circuit is disconnected
Removal from contact point by natural reflexes
|8 or less||Shock sensation, but certainly not severe|
Table 1. Electrical shock results in any time a body becomes part of an electrical circuit.
* in mA
**effects vary depending on time, path, amount of exposure, and condition of the body
Prevention is the best medicine for electrical shock. Anyone working on electrical equipment should have respect for all voltages, have knowledge of the principles of electricity, and follow safe work procedures. All technicians should be encouraged to take a basic course in cardiopulmonary resuscitation (CPR), so they can aid a coworker in emergency situations.
A person's body becomes a part of an electrical circuit during an electrical shock. The body of a person offers varied resistance to the flow of current. Sweaty hands have less resistance than dry hands. The resistance of a wet floor is less than that of a dry floor. The lower the resistance, the higher the current flow. As the current flow increases, the severity of the electrical shock increases.
If a person is receiving an electrical shock, power should be removed as quickly as possible. If power cannot be removed quickly, the victim must be removed from contact with live parts. Action must be taken quickly and cautiously. Delay may be fatal. Individuals must also avoid being a casualty while attempting to rescue another person. If the equipment circuit disconnect switch is nearby and can be operated safely, shut OFF the power. Excessive time should not be spent searching for the circuit disconnect. In order to remove the energized part, insulated protective equipment such as a hot stick, rubber gloves, blankets, wood poles, plastic pipes, etc., can be used if such items are accessible.
After the victim is freed from the electrical hazard, help should be called, and first aid (CPR, etc.) begun as needed. The injured individual should not be transported unless there is no other option and the injuries require immediate professional attention.
Grounding is the connection of portions of the distribution system to earth in order to establish a common electrical reference and a low impedance fault path to facilitate the operation of overcurrent protective devices. Grounding provides an electrically conductive path designed and intended to carry current under fault conditions from the point of a fault on a wiring system to the electrical supply source. Grounding facilitates the operation of overcurrent protection devices.
For systems that are solidly grounded, grounding provides a means to limit the voltage to the ground during normal operation and to prevent excessive voltages due to lightning, line surges, or unintentional contact with higher-voltage lines and to stabilize the voltage to the ground during normal operation.
The non-current-carrying metal parts of a transformer installation are required by the NEC® to be effectively grounded. Conductive materials enclosing conductors or equipment are grounded to prevent a voltage or difference of potential on these materials. Circuits and enclosures are grounded to allow overcurrent devices to operate in the event of insulation damage or ground faults.
The electrical distribution system is grounded by connecting it to a metal underground water pipe, a building's metal frame, a concrete-encased electrode, or a ground ring. To prevent problems, a grounding path must be as short as possible and of sufficient ampacity, never be fused or switched, be a permanent part of the electrical circuit, and be continuous and uninterrupted from the electrical circuit to the ground.
The ground is provided at the main service equipment or at the source of a separately derived system (SDS). A separately derived system (SDS) is a system that supplies electrical power derived or taken from transformers, storage batteries, solar photovoltaic systems, or generators. See Figure 2. The majority of separately derived systems are produced by the secondary of a distribution transformer.
Figure 1. A separately derived system (SDS) is a system that supplies electrical power derived or taken from transformers, storage batteries, solar photovoltaic systems, or generators. Image Courtesy of Sask Power
The neutral ground connection must be made at the transformer or at the main service panel only. The neutral ground connection is made by connecting the neutral bus to the ground bus with a main bonding jumper. The main bonding jumper (MBJ) is a connection at the service equipment that connects the equipment grounding conductor, the grounding electrode conductor, and the grounded conductor (neutral conductor). The purpose of the main bonding jumper is to bond the neutral and equipment grounding conductor together with the enclosure to create a common reference potential.
An equipment grounding conductor (EGC) is an electrical conductor that provides a low-impedance ground path between electrical equipment enclosures within the distribution system and takes current back to the source. A grounding electrode conductor (GEC) is a conductor that connects grounded parts of a power distribution system (equipment grounding conductors, grounded conductors, and all metal parts) to the grounding system.
A grounded conductor is one that has been intentionally grounded. The grounded conductor is commonly a neutral conductor. However, not all electrical distribution systems use the grounded conductor as a neutral. For example, corner-grounded delta systems contain a grounded conductor that is not a neutral conductor. Therefore, it is not correct to refer to all grounded conductors as neutral conductors, although that is the case in the majority of electrical distribution systems.
Ground Fault Circuit Interrupters
A ground fault circuit interrupter (GFCI) detects an imbalance of current in the normal conductor routes and opens the circuit to safeguard against electrical shock. A GFCI opens the circuit when the current in two conductors of an electrical circuit differ by more than 5 mA. A GFCI is designed to trip quick enough (1/40 of a second) to avoid electrocution (1/4 of a second).
A potentially dangerous ground fault is any quantity of current above the level that might cause a harmful shock. Any current more than 8 mA is regarded as potentially harmful -- depending on the path the current follows, the physical state of an individual receiving the shock, and the length of time the individual is exposed to the shock. GFCIs are therefore necessary for places like homes, hotels, resorts, industrial sites, receptacles around swimming pools, and other places where a person may encounter a ground fault.
A GFCI compares the current flowing through the ungrounded (hot) conductor with the current flowing through the neutral conductor. A ground fault occurs if the current in the neutral conductor falls below the current in the hot conductor. The missing current is returned to the source via some path other than the intended one (fault current).
GFCI protection can be installed at various points throughout a circuit. Ground fault protection is provided at the point of installation with direct-wired GFCI receptacles. GFCI receptacles can also be used to protect all other receptacles installed downstream along the same circuit. When implemented in a load center or panel board, GFCI circuit breakers offer GFCI protection as well as conventional circuit overcurrent protection for all branch-circuit elements connected to the circuit breaker.
Plug-in GFCIs protect against ground faults for devices that are plugged into them. These plug-in devices are frequently used by personnel working with power tools in areas without GFCI receptacles.
Portable GFCIs are designed to be easily moved from one location to another (see Figure 2). Portable GFCIs commonly contain more than one receptacle outlet protected by an electronic circuit module. Portable GFCIs should be inspected and tested before each use. GFCIs have a built-in test circuit to ensure that the ground fault protection is operational.
Figure 2. A portable GFCI can be used on a job site to protect workers. Image Courtesy of Cable Organizer
A ground fault circuit interrupter (GFCI) safeguards against the most common type of electrical shock hazard, the ground fault. Line-to-line contact hazards, such as a technician holding two hot wires in each hand, are not protected by GFCIs. GFCI protection is mandatory in addition to NFPA grounding requirements.