Electrical Safety and Design

Overview

This article provides a brief overview of the most important and common hazards associated with the use of electricity. It then describes some methods and systems available in electrical engineering to reduce and mitigate these risks to acceptable safe levels.

Hazards associated with the use of electricity

Fire and ignition

Electric current flowing through cabling systems is partially converted to heat. Since the amount of heat generated evolves quadratically to the current intensity, the resulting temperature rise will be more significant during circuit overload, and can be very dramatic in short circuit conditions. Depending on the environmental conditions and the materials in contact with the cabling system, such elevated temperatures can act as an ignition source, causing fire.

The operation of electrical circuits – and in particular switching elements in these circuits – can cause electric sparks. This is explained by the inductive properties of the circuit that cause the current to continue to flow for a period of time after the opening of the switch contacts. When a flammable or explosive atmosphere is present, such sparks can act as an ignition source.

Other thermal hazards

Hot surfaces can cause burns and scalds. Electric arcs radiate vast amounts of heat, with the potential for causing severe burns. Thermal effects can also cause damage to installations and buildings.

Electrification and electrocution

Electrification refers to a person (or other living organism) being subjected to electrical current. Electrocution refers to electrification with lethal consequences. The effects a person experiences are not directly related to the voltage, but the current, and depend on numerous factors. These include the path of the current through the body, and the length of time the current flows through the body. DC and AC of various frequencies have significantly different effects. Allowing currents of over 30 mA to travel through a person cannot be considered safe, and depending on circumstances even currents as low as 5 mA can constitute a hazard.

Electromagnetic fields and radiation

Potential hazards are from radio frequency and microwave heating applications. Persons entering the effective area or radiation escaping the technical installation due to faulty shielding can lead to localized overheating of tissue, resulting in specific types of burns and scalds. An electric arc will, in addition to the significant amount of heat, emit a broad spectrum of electromagnetic radiation. This intense radiation can affect a person’s health and safety through effects on the eyes and skin.

Hazard and risk

Communications concerning safety requires clear and unambiguous definitions of a number of concepts and terms:

• Safety: Freedom from unacceptable risk.
• Risk: Combination of the probability of occurrence of a hazard and the severity of that hazard.
• Hazard: Potential source of harm.
• Tolerable risk: Risk which is accepted in a given context based on the current values of society.
• Residual risk: Risk remaining after protective measures have been taken.
• Electrical hazard: Potential source of harm when electric energy is present in an electrical installation.

The risk associated with the electric power system is made greater or smaller through design and construction. For the electrical engineering domain, most national authorities have established specific sets of rules governing electrical safety.

Risk reduction by design

Proper design of an electrical system can reduce the risk associated with the use of electricity to an acceptable level. To achieve a safe electrical system, the design effort needs to identify and account for all existing hazards, and aim to either reduce the possible consequences or reduce the probability of occurrence of hot surfaces, incendiary sparks, electrification of persons by direct or indirect contact, etc. Examples include:

• Reduction of voltage levels.
• Avoiding contact by screens, guards and shields.
• Conductor sizing to limit temperature during normal operation.
• Detection and elimination of fault conditions.
• Limiting exposure to the hazardous condition.

Extra-low voltage systems

The IEC has adopted standard voltage values that are considered sufficiently safe. Extra-low voltage (ELV) is defined as not exceeding the relevant voltage limit of 50 V AC RMS and 120 V DC. In a dry skin condition, this extra-low voltage can be assumed safe to touch, for an indefinite period of time. In wet skin conditions, lower voltage levels need to be observed since the contact resistance is significantly lowered (25 V AC and 60 V DC).

Grounding, protective earth, and earth connection systems

The protective earth concept aims at reducing the risk resulting from indirect contact. The basic intent is to avoid elevating the electrical potential of the casing of an electrical apparatus to a dangerous voltage level with reference to the earth surface. Main factors to be considered are:

• The earth connection system used.
• Type of protective system used.
• Power supply type and voltage level.
• Cable design calculation results.
• Environmental conditions with respect to exposed persons.

Short circuit and overload protection

Short circuit and overload protection of electrical circuits plays an important role in risk reduction as it relates to thermal hazards. Additionally, short circuit protection limits the consequences of high mechanical forces occurring due to magnetic effects in short circuit conditions. For a technical component (cable, bus bar, switch gear, or other device) to survive a heat surge and the related mechanical stress, the time required to switch off the current needs to be very short. Common operating times range from a few milliseconds up to a few seconds.

Insulation

The primary function of electrical insulation is to separate current carrying conductive elements from other parts, components, and elements. Levels of insulation include:

• Insulation: All the materials and parts used to insulate conductive elements of a device.
• Functional insulation: Insulation between conductive parts, necessary for the proper functioning of the equipment.
• Basic insulation: Insulation of hazardous live-parts.
• Supplementary insulation: Independent insulation applied in addition to basic insulation.
• Double insulation: Insulation comprising both basic insulation and supplementary insulation.
• Reinforced insulation: Insulation of hazardous live-parts providing a degree of protection against electric shock equivalent to double insulation.

Whether or not any of the various types of can considered for safety purposes depends upon the circumstances and environment.

Arc flash protection

An arc flash protection strategy will encompass a number of techniques such as:

• Arc flash containment/arc-resistant equipment.
• Current limiting fuses.
• Arc flash detection and high-speed switching.
• Arc flash quenching.
• Remote operation.
• Strict application of safe work procedures.
• Personal protective.
• Labeling and marking.

Overvoltage protection

The design of overvoltage protective systems is based on both the coordination of the maximal voltage surge that equipment can withstand, and methods to limit the voltage surges passed on to the equipment. Strategies include:

• Preventing atmospheric overvoltages from entering the electrical system.
• Prevention of overvoltage originating from switching actions.
• Automatic supply disconnection.
• Surge suppression.

Conclusion

Compared to certain other forms of energy, electricity has the benefit that the occurrence of hazardous events is influenced relatively easily by proper design of the electrical systems and devices. This effectively reduces the risk to an acceptable level.

Further information

Application note: Design for Electrical Safety: http://www.leonardo-energy.org/good-practice-guide/design-electrical-safety