Intrinsic safety is a protection technique for safe operation of
electrical equipment in hazardous areas by limiting the energy available for
ignition. In signal and control circuits that can operate with low currents and
voltages, the intrinsic safety approach simplifies circuits and reduces
installation cost over other protection methods. Areas with dangerous
concentrations of flammable gases or dust are found in applications such as
petrochemical refineries and mines. As a discipline, it is an application of
inherent safety in instrumentation. High-power circuits such as electric
motors or lighting cannot use intrinsic safety methods for protection.
Operating and Design Principles In normal use, electrical equipment
often creates tiny electric arcs in switches, motor brushes, connectors, and
in other places. Compact electrical equipment generates heat as well, which
under some circumstances can become an ignition source.
There are multiple ways to make equipment explosion-proof, or safe for
use in ex-hazardous areas. Intrinsic safety is one of a few methods available
for ex-hazardous areas. Others include explosion proof enclosures, venting, oil
immersion, powder and sand filling, and hermetic sealing. For handheld
electronics, intrinsic safety is the only realistic method that allows a
functional device to be explosion-proof. A device termed intrinsically safe is
designed to be incapable of producing heat or spark sufficient to ignite an
explosive atmosphere. There are several considerations in
designing intrinsically safe electronics devices: reducing or eliminating
internal sparking, controlling component temperatures, and eliminating component
spacing that would allow dust to short a circuit. Elimination of spark potential
within components is accomplished by limiting the stored energy in any given
circuit and the system as a whole. Temperature, under certain fault
conditions such as an internal short in a semiconductor device, becomes an issue
as the temperature of a component can rise to a level that can ignite some
explosive gasses, even in normal use. Safeguards, such as current limiting by
resistors and fuses, must be employed to ensure that in no circumstance can a
component reach a temperature that could cause autoignition of a combustible
atmosphere. In the highly compact electronic devices used today PCB's
often have component spacing that create the possibility of an arc between
components if dust or other particulate matter works into the circuitry, thus
component spacing, siting and isolation become important to the design.
The primary concept behind intrinsic safety is the restriction of available
electrical and thermal energy in the system so that ignition of a hazardous
atmosphere cannot occur. This is achieved by ensuring that only low
voltages and currents enter the hazardous area, and that no significant
energy storage is possible. One of the most common methods for
protection is to limit electric current by using multiple series resistors; and
limit the voltage with multiple zener devices to ground. Sometimes an
alternative type of barrier known as a galvanic isolation barrier may be used.
Certification standards for intrinsic safety designs, which vary by device
type, generally require that the barrier not exceed approved levels of voltage
and current with specified damage to limiting components.
Equipment or instrumentation for use in a hazardous area will be designed to
operate with low voltage and current, and will be designed without any large
capacitors or inductors that could discharge in a spark. The instrument
will be connected, using approved wiring methods, back to a control panel in a
non-hazardous area that contains safety barriers. The safety barriers ensure
that, no matter what accidental contact occurs between the instrument circuit
and outside power sources, no more than the approved voltage or current enters
the hazardous area. For example, during marine transfer
operations when flammable products are transferred between the marine terminal
and tanker ships or barges, two-way radio communication needs to be
constantly maintained in case the transfer needs to stop for unforeseen
reasons such as a spill. The United States Coast Guard requires that the two
way radio must be certified as intrinsically safe.
Another example is intrinsically safe or explosion-proof mobile phones used in
explosive atmospheres, such as refineries. Intrinsically safe mobile
phones must meet special battery design criteria in order to achieve UL, ATEX
directive, or IECEx certification for use in explosive atmospheres.
No single field device or wiring is intrinsically safe by itself, but is
intrinsically safe only when employed in a properly designed IS system. Such
systems are usually provided with detailed instructions to ensure safe use
and maintenance. Certifying agencies
Several different agencies develop standards for intrinsic safety, and
evaluate products for compliance with standards. Agencies may be run by
governments or may be composed of members from insurance companies,
manufacturers, and industries with an interest in safety standards. Certifying
agencies allow manufacturers to affix a label or mark to identify that the
equipment has been designed to the relevant product safety standards.
Examples of such agencies in North America are the Factory Mutual Research
Corporation, which certifies radios, Underwriters Laboratories that certifies
mobile phones, and in Canada the Canadian Standards Association. In the
EU the standard for intrinsic safety certification is the ATEX directive,
while in other countries around the world the IECEx standards are followed.
To facilitate world trade, standards agencies around the world engage in
harmonization activity so that intrinsically safe equipment
manufactured in one country eventually might be approved for use in another
without redundant, expensive testing and documentation.
See also ATEX directive
References Intrinsic safety on-line assessment tool
Further reading Redding, R.J., Intrinsic Safety: Safe
Use of Electronics in Hazardous Locations. McGraw-Hill European
technical and industrial programme. 1971. ISBN 978-0-07-094224-0
Paul, V., '"The earthing of intrinsically safe barriers on offshore
transportable equipment". IMarEST. Proceedings of IMarEST - Part A -
Journal of Marine Engineering and Technology, Volume 2009, Number 14,
April 2009, pp. 3–17(15) Intrinsic Safety in Cellphones,
Smartphones and PDA's, Airo Wireless . "AN9003 - A Users Guide to Intrinsic
Safety". Cooper Crouse Hinds. Retrieved 25 Sep 2012.
Essential Concepts of Intrinsic Safety, Spark Institute .