LINEAR LT4356 User Guide and Manual

DESIGN FEATURES L

Surge Stopper IC Simplifies Design of Intrinsic Safety Barrier for Electronics Destined for Hazardous Environments

by Murphy Pickard, Hach Co.

Introduction

As applications for electronic instru- GAS, VAPOR

OR POWDER About the Author

mentation proliferate, an increasing

Murphy Pickard is an Electronic

number of applications require equip-

Engineer in the Flow & Sampling

ment safe enough to operate in

Business Unit of Hach Company

hazardous environments. Chemical

(www.hach.com) of Loveland, CO.

COMBUSTION

plants, refineries, oil/gas wells, coal,

If you have questions about this

and textile operations are all examples

article or intrinsic safety bar-

of potentially explosive environments

OXIDIZER IGNITION SOURCE rier design, feel free to contact

that use electronic instrumentation. AIR OR THERMAL OR

the author at 800-227-4224 or

OXYGEN ELECTRIC

In order to operate safely in such en-

[email protected].

vironments, instrumentation must be Figure 1. The ignition triangle made explosion proof. Companies that supply appara- quirements. In particular, the LT4356 tus to these markets must integrate LT4356 series surge stopper series of overvoltage/overcurrent pro- protection into the design. It falls to tection devices offers an efficient and

IC can be used to design

the electronic designer to consider elegant means of creating protection

an active barrier with

available safety measures and imple- barriers in electronic apparatus. To

parameters that can be

ment them with minimum cost and fully understand the requirements

easily altered to quickly

impact on proper circuit operation. and solutions, one must become mod-

This is a daunting task from a design produce custom barriers. erately acquainted with the standards

standpoint, made even more difficult themselves, and the agencies that

Since the fundamental

by the number of hazardous environ- enforce them.

ment standards that must be met to circuit topology won’t

satisfy global or domestic markets. Intrinsic Safety and the

be changing much, once

Although the various standards are Classification of Hazardous

such an active design is

moving slowly to harmonization, in Environments

approved, it will be more

some cases they still contradict them- Simply put, in a hazardous environ-

readily approved when only

selves and each other. ment, the designer’s task is to prevent

component value changes

This article discusses the essential an ignition source from meeting an

requirements of safety standards, and are made. explosive atmosphere. There are sev-

methodologies for meeting these re- eral techniques for achieving this end,

and this article focuses on a design discipline referred to as intrinsically

Table 1. Established protection techniques

safe (IS) design. Figure 1 depicts the

‘Ex’ Designation Technique Description Application

ignition triangle, illustrating that a ‘p’ Separation: Gas Pressurization Equipment Rooms fuel, an oxidizer and an ignition source must all be present for an explosion to

‘o’ Separation: Liquid Oil Fill Transformers

occur. Several techniques simply pre- ‘q’ Separation: Semi-Solid Sand Fill Instrumentation vent an existing ignition source from contacting an explosive atmosphere,

‘m’ Separation: Solid Encapsulation Instrumentation

while Intrinsically Safe design actually

‘n’ Construction Nonincendive Switchgear eliminates the ignition source. The

principal protection techniques are

‘e’ Construction Increased Safety Lighting, Motors

listed in Table 1.

‘d’ Containment Flameproof Pumps

Separation techniques are well suited for many applications but

‘i’ Electrical Design Intrinsic Safety Instrumentation

require special sealing methods and

Linear Technology Magazine • September 2009