SURGE PROTECTION FOR INTRINSICALLY SAFE SYSTEMS

INTRODUCTION

Electronic systems used on process plants or telemetry/monitoring networks
associated with these are always at risk from surges and transients caused by
power faults or nearby lightning strikes. These transients are just as likely to
affect systems located in or connected to hazardous areas as those in safe
areas. However, the certification and approvals needed before electrical and
electronic systems can be used in potentially explosive atmospheres makes the
application of surge protection a little more complicated. This Application Note
describes the interaction of surge protection devices (SPDs) with certified and
approved intrinsically safe systems for hazardous areas.

The need for surge protection
Surge protection is needed because modern electronic systems rely on high
performance electronic components. However, design considerations for
modern systems include faster operation, smaller and cheaper components
and lower power consumption; factors which also impair their ability to
dissipate any significant impressed energy, so increasing their vulnerability to
induced overvoltages and surge currents.

The requirements of systems integrity in hazardous locations are often more
stringent than in other locations. For example, emergency shutdown systems
are designed to cope with failures in equipment and power supplies Ð often
using duplicated or triplicated sensors, interface cards, processors and even
actuators. These systems are generally connected to process sensors by
cables which are potential entry routes for surges and transients. Surge
protection devices (SPDs) are therefore designed to improve the resilience of
such systems to induced transients.

One aspect of surge protection not always appreciated is that an SPD operates
locally, i.e. it protects only that part of a loop provided with common grounding.
Typically, an SPD mounted at the back of a panel will protect systems within the panel; but, if the field devices also need protecting, this must be done with
additional SPDs at the field device locations.

INTRINSICALLY SAFE SYSTEMS

Ignition of a potentially explosive atmosphere can be prevented by limiting the
available electrical energy to levels below which ignition can take place.
Intrinsically safe systems achieve this by one or both of two methods. One
method intersperses energy limiting interfaces (such as shunt-diode safety
barriers or galvanic isolators) at the safe-area end of each loop while the other
makes use, wherever possible, of hazardous-area devices designed to neither
store nor generate sufficient energy to cause ignition. The combination of
approved hazardous and safe-area devices has proved extremely successful in
instrumentation and control applications. Figure 1 illustrates the basic design of
an intrinsically safe (IS) loop.

Intrinsically safe SPDs
Surge protection devices in intrinsically safe circuits must meet the same
standards of design and construction as the intrinsically safe equipment, i.e.
they must either be considered to be simple apparatus with respect to the
performance of energy-storing or voltage-generating components or must be
certified as being within the safety parameters of the intended application.
Approved apparatus or simple apparatus.