Introduction

Many loads in the life safety industry are inductive in nature .  Inductive loads are devices which use coils of wire to generate a magnetic field - these include magnetic locks, door strikes, solenoids, crash bars, and electric motors as well as other devices .  Unlike a simple resistive load, inductive loads can present some challenges when integrating them into a DC power system. 

Inductive Loads

To understand the effects of an inductive load, we must first understand the basic operation of an inductive load .  Ignoring the function of the load, we will just view it as a simple coil of wire for this discussion . When an inductive load is powered, a magnetic field is generated in the coil of wire .  Faraday‚Äôs Law states that a change in the magnetic field around a circuit will generate an electromotive force (voltage) in that circuit .  This means that the magnetic field generated causes the inductor to resist changes in current through it by generating a voltage to offset the current change (called a back electromotive force, or backEMF) .  This voltage is determined by the amount of change in current and the time in which the change occurs - increasing the change in current or shortening the time period in which the change occurs will both result in an increased back-EMF.

In a life safety application, devices such as magnetic locks are typically controlled by a relay contact .  When the relay contact in the circuit of a simple magnetic load is opened, the current instantly drops to zero .  This large, fast change in current causes the magnetic field to collapse, briefly causing the coil to become a high voltage source and feeding a short duration, negative high voltage spike back into the wiring . 

This returned high voltage spike can cause arcing of relay contacts as the high voltage looks for a path to dissipate, greatly shortening relay life .  The negative voltage can also travel to other sensitive devices in the system, causing problems such as lockup of microprocessors, false triggering of overcurrent protection, and possibly damaging devices within the system. 

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