Excessive current events can lead to failures in electronic circuits. These failures can result in safety hazards such as fire, shock or explosion. Common types of overcurrent threats include:
Overloads occur when more current is allowed to flow through a circuit path than it was designed to carry. This excess current can generate and accumulate heat and result in complete circuit destruction and possible fire, electrocution or explosion. Sources of overload can include:
• Construction hazards cutting across power mains
• Equipment failure in the power grid
• Environmental hazards on the power grid
• Short spikes of energy within the circuit as a result of turning equipment on or off
Short circuits occur when one conducting path comes in contact with another conducting path or with ground, such as may occur due to a loose wire, insulation breakdown, or contact with water. These conditions can increase the likelihood of arcs, shock or fire hazard.
The principal forms of protection against over current conditions include fuses and resettable PTC thermistor fuses.
Their function is to limit current to acceptable levels and prevent catastrophic events, and during acceptable conditions act dormant with a minimal amount of resistance to the circuit.
Fuses will completely stop the flow of current when opened, which may be desired with sensitive, expensive or critical applications.
PTC’s offer the ability to re-set for withstanding most minor, common and recurring overcurrent events. They will allow safe levels of current to pass continuously, and during major overcurrent events, increase in resistance as they heat, to restrict the flow of current. When the overcurrent event ends, the device cools and resets to it's normal operating state.
Over voltage events (or transients) are short duration surges or spikes. Unsuppressed, they may damage circuits and components, and result in complete system failure. Below are descriptions of common types of over voltage conditions, and technologies to reduce their effects:
Electrostatic Discharge (ESD)
Damage from ESD is generally caused by transfer of static electrical charge from a body to an electronic circuit. It may result in faulty circuit operation, latent defects, and even catastrophic failure of sensitive components. ESD suppressors must have very fast response times and handle high peak voltages and currents for short durations. There are many types of products designed to suppress these types of events including Varistors & ESD protectors.
Inductive Load Switching
Switching of inductive loads, such as those that occur with transformers, generators, motors and relays, can create voltage spikes up to hundreds of volts and amps, and can last as long as 500 milliseconds, affecting both AC and DC circuits. For these applications, commonly used suppression devices include Varistors, Gas Discharge Tubes (GDT), and Transient Voltage Suppression (TVS) Diodes.
Lightning Induced Spikes
Most transients induced by nearby lightning strikes result in an electromagnetic disturbance on electrical and communication lines connected to electrical equipment. Devices that protect against these transients must have a fast response time and must be able to dissipate a large amount of energy. Products typically used to protect against these events include Varistors, TVS Diodes, GDT.