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Power Supplies & Circuit Protection

April 4, 2008
An Overload on Any Circuit That Increases the Total Current to More Than 11A Will Cause the Power Supply to Go into Its Shutdown or Hiccup Mode
By Dan Hebert, Senior Technical Editor

Electromechanical and analog technologies once dominated the world of automation and electrical systems. Automation systems migrated to electronic digital technologies a few decades ago as PLCs and DCSs replaced relays, timers and analog instruments.

The electrical systems that power and protect process plant automation systemsfollowed suit. Among the first to transition was the power supply. Switch-mode power supplies have now replaced transformer-based power supplies in many applications, but in some instances switch-mode power supplies don’t work well with thermal and thermal-magnetic circuit breakers. E-T-A Circuit Breakers illustrates why in a white paper summarized below.

The white paper refers to an automotive plant application where a switch-mode, 24-VDC power supply feeds a number of loads. Process plants have many similar applications, such as 24-VDC instrument circuits.

When a transformer-based power supply sees a load change, it reacts based on basic electrical principles. A current overload due to an increase in load will cause a gradual drop in voltage output. A voltage drop may disable control components powered by the supply, as they typically require at least 18 volts to operate.

Switch-mode power supplies, on the other hand, are designed to shut down when the total load current reaches 110% of nominal current. Shutdown is typically via a quick-operating electronic switch.

Most of the components that make up the power supply load have capacitive behavior when switching on, so initial current inrush can easily exceed the range of nominal current, causing the power supply to shut down. Switch-mode power supplies can handle this condition. If the overload is still present after initial inrush, the power supply will repeatedly try to power up and then immediately shutdown or “hiccup.”

If a power supply is supplying multiple circuits, an overload condition on just one circuit that is not cleared quickly will cause the power supply to shut down. To protect individual circuits fed from one power supply, the system designer will often place a thermal or thermal-magnetic circuit breaker between each individual circuit under load and the power supply.

Let’s look at the behavior of a typical 10-A, 24-VDC switch-mode power supply. Let’s say it has four circuits drawing 2A each for a base load of 8A. The maximum current the switch-mode power supply will provide is 11A or 110% of nominal current. Therefore, an overload on any circuit that increases the total current to more than 11A will cause the power supply to go into its shutdown or hiccup mode.

Conventional circuit breakers are effective in many applications. But because a switch-mode DC power supply output is current limiting, no conventional circuit breaker is able to limit the current to a safe value that avoids shutting down the power supply while distinguishing between a momentary inrush and a short circuit.

Electronic switches overcome the limitations of thermal and thermal magnetic breakers in this and many other applications. These switches give more control in defining which current overload conditions trigger a trip.

For example, E-T-A’s smart electronic circuit protectors tolerate a momentary inrush and at the same time limit the maximum current to no more than 1.8 times their rating. These circuit protectors can provide advanced circuit protection of automation components, such as sensors, actuators and fieldbus couplers.

Electronic circuit protectors use a mechanical circuit breaker in series with an electronic switch. This allows isolation of a faulty circuit while still aggressively protecting control system components. 

Precise trip-current monitoring, remote operation capability and remote status indication make electronic circuit protectors a good match for modern solid-state automation and electrical systems.

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