Minimize downtime with electronic circuit protection

Switch-mode power supplies have many advantages, but when used to power industrial controls they can have one serious drawback. When a fault or overload occurs on one control circuit fed by a 24 V dc switch-mode power supply, the supply may shut down, removing power from all attached components and causing expensive downtime.

When a simple transformer-based power supply is used, an overload will cause a gradual drop in voltage output (see “Basic power supply” graphic), but the power supply will continue operating unless the overload is severe or persists. Switch-mode power supplies, on the other hand, quickly shut down when load current reaches 110% of nominal (see “Switch-mode power supply” graphic). If the load includes capacitors or motors, inrush current can cause a shutdown. Switch-mode power supplies are designed to restart after a short pause. If the overload is still present, the supply may repeatedly power up and shut down, a condition called the “hiccup” mode.

Selective circuit protection

The obvious approach to solving this problem is to add a circuit breaker to each load circuit so an overload on one circuit will not shut down the supply. Unfortunately, conventional circuit breakers will not help in this situation.

The 24 V dc transformer-based power supply normally provides 10 Amps of current. Let’s say it has four circuits, each drawing 2 A (and protected by a 2 A thermal-magnetic circuit breaker), for a base load of 8 A. If one circuit overloads, it will trip the breaker at six times rating, or 12 A, within one second. The power supply must deliver 18 A (12 A + 2 A + 2 A + 2 A) to trip the breaker. A transformer-based power supply can do this, although at reduced voltage.

The maximum current provided by a 10 A switch-mode power supply is 11 A (110% of nominal). If an overload greater than 2 A occurs on one circuit, the supply will shut down before the circuit breaker detects enough current to trip.

A possible solution might be fast-acting circuit breakers, which trip at two times rating, rather than at six times. But a fast-acting circuit breaker can trip on inrush currents, which are often six to ten times the normal load and last for several milliseconds.

When a transformer-based power supply is overloaded, its output voltage decreases, but will not shut off abruptly.

A better solution is a new class of circuit breaker that allows selective protection of equipment connected to a switch-mode power supply, while avoiding nuisance tripping and supply shutdowns. New “smart” electronic circuit breakers tolerate a momentary inrush and limit the maximum current to no more than 1.8 times their rating. And, like conventional circuit breakers, they open the circuit when the current is too high for too long.

A new generation

Electronic circuit breakers are transistor-based, and can provide “active current limiting” —the transistor measures the load current through a shunt, compares it with a pre-programmed value, and electronically adds the necessary impedance to reduce it to a safe value before tripping its internal circuit breaker.

When the overcurrent reaches 110% to 180%, a smart breaker will disconnect in three seconds or less. It will also disconnect high overloads that last more than 50 milliseconds. In addition, it physically isolates the load when tripped or when the manual on/off button is pressed.

Electronic circuit breakers often combine a mechanical circuit breaker and an electronic switch. Physical disconnection typically follows electronic disconnection in five seconds. Fault status is communicated via LED, and the device can send a signal to a logic controller or system software.

For failsafe protection, the breaker also includes a thermal technology mechanism, which will trip the circuit breaker in cases of electronic failure or in case of high amperage faults.

Switch mode power supplies have high electrical efficiency that helps to minimize heat buildup in control cabinets, but the way they react to overloads can cause unnecessary downtime. New electronic circuit breakers can protect individual load circuits and, by limiting fault currents, help keep power supplies from shutting down.

Author Information

Kenneth Cybart is senior applications engineer at E-T-A Circuit Breakers,

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