When switched mode power supplies are used to energize control circuits in an industrial application, some form of circuit protection is typically applied between the power supply and the control-circuit loads to protect the equipment on the control circuit, should a fault occur.
Power supplies have built-in protection, but this protection is designed to protect the power supply itself. Control circuit faults can occur in such a way as to damage sensitive electronic components on the circuit long before the power supply shuts down-if it ever does.
Traditional Circuit Protection (Fuses and Thermo-magnetic circuit breakers) have often been applied to protect the control circuits. This technology is typically quite effective if a short occurs while the circuit is energized.
But, not all faults follow this pattern.
On an installation or repair, a wiring problem can create a short circuit that will only be discovered once the circuit is powered up.
Sometimes the load on a circuit increases beyond its design due to mechanical equipment failure. For example a bearing failure causes a motor to overload, drawing additional current.
And sometimes, there is the opposite problem: normal current-inrush when energizing inductive loads can cause nuisance tripping, when no actual fault occurred.
The nature of Traditional Circuit Protection (Fuses and Thermo-magnetic circuit breakers) is that they "assume" infinite current available on a circuit. In the real-world on non-infinite budgets, however, power supplies are a specified based on the expected demands of the circuit they will be energizing. In fault or overload conditions, the current observed on the circuit can go up, while the voltage falls. This condition of high current and low-voltage can remain long enough-before the thermal protection of traditional circuit breakers kicks in-to damage components on the circuit.
Electronic Circuit Protection monitors both current and voltage and permits only so much current on the outputs that the input voltage of this unit (which corresponds to the output voltage of the power supply) does not fall below 21V. This ensures a reliable supply voltage for sensitive equipment, such as PLCs, controls or sensors, when they are connected directly to the same power supply as the Bulletin 1692 protection module.
The protection module has one 24V input and four output channels to which the current is distributed. (By distributing the current of a large power source to four lower current output channels, smaller wires can be used.)
Each output channel is equipped with independent over-current protection. All four output channels will shut down simultaneously, if the current of one individual channel or the maximum allowed current for the protection module is exceeded.
A voltage monitor circuit in the input stage of the Bulletin 1692 module works like a valve. It permits only so much current that the input voltage does not drop below 21V. In case the input voltage would fall below this value (e.g. due to overloads, too small of a power supply or high inrush currents such as from starting a motor), all four output channels will be actively current limited and will shut down after a certain period of time.
In order to illustrate what happens in these kinds of conditions, we constructed a demonstration board that allows uses to observe how thermomagnetic circuit breakers behave under a variety of fault conditions compared to a Allen-Bradley Bulletin 1692 Electronic Circuit Protection Module.
You can use a virtual version of this board to test the performance of the Allen-Bradley Bulletin 1692 Electronic Circuit Protection Module under different fault conditions by clicking on the fault scenarios below. You can run a real-time video of each test on the actual hardware by clicking on the "play" near each circuit protection device.
Fuses are available with various tripping characteristics. Thermo-magnetic circuit breakers, too, are available with various trip characteristics, often expressed in graphical form as "trip curves." The typical "C curve" breaker has a magnetic trip activated at 5 ... 10 times the rated current of the circuit breaker and is intended to accommodate inrush current from inductive loads. "B curve" breakers have a magnetic component at lower thresholds and are typically used on resistive loads. "Z curve" breakers (available from other manufacturers) are often stated to protect solid state circuits, and have an even lower tripping threshold. Note, all have similar thermal characteristics.