Question

if a system is able to procude 5v to 45 v max. 2 amps to 20 amps max. how do you prevent malfunction of this systsm through GFI or circuit breaker? for example. if the max current is 20 amps and the system is drawing 20.1 amps. how do we degrade the system so that it will go back to 20 amps. draw a circuit of GFI or circuit breaker to explain please. Please dont shutdown the system if it goes over the limit, but reverse it back to the right amps or voltage without ahutting down the system.

Answer 1

The circuit breaker must first detect a fault condition. In small mains and low voltagecircuit breakers, this is usually done within the device itself. Typically, the heating or magnetic effects of electric current are employed. Circuit breakers for large currents or high voltages are usually arranged with protective relay pilot devices to sense a fault condition and to operate the opening mechanism. These typically require a separate power source, such as a battery, although some high-voltage circuit breakers are self-contained with current transformers, protective relays, and an internal control power source.

Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit; This is commonly done using mechanically stored energy contained within the breaker, such as a spring or compressed air to separate the contacts. Circuit breakers may also use the higher current caused by the fault to separate the contacts, such as thermal expansion or a magnetic field. Small circuit breakers typically have a manual control lever to switch off the load or reset a tripped breaker, while larger units use solenoids to trip the mechanism, and electric motors to restore energy to the springs.
Thermal-Magnetic Trip Unit :

In addition to providing a means to open and close its contacts manually, a circuit breaker must automatically open its contacts when an overcurrent condition is sensed.

The trip unit is the part of the circuit breaker that determines when the contacts will open automatically.

In a thermal-magnetic circuit breaker, the trip unit includes elements designed to sense the heat resulting from an overload condition and the high current resulting from a short circuit. In addition, some thermal magnetic circuit breakers incorporate a “PUSH TO TRIP” button.

Trip Mechanism

The trip unit includes a trip mechanism that is held in place by the tripper bar. As long as the tripper bar holds the trip mechanism, the mechanism remains firmly locked in place.

Trip Unit with Trip Mechanism

The operating mechanism is held in the “ON” position by the trip mechanism. When a trip is activated, the trip mechanism releases the operating mechanism, which opens the contacts. Note: the drawings in this section show an AC power source; however, a DC source could also be used.

[The operating mechanism is held in the “ON” position by the trip mechanism.]
The operating mechanism is held in the “ON” position by the trip mechanism.

Manual Trip

Some molded case circuit breakers, especially larger breakers, can be manually tripped by pressing the “PUSH TO TRIP” button on the face of the circuit breaker. When the button is pressed the tripper bar rotates up and to the right. This allows the trip mechanism to “unlock” releasing the operating mechanism.

The operating mechanism opens the contacts.

The “PUSH TO TRIP” button also serves as a safety device by preventing access to the circuit breaker interior in the “ON” position. If an attempt is made to remove the circuit breaker cover while the contacts are in the closed (“ON”) position, a spring located under the pushbutton causes the button to lift up and the breaker to trip.

Manual trip mechanism

Overload Trip

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions. When sufficient overcurrent flows through the circuit breaker’s current path, heat build up causes the bi-metalic strip to bend. After bending a predetermined distance, the bi-metalic strip makes contact with the tripper bar activating the trip mechanism.

[Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.]
Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.
[Circuit breaker contacts]
Circuit breaker contacts

A bi-metalic strip is made of two dissimilar metals bonded together. The two metals have different thermal expansion characteristics, so the bi-metalic strip bends when heated. As current rises, heat also rises.

The hotter the bi-metalic becomes the more it bends. After the source of heat is removed, as when the circuit breaker contacts open, the bi-metalic strip cools and returns to its original condition. This allows a circuit breaker to be manually reset once the overload condition has been corrected.

Short Circuit Trip

As previously described, current flow through a circuit breaker’s blow-apart contacts creates opposing magnetic fields. Under normal operating conditions, these opposing forces are not sufficient to separate the contacts. When a short circuit occurs, however, these opposing forces increase significantly.

The current that flows through the contacts also flows through a conductor that passes close to the circuit breaker’s trip unit. At fault current levels, the magnetic field surrounding this conductor provides sufficient force to unlatch the trip unit and trip the breaker.

[Short Circuit Trip]
Short Circuit Trip

The combined actions of magnetic fields forcing contacts apart while simultaneously tripping the circuit breaker result in rapid interruption of the fault current. In addition, because the magnetic forces are proportional to the current, the greater the fault current, the shorter the time it takes to interrupt the current.