Tag Archives: Circuit Breakers

What are Digital Circuit Breakers?

We need protection from fires resulting from an electrical overload caused by a faulty device or an accidental short circuit. The huge current from the overload heats up wires and their insulation may go up in flames. There are several ways to activate this protection.

The oldest method consists of a fuse wire. Usually, this is a thin wire enclosed in a casing. The material of the fuse wire is carefully chosen to heat up and melt (blow) when a certain current level is exceeded. Melting of the wire disconnects the circuit and interrupts the current, preventing heat buildup. Once a fuse wire blows, it has to be replaced by a similar wire to continue protection and reestablish electrical operation.

Nowadays, it is common to see switchboards where the fuse holder has been replaced by a miniature circuit breaker (MCB). The device has a bi-metallic spring holding pair of mechanical contacts, which can establish connection by throwing an external switch. An electrical overload causes the bi-metallic spring to trip and the contacts open up, disconnecting the fault from the rest of the circuit. Once the fault has been cleared up, the MCB can simply be rearmed by flipping the external switch.

Although simpler to operated compared to the fuse wire, MCBs have their own disadvantages of being slow to react and expensive, with their cost going up proportional to their trip current. Over time, the bimetallic strip tends to deform, reducing the current capacity of the breaker and its accuracy. The mechanical construction of an MCB makes it prone to wear and tear.

Opening mechanical contacts to interrupt high currents often causes an arc flash to jump across the contacts. It is necessary to quench the arc flash within a short time to prevent incidence of fires.

For overcoming the above problems, using a digital circuit breaker offers the most convenient solution. The device has an all-electronic construction involving an electronically controlled automatic switch. There are no mechanical components involved, no bi-metallic strips, and no electromagnetic coils inside.

Atom Power is proposing a solid-state digital circuit breaker to replace the traditional types and thereby avoiding the related problems. Currently awaiting approval from the Underwriters Laboratory (UL), Atom Power has two models, one each for AC and DC circuits.

So far, Atom Power was producing only a few numbers of their digital circuit breakers, using their in-house 3-D printers for producing the plastic parts of the housing. With increase in production, they will use the resources of an external rapid manufacturing company, and will move to injection molding for higher volumes of commercial operations.

The Atom Switch, within the breaker, responds to a digital signal generated whenever the current exceeds a certain level, whether due to overload or short-circuits. With tripping speeds exceeding 16,000 times those of its mechanical counterparts, the arc flashes simply do not happen.

Another technique used to prevent arc flashes is to switch the device off when the AC voltage passes through zero. This is called zero voltage switching or ZVS, and is a very useful technique to prevent arcing across the open ends of the circuit.

How Long Does it take for a Circuit Breaker to Operate?

Electrical power systems all over the world use circuit breakers as important and critical components. As they play a key role, engineers periodically test circuit breakers. One of the most important test methods is the timing test, which measures the mechanical operating time of the breaker’s contacts. A timing test averts damage to a circuit breaker, as the incorrect operation of a circuit breaker prevents fatal consequences on connected equipment and substation personnel.

Various measuring devices have evolved for measuring the operation times of a breaker. For instance, although are no longer in use, first-generation devices used the oscillographic mode of recording curves. The present methods of testing, the second generation, are based on digital timers converting pulses to time. The latest concept for testing circuit breakers is through the analysis of signals from mechanical vibration.

Circuit Breaker Operation Times

The technically operational quality of a three-phase circuit breaker is an important parameter of its operation times—characterizing the process of opening and closing of its contacts. The international standards of the International Electrotechnical Commission, the IEC 56.3.105, define these time parameters. The standard quantitatively describes the switching on and off times of the process as:

Time discrepancy between contacts
— the diverging interval characterizing the disconnection or connection of the breaker contacts during non-simultaneous switching.

Closing time — the interval from the time of energizing the closing circuit (the circuit breaker being in the open position), to the moment the contacts strike the poles.

Opening time of the breaker — the interval from the time of energizing the opening release (the circuit breaker being in the closed position), to the moment the contacts separate at all the poles.

Worth noting is the time discrepancy between the contacts of the breaker. This should be within specified tolerance limits—usually 5 ms. This is important, as the time difference in closing or opening of all contacts may cause huge voltage spikes with a potential to damage the network and its equipment.

Digital Timer Meter

Digital timer meters are typically built with interconnected functional blocks such as input circuits, the micro-controller, a display, and a keyboard. The micro-controller is the central unit of the meter, while the input circuit eliminates disturbances and protects the unit.

The input circuit usually includes a converter with an output voltage of about 50 V. An opto-isolator ensures the optical isolation between the parts of the device directly connected to the breaker under test, and the other circuits of the meter. This protects components and digital circuits to ensure standardization of the signal for further digital processing.

Principle of Operation

The measuring process employs a principle known as the time-pulse method. This allows counting of pulses of a regular frequency in time intervals. The software in the micro-controller allows counting the pulses initiated by an external signal obtained from the breaker’s drive system.

A signal corresponding to the opening or closing of each contact of the breaker closes the time gates. As soon as the signals for the contact closure appear, the micro-controller copies the contents of the counter into its memory. The number of pulses the micro-controller counts is proportional to the interval elapsed from the moment of triggering to the moment of closing.