Beatrice Lang
The interruption of electrical circuits is a complex process that involves various factors. At its most basic, an electrical circuit is interrupted when a switch is moved to the open position, disrupting the flow of electrons. This can be achieved through different types of switches, such as mechanical switches or solid-state switches, which rely on the rapid cooling of an arc medium or the use of a semiconductor to break the circuit. In some cases, electrical circuits may be interrupted at natural or forced current-zeros, and the presence of an interruption device can influence this process. Understanding the physics behind electrical current interruption is crucial for ensuring safety and managing electrical currents effectively.
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What You'll Learn
- Switches are binary: either on (closed) or off (open)
- Mechanical switches rely on rapid arc medium cooling to open circuits
- Solid-state switches use semiconductors to conduct current
- Relays control large amounts of current/voltage with a small signal
- Normally open switches default to open, requiring activation to close the circuit
Switches are binary: either on (closed) or off (open)
An electrical switch is a device that interrupts the flow of electrons in a circuit. Switches are binary, meaning they are either on ("closed") or off ("open"). When a switch is on, two electrical conductors are in contact with each other, completing the circuit. When a switch is off, the circuit is interrupted, and no current flows.
The simplest type of switch involves two electrical conductors that are brought into contact by the motion of an actuating mechanism. However, there are many different types of switches, ranging from simple mechanical switches to more complex electronic switches. Some switches contain electronic circuits that can turn on or off in response to a physical stimulus, such as light or a magnetic field.
In a direct current (DC) circuit, as the switch contacts begin to move apart, the current density becomes high enough to melt portions of the metallic surfaces due to resistive heating. This creates a liquid metal vapour plasma state that temporarily maintains the electrical conducting path. As the contacts move further apart, electrons are emitted from the cathode contact due to thermal and field emission. These electrons collide with air molecules, creating an arc that enables circuit interruption.
Solid-state switches, unlike mechanical switches, do not rely on an arc to break a circuit. Instead, they use a semiconductor material that can conduct current when mobile carriers (electrons and holes) are supplied from injection regions. If the injection of mobile carriers is turned off, the semiconductor reverts to an insulating state, blocking the current.
In summary, switches are binary devices that control the flow of electricity in a circuit. They are either on ("closed"), allowing current to flow, or off ("open"), interrupting the circuit and preventing current flow. This binary nature of switches is fundamental to their function in electrical circuits.
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Mechanical switches rely on rapid arc medium cooling to open circuits
An electrical switch is a device used to interrupt the flow of electrons in a circuit. Switches are binary devices, meaning they are either completely on ("closed") or completely off ("open"). Mechanical switches are a type of electrical switch that relies on the rapid cooling of the arc medium to open an electrical circuit.
When a switch is turned off, the current density becomes high enough that portions of the metallic surfaces melt due to resistive heating. As the contacts move apart, electrons from the cathode contact are emitted into the intercontact space region due to thermal emission and field emission. As the contacts pull further apart, these electrons collide with air molecules within the contact gap and ionize them. This creates a liquid metal vapour plasma state that continues the electrical conducting path as the contacts physically part.
The voltage drop across the arc is proportional to the arc length and inversely proportional to the arc cross-sectional size. As the arc is lengthened by the physical movement of the contacts, the arc voltage grows and the arc cross-section diminishes as it is cooled by contact with un-ionized air molecules. In low-voltage DC circuits, the arc voltage can exceed or match the source voltage in the circuit, driving the circuit current to zero. At this point, no further arc ionization takes place, and the arc is cooled even more rapidly, completing the interruption process and opening the circuit.
Mechanical switches depend on this rapid cooling of the arc medium to open the circuit. In contrast, solid-state switches do not require an arc to break a circuit, as they use a semiconductor material that can be turned off to block the flow of current. The ease of control and reliability of solid-state switches due to their lack of mechanically moving parts suggest a promising future for these switches and circuit breakers.
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Solid-state switches use semiconductors to conduct current
Solid-state switches, also known as solid-state relays (SSR), are a type of electrical switch with no moving parts. Unlike standard electro-mechanical relays and contactors, which rely on coils, magnetic fields, springs, and various mechanical contacts to control and switch power supplies, SSRs use semiconductors to conduct current.
Semiconductors are materials that can conduct current when supplied with mobile carriers (electrons and holes) from injection or supply regions within the device. When the injection of mobile carriers is turned off, the semiconductor material reverts to an insulating state, blocking the current. This property allows SSRs to perform input-output isolation and switching functions.
SSRs provide complete electrical isolation between their input and output contacts. When non-conducting (open), they exhibit very high resistance, while when conducting (closed), they exhibit very low resistance. This binary nature of SSRs makes them ideal for ON/OFF switching applications.
SSRs can be designed to switch both alternating current (AC) and direct current (DC) by employing semiconductor switching elements such as thyristors, triacs, transistors, and MOSFETs. They are available in a range of output switching capabilities, from a few volts or amperes to hundreds of volts and amperes.
One of the main advantages of SSRs over electromechanical relays is their speed. SSRs switch faster due to the absence of moving parts, with switching times ranging from microseconds to milliseconds. Additionally, SSRs offer a longer lifetime as there are no mechanical components to wear out or contacts to pit or build up carbon.
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Relays control large amounts of current/voltage with a small signal
An electrical switch is a device that interrupts the flow of electrons in a circuit. Switches are binary, meaning they are either completely on ("closed") or completely off ("open").
Relays are a type of electrical switch that can control large amounts of current and/or voltage with a small electrical signal. They are extremely useful when there is a need to control a high-power or high-voltage circuit with a low-power circuit. The relay coil, which produces a magnetic field, consumes only a fraction of a watt of power, while the contacts closed or opened by that magnetic field may be able to conduct hundreds of times that amount of power to a load.
For example, a relay with a low-voltage (12 VDC) source can control a high-voltage (480 VAC) circuit. Similarly, relays with small currents can control larger currents, such as in the case of railway signalling relays, which use small voltages and currents to switch larger currents.
Transistors are another type of electronic device that can work as a switch and control a large current with a smaller current, similar to relays. Unlike relays, transistors are not mechanical and can operate much faster. They also have the advantage of being able to handle reverse voltages, which can be created by motors when spinning down, without the need for a protection diode in parallel. However, relays have the benefit of being able to handle AC and DC loads, while transistors can only handle current flowing in one direction.
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Normally open switches default to open, requiring activation to close the circuit
An electrical switch is a device used to interrupt the flow of electrons in a circuit. Switches are binary devices, meaning they are either completely on ("closed") or completely off ("open").
Normally open (NO) switches are a type of switch that remains open in its resting state, only completing the circuit when actuated. In other words, the switch keeps the circuit open at rest, preventing the flow of electric current. When activated, the current can pass through by closing momentarily. This activation is typically done by pressing or toggling the switch.
NO switches are commonly used in applications where a temporary connection is required, such as doorbells, push-button switches, and emergency stop buttons. They are also used in safety systems, requiring deliberate activation. For example, in a doorbell, pressing the button temporarily closes the NO switch, activating the circuitry. Similarly, in a light switch, an NO switch allows the light to turn on only when the switch is pressed.
NO switches are ideal for applications requiring momentary actions, energy efficiency, and precise circuit control. They are also useful in simple push-button controls and complex systems, providing flexibility in designing electronic circuits. However, they are not suitable for applications requiring a high degree of safety as they are more prone to accidental activation.
Understanding the difference between NO and normally closed (NC) switches is essential for designing control systems in electrical engineering. NC switches, in contrast, maintain a closed circuit at rest and break the circuit upon activation. NC switches are used in scenarios where circuit interruption signals an abnormal condition, such as in safety switches.
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Frequently asked questions
An electrical point is a switch that uses a secondary, low-voltage power source to complete or interrupt circuits that affect an electrical current.
When an electrical point is open, it is in the "off" position, meaning that no electrical current runs through it. This interrupts the flow of electrons in a circuit.
A box at a point of electrical wiring interruption serves as a junction point for electrical connections, providing a protected area for safely managing the flow of electricity through wires. It houses connections and splices, ensuring safety and providing a secure location for managing electrical currents.
