Trystan Casey
Undesired electric arcing can have detrimental effects on electrical contacts and components, causing damage, melting, destruction of insulation, and even fire. This is due to the high temperatures and voltages involved, which can lead to material loss, evaporation, splattering, and corrosion. To prevent arcing in electrical contacts, several methods can be employed, including the use of contact arc suppressors, RC snubbers, immersion in transformer oil, dielectric gas, or vacuum, and the use of more durable metals or larger contact surfaces. Additionally, a diode can be connected across the inductive load to limit voltage spikes and suppress arcing. Understanding the mechanisms of arcing, such as the release of charged particles, gas ionization, and electric field effects, is crucial for developing effective prevention strategies.
Characteristics and Values for Preventing Arcing in Electrical Contacts
| Characteristics | Values |
|---|---|
| Arc suppressors | Contact arc suppressors, RC snubbers, capacitors, diodes |
| Arc suppression techniques | Immersion in transformer oil, dielectric gas, or vacuum |
| Relay contacts | Silver-cadmium-oxide contacts with appropriate arc suppression |
| Cathode and anode | Connect the diode across the inductive load with the cathode connected to the positive supply and the anode to the negative side |
| CEMF voltage spike | Limit to less than 1 volt to avoid negative voltage spike |
| Contact protection | Larger contacts, more durable metals or metal alloys such as tungsten |
| Transient suppressing electronic components | Capacitors, snubbers |
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What You'll Learn
Use contact arc suppressors
Arc suppressors are a crucial tool in preventing electrical arcing and mitigating its destructive effects on electromechanical power switches, relays, and contactors. The use of contact arc suppressors offers a range of benefits, including reduced contact damage, decreased maintenance and repair needs, improved contact reliability, and reduced heat generation.
Contact arc suppressors are designed to be connected across the protected contact or, in some cases, to the coil of the contactor as well. This connection provides the suppressor with valuable input about contact operation, allowing it to effectively manage the arc. By reducing the electric arc energy that occurs when current-carrying contacts are opened and closed, suppressors prevent the metal on the contact surfaces from melting, pooling, and migrating with the current.
There are various types of arc suppressors available, including semiconductor-based suppressors and those that utilize a high-speed, solid-state switch element. For example, a silicon-controlled rectifier (SCR) or a high-power transistor can be used to counteract the arc. Additionally, specific products like TRIPflash™ and TRIPfault™ arc suppressors are designed to stop arc-flash and arc-faults, respectively, by limiting their impact to that of a blown fuse or tripped circuit breaker.
In some cases, a simple fix for arc suppression can be a diode or resistor/capacitor 'snubber' across the contacts or the coil driving the contacts. This setup is commonly used in a 6-volt analog clock to prevent arcing and damage to the points of contact. When using a diode for arc suppression, it is important to connect it correctly, ensuring the cathode is connected to the positive supply of the relay's coil and the anode to the negative side.
Overall, the use of contact arc suppressors is an effective method to prevent arcing in electrical contacts, reduce maintenance needs, improve reliability, and enhance safety for both electrical components and personnel.
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Use RC snubbers
RC snubbers, or resistor-capacitor circuits, are a highly effective method of preventing electrical contact arcing. When a switch contact is opened, the current flows through the snubber circuit, and the applied voltage goes through the capacitor, not the contacts. This prevents arcing across the contacts by limiting the current.
The use of RC snubbers can extend the life of electrical contacts by reducing contact degradation due to arcing. The R value, or resistance, must be minimised to prevent contact degradation and increased to reduce arcing caused by inrush current. The capacitor should be selected first, beginning with a standard value of C = 0.1μF or 100 nF, and then increased until sufficient capacitance is achieved.
The resistor is then introduced in series with the capacitor, acting as a current limiter by absorbing the inrush current and reducing the arc. The resistance and capacitance values required for the RC network can be calculated using the formula developed by C.C. Bates: C = I^2 /10, and Rc= Vo /[10I{1+(50/Vo)}].
RC snubbers can be used in combination with a bidirectional TVS diode or MOV (metal-oxide varistor) for increased efficiency. This type of circuit is particularly useful for DC inductive circuits, where a diode can be used to avoid high voltage.
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Immerse contacts in transformer oil
The use of transformer oil, also known as oil-filled transformers, is a method to prevent arcing in electrical contacts. This is achieved by immersing the transformer coil and core in transformer oil. This immersion technique offers several advantages and benefits.
Firstly, the oil serves as both an insulating and cooling medium. By cutting off the cooling coil from the air, the oil prevents moisture in the air from causing corrosion to the transformer's core. This is particularly important in dry transformer designs, where the core and coil are directly exposed to the air.
Secondly, the oil-immersed transformer has a three-dimensional wound iron core with no air gap in its magnetic circuit. This is achieved through the arrangement of its three core columns in an equilateral triangle, resulting in tighter winding and improved performance. The three magnetic circuits have the same length, reducing loss, noise, and the third harmonic component.
Additionally, the oil-immersed transformer offers a reasonable structure, excellent performance, economy, and low noise. It is well-suited for power grid transformations in urban, rural, industrial, and mining enterprise settings.
However, it is important to note that mineral oil-immersed transformers present a potential fire hazard due to the flammability of the oil. To mitigate this risk, transformers are typically protected by overcurrent devices that can clear arcing faults within 0.5 seconds or less. Additionally, indoor applications may require dry-type construction or non-flammable oil-cooled transformers to ensure safety.
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Use a diode or resistor/capacitor 'snubber'
A diode or resistor/capacitor snubber can be used to prevent arcing in electrical contacts. This is a common fix for arcing on contact points.
A diode can be placed across the inductive load (the coils of the relay) with the cathode connected to the positive supply of the relay's coil and the anode connected to the negative side. The diode will not conduct normally, but when the relay's coil is disconnected, the energy stored in the coil will be released in the opposite direction. This is called a CEMF voltage spike, and the purpose of the diode is to limit this spike to less than 1 volt, thus avoiding damage to the activating component.
A resistor/capacitor snubber can also be connected across the two arcing contacts. When the contacts in a snubber circuit open, voltage is applied across the capacitor instead of the relay contacts, and no arcing occurs because the capacitor charges faster than it takes for the contacts to open. When the contacts close, the current from the charged capacitor and the source can exceed the safe conductance of the contacts, and the resistor in the network limits the inrush of current, reducing arcing and extending the service life of the contacts.
The resistor value will not cause harmful current or voltage but will affect the snubber's efficiency. It is recommended to choose a capacitor with a dielectric strength between 200 and 300 V. For an AC circuit, select a capacitor with no polarity.
RC snubber circuits can be utilized in combination with a bidirectional TVS diode of MOV. RC suppression is ideal for limiting the switch contact voltage during switch opening when the contact gap is small. RC suppression can also be implemented to lessen arcing and improve life in resistive loads.
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Make contacts out of durable metals or alloys
Undesired or unintended electric arcing can have detrimental effects on electric power transmission, distribution systems, and electronic equipment. Devices that may cause arcing include switches, circuit breakers, relay contacts, fuses, and poor cable terminations.
Arcing occurs when charged particles accelerate within the contact gap from one electrode to another under the influence of an electric field. The kinetic energy of the accelerating charges can then ionize additional gas molecules within the gap, causing a cascading, positive feedback loop that results in a self-sustaining, energetic arc.
One way to prevent arcing in electrical contacts is to make the contacts out of durable metals or alloys. Historically, this has been one of the most common approaches to contact protection. By using durable materials, such as tungsten, the life expectancy of power relays and contactors can be increased. This is because the contacts are less likely to suffer from material loss, deformation, and corrosion caused by the high temperatures and currents associated with arcing.
Additionally, the use of durable metals or alloys can help to prevent the micro-welding of contacts, which can occur when melting material solidifies while the contacts are closed. This can cause the contacts to stick closed, resulting in a failure to break the circuit.
It is important to note that while using durable metals or alloys can help to prevent arcing and extend the life of electrical contacts, it may not be effective in all situations. Other factors, such as gas pressure and voltage, can also influence the formation of an electric arc.
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Frequently asked questions
Electrical arcing is a form of electric discharge with the highest current density. Undesired arcing can damage electrical contacts and equipment.
Electrical arcing occurs when a low-resistance channel forms between places with different voltages. The ionized air conducts extremely high currents, causing a short circuit.
Electrical arcing can lead to detrimental effects on power transmission, distribution systems, and electronic equipment. It can cause melting of conductors, destruction of insulation, and even fires.
Electrical arcing can be prevented by using contact arc suppressors, RC snubbers, or techniques like immersion in transformer oil, dielectric gas, or vacuum. Additionally, contacts can be made larger or from more durable metals like tungsten.
