What is Internal Arcing Fault Clearing Time
Switchboard rating for internal arcing fault clearing time is defined as the amount of time a switchboard or panelboard can withstand an internal arcing fault before it is safely cleared by protective devices like circuit breakers or fuses. Internal arc fault containment testing ensures that electrical equipment can handle and confine such faults without endangering people or property.
IEC 61439-1 and IEC 61641 has described internal arcing fault performance. The time duration for which the switchgear can contain and clear an internal arcing fault, without causing harm outside its enclosure, is usually stated in seconds commonly "1s" or "0.1s" for Low Voltage Switchboards while High Voltage Switchboards are rated with longer time durations.
Switchboards or panelboards that pass such tests get a specific rating that indicates they can contain an arcing fault for the specified duration without external harm. For instance, a rating might be denoted as "AF [Arcing Fault] 1s", meaning the switchgear can contain an internal arcing fault for 1 second.
When specifying, designing, or purchasing switchgear, it's crucial to consider the internal arcing fault rating, especially for installations in areas where human safety is paramount. Always refer to the manufacturer's documentation and the relevant standards to ensure that the switchgear meets the necessary requirements.
Switchboard Rating and Internal Arcing Fault Clearing Time are two distinct concepts related to electrical systems and safety.
Switchboard Rating refers to the maximum current-carrying capacity or current-handling capability of an electrical switchboard or panel. It is usually expressed in terms of amperes (A) and indicates the maximum load that the switchboard can safely handle without overheating or causing damage. The rating takes into account factors such as the size of conductors, insulation, and operating temperature.
Internal Arcing Fault Clearing Time is the duration it takes for protective devices (such as fuses or circuit breakers) to detect the fault and interrupt the circuit, thereby stopping the arc. This time is crucial for preventing extensive damage and ensuring the safety of personnel. An internal arcing fault occurs when an electrical fault (such as a short circuit or ground fault) creates an arc within the enclosure of an electrical switchboard or panel. These arcs can generate intense heat, gases, and potentially cause explosions or fires.
The internal arcing fault clearing time depends on factors such as the type and settings of protective devices, the design of the switchboard, and the fault current. It's usually specified by electrical standards and regulations to ensure the safety of electrical installations and the people working around them.
It's important to note that both switchboard rating and internal arcing fault clearing time are critical considerations for electrical system design, particularly in industrial settings where larger electrical loads and potential hazards are present. Ensuring that switchboards are properly rated and equipped with effective protective devices is essential for maintaining electrical safety.
What is Switchboard Peak Current Withstand?
Switchboard Peak Current Withstand also referred to as Short-Time Withstand Current or Short-Circuit Withstand Current is a measure of the maximum instantaneous current that an electrical switchboard or panel can safely handle without sustaining damage due to short-circuit conditions. It is a vital factor when designing or specifying switchgear and other electrical distribution equipment.
When there is a short-circuit fault in an electrical system, an extremely high magnitude of current, typically many times higher than the normal operational current, flows through the system for a brief duration until the fault is cleared. This sudden, high magnitude current is termed as "fault current" or "short-circuit current."
The Switchboard Peak Current Withstand is important for a few reasons:
- Mechanical Stress
High magnitude fault currents can generate significant mechanical forces within switchgear and associated equipment. The equipment must be designed to endure these forces without suffering mechanical deformation or damage. - Thermal Stress
The high currents can also produce substantial heat, which can potentially damage insulating materials and even conductive parts if the current magnitude and duration exceed the equipment's ratings. - Safety
Ensuring that the switchgear can withstand short-circuit conditions without catastrophic failure is crucial for the safety of both the electrical equipment and personnel. Failure of switchgear under fault conditions can lead to arcs, explosions, fires, and other hazardous situations.
For these reasons, manufacturers test their switchboards and switchgear to certify their peak current withstand rating. The tests involve subjecting the equipment to simulated fault conditions to verify that it can handle the mechanical and thermal stresses induced by high fault currents.
When selecting or designing switchgear for an application, it's imperative to ensure that the peak current withstand rating (and other related ratings like short-time withstand current) is higher than the estimated maximum fault current of the system in which the switchgear will be installed.
Switchboard Peak Current of an electrical switchboard could be caused by various factors, including inrush currents during motor start-up, short-circuit currents, or switching events.
- Inrush Currents
When electric motors start, they can draw a high current for a short period until they reach their operating speed. This inrush current is typically higher than the motor's normal operating current and can stress the components within the switchboard. - Short-Circuit Currents
In the event of a short circuit, an extremely high current flows through the system for a brief period until protective devices, such as circuit breakers or fuses, operate to clear the fault. The switchboard must be designed to handle these high short-circuit currents without suffering damage. - Switching Events
Switching events, such as opening or closing switches or circuit breakers, can also lead to transient currents. These currents can result from the sudden change in the electrical circuit's configuration and can potentially cause arcing or stress on the switchboard components.
The Switchboard Peak Current Withstand capability is determined by various factors, including the design of the switchboard, the type and quality of components used, the insulation and cooling systems, and the overall construction. Manufacturers often specify this value as a part of the switchboard's technical specifications.
Properly designing switchboards to handle peak currents is crucial for preventing damage to the equipment, reducing downtime, and ensuring the safety of personnel working in proximity to the switchboard. It's also important to adhere to relevant electrical codes and standards that provide guidelines for designing and testing electrical switchboards to withstand these transient events.
Calculation
The Electrical Engineer should always be able to backup with calculation the selected Switchboard Rating for Internal Arcing Fault Clearing Time and Peak Current Withstand. I have prepared a publication on this matter based on Australian / New Zealand Standards. his however could be used for other standards as well. The publication could be downloaded below.
