Understanding On Load and Off Load Devices in Electrical Systems
When discussing electrical systems, especially those involving switchgear and circuit protection, it is vital to understand the differences between on load devices and off load devices. These devices play crucial roles in managing the operation and safety of electrical circuits. This article aims to clarify these concepts and explain the practical applications through detailed descriptions and real-world examples.
Introduction to Arcing and Flashover
When a switch is activated on or off to create or break an electric current in a circuit, an arc is produced during the closing and opening operations of the switch. This arc is essentially the result of an electric discharge. If the medium where the contacts make or break the connection is not capable of extinguishing this arc, a dangerous condition known as flashover can occur. To prevent such scenarios, specialized arc-quenching mediums such as oil, vacuum, and sulfur hexafluoride (SF6) gas are used. Devices designed with these arc-quenching mechanisms are categorized as on load devices.
On Load and Off Load Devices
On load devices are those intended for use under load, meaning they can handle the current flow and associated arcing during operations. A common example of an on load device is a circuit breaker. Circuit breakers are typically used to open and close electrical circuits and are equipped with arc-quenching features to safely manage the process.
In contrast, off load devices are designed to be operated only when the circuit current is zero. This means these devices should never be operated while the circuit is under load. Off load devices generally have a lower current-making and interrupting capacity compared to on load devices. Typical examples of off load devices include isolator switches and stand-alone gang-operated switches.
Operating Procedures for Off Load Devices
The process of operating an off load device involves a sequence of steps to ensure safety and efficiency. Here’s a detailed explanation using an isolator switch as an example:
Switch off the breaker to ensure the load current is zero.
Isolate the circuit by opening the gang-operated switch.
Ensure the breaker is in the off position when you close the isolator switch.
This sequence ensures that all operations are performed under conditions where the circuit is not under load, thereby minimizing the risk of dangerous electrical discharges and flashovers.
Transformer Tap Changers: On Load vs Off Load
Transformers are equipped with tap changers to adjust voltage levels, and there are two main types of tap changers: on load tap changers and off load tap changers. On load tap changers are used in transformers connected to the grid supply because they allow voltage adjustment while the transformer is in operation. This is crucial for maintaining stable voltage levels as grid conditions fluctuate.
Off load tap changers, on the other hand, are typically found in distribution transformers. Since these transformers are not continuously energized, it is safe to make tap changes during periods when the transformer is not in use, thus the term "off load."
Practical Applications and Combinations
The distinction between on load and off load devices is not rigidly defined. In many practical scenarios, you can combine both types of devices in a circuit, with at least one on load device to ensure complete circuit operation. This setup allows for safe and efficient management of electrical loads and ensures that off load devices can be safely operated under predefined conditions.
Here are two possible combinations:
An on load circuit breaker and an off load isolator switch.
Two on load circuit breakers and an off load isolator switch.
These combinations illustrate how on load and off load devices can be integrated to provide comprehensive electrical protection and management.