Use Disconnect Switches
Disconnect switches (also known as knife switches or disconnects) are one of the most widely used switching devices in power system. Their role is clear: they are not designed to cut off electricity, but to "isolate it." They are a lifeline for maintainers and a central tool in power system switching operations.
Purpose 1: Isolation of power-that's what it's all about
This is the most critical and irreplaceable role of a disconnect switches.
When electrical equipment needs repair, the equipment and power supply must be completely disconnected. While high-voltage circuit breakers can cut electricity, their contacts are sealed inside the arc extinguishing chamber, making it impossible to see the break and determine if the power is actually cut.
However, when the disconnected switch is turned on, there is a noticeable gap between the knife switch and the fixed contact point, immediately confirming that "the power has been cut." This glaring gap is a "lifeline" for maintenance workers.
So, in substations, a set of disconnect switches are installed on either side of the circuit breaker-the front switch isolates the power supply and the rear set prevents the backflow and provides double protection. Repair crews are afraid to work without breaking the switch.
Application 2: Switching operation-changing the Mode the power system works
In a double busbar system, it is necessary to switch the line from busbar A to bus B. This is done by using a circuit breakers to connect the switch and "move" the line sequentially to the other busbar. This operation, called switching, is the second largest application for disconnect switches.
Specifically:
When the a standby busbar is activated, disconnect switches is used to switch the line to the backup bus.
When a bypass busbar is activated, transfer the load to the bus with a disconnected switch.
When changing the method the system is wired, reassemble the circuit with a disconnected switch.
All this is performed by disconnect switches the circuit breakers, which is only responsible for interrupting and connecting currents during the operation.
Purpose 3: Switching on and off Small Current Circuits-not completely out of current
While disconnecting switches cannot cut the load current or short-circuit current, they can safely switch on very small current, which is one of their everyday applications:
Switch voltage transformer circuits
Switching surge arrester circuits
Switching Power Supply for unloaded busbars
Switching excitation current for an unloaded transformers not exceeding 2A (outdoor type limited to 560kVA and below, indoor limited to 320kVA and below)
Switching capacitor current an unloaded lines does not exceed 5A (10kV and below, length does not exceed 5km)
These currents are so small that they don't create powerful arcs, and broken switches can be handled well.
App 4: Work with a circuit breaker-the 'best partner'
In a power system, circuit breakers are never used alone, they always work with circuit breakers.
Circuit breakers are responsible for "cutting" -cutting off load current and short-circuit current; disconnecting switches are responsible for "isolating" -when circuit breaker cuts off the current, it completely isolates the power supply and equipment.
The division of labour is very clear:
Circuit breaker = can operate under load, with arc extinguisher, break point not visible.
Disconnect switch = cannot operate under load, no arc extinguisher, break point clearly visible.
So the sequence of operations is very strict: when electrified, switch off the circuit breaker, then switch off the circuit breaker; when electrified, switch on the circuit breaker, then switch on the circuit breaker. Reversing the sequence would lead to the opening of a disconnect switch under load, which would create a strong arc of electricity that could lead to a short circuit or even an explosion, a major incident in the power system.
Objective 5: To provide a clear point of disconnection-a safety symbol.
When the disconnect switch is turned on, the visible breakpoint is a clear safety sign not only for maintenance personnel but for the entire electrical system: the circuit has been disconnected from the power supply, and no one should mistakenly shut it down to electrify the system.
This is especially important in multi-person collaborative maintenance scenarios. Disconnect switches typically have ground lines and warning signs, plus visible breakpoints to form a complete safety system.
In short, the role of a a disconnecting switch can be summed up in one sentence: it does not cut the current, it isolates the power supply; it does not provide protection, it provides isolation; it is not the main component, it helps. It is the perfect partner for circuit breakers, a lifeline for maintenance workers and a core tool for power system switching operations. Without it, high voltage equipment simply cannot be maintained safely, and power system cannot flexibly switch operating modes.
