This informal CPD article Capabilities of High Voltage Withdrawable Circuit Breakers was provided by Horizon Engineering Solutions, established in 2015 to fill the gaps in the marketplace within Northern Ireland for the provision of specialist Electrical and Mechanical Engineering support services to major industrial clients. This article looks at the capabilities of withdrawable HV circuit breakers which are commonly found in industrial HV Networks.
A withdrawable circuit breaker assembly will normally incorporate:
A Controls Cubicle containing:
- Protection relays
- Current Transformers (CT’s) and a Voltage Transformer (VT)
- Local operating controls (open, close, local, remote)
- Electrical system metering (Voltage, current, power (kW), reactive power (kVAr), frequency, power factor, harmonics)
A Circuit Breaker Cubicle containing:
- The circuit breaker carriage
- Busbar and circuit orifices and associated shutters
- Interlocking mechanisms
- Circuit earth switch (if fitted and depending on design)
An Earth Switch Cubicle (if fitted and depending on design) containing:
- The circuit earth switch
General Capabilities of withdrawable circuit breakers
Withdrawable circuit breakers are designed to make and break all currents within their designed ratings from very small load currents right up to full short circuit and full earth fault currents. However, they are not generally designed for frequent operation. When frequent switching of an HV circuit is required, a Contactor is often used. Circuit breakers do not require a power supply to be available to maintain their selected open or closed position. For this reason, they are often termed as latched devices.
Operating Principle
Withdrawable circuit breakers comprise two contacts, the fixed contact, and the moving contact. The moving contact is used to make the circuit using the stored energy of a closing spring and to break the circuit using the stored energy of the opening spring. The springs control the speed of the moving contact during the opening and closing of the circuit breaker.
Safety Interlocking
Withdrawable circuit breakers are provided with mechanical and electrical safety interlocking devices to prevent mistakes being made. Mechanical interlocks are usually provided by means of mechanical latches, bolts, or shutters. Electrical interlocks are usually provided to ensure certain closing and tripping functions take place in a particular sequence.
The most important interlock associated with withdrawable circuit breakers is the isolating interlock which prevents the withdrawal of the circuit breaker when it is in the Closed-(ON) position.
Depending on design other interlocks which are normally provided prevent:
- Insertion of the circuit breaker carriage when the circuit breaker is in the Closed-(On) state
- Opening of access doors or panels before the circuit breaker has been set to its Open-(Off) state
- Gaining access to main conductors and contacts whilst they are energized
- Gaining access to busbar and circuit orifices when the circuit breaker has been withdrawn
- Closing of earthing switches onto live circuits or busbars
Isolation
Isolation is achieved by fully withdrawing the circuit breaker carriage from its SERVICE position and securing the cubicle to prevent the circuit breaker carriage from being re-inserted. To achieve this the circuit breaker must first be in the Off-(OPENED) state. Withdrawal of the carriage from the switchboard busbar and circuit orifices can then commence by moving or racking out the carriage utilizing the operating handle, and associated isolating mechanisms and interlocks. Movement of the carriage can either be Vertically or Horizontally depending on the design of the circuit breaker. When the carriage is fully withdrawn and removed from its cubicle it is essential that the empty cubicle is secured.
Suitable methods to secure the cubicle include:
- Applying a safety lock with Danger Notice to the locking mechanism of the cubicle door
- Applying a safety lock with Danger Notice to the busbar shutters
- Applying a safety lock with Caution Notice to the circuit shutters (but only after proving DEAD)
Earthing
When working on HV systems it is essential to Prove Dead before applying an earth to busbars or circuits and to ensure the application of that earth is made with switchgear which is designed for earthing and which can withstand any potential short circuit which it may create if it is inadvertently applied to a circuit which is live.
Vertically Isolated
This type of circuit breaker utilizes the circuit breaker carriage to apply the required earth. The carriage can be located within the cubicle in the:
- Service position
- Busbar Earth position or
- Circuit Earth position
When the carriage is withdrawn a mechanical selector gate within the cubicle can be accessed which enables the carriage to be set into the required position. Once the required position has been selected and locked the circuit breaker carriage can then be racked in and the circuit breaker closed to apply the required earth which must then be secured with a safety lock and Caution Notice.
Horizontally Isolated with Separate Earth Switch
- This type of circuit breaker utilizes a separate earth switch to apply a Circuit Earth
- Earth switches of this type are normally located in the same cubicle as the circuit breaker carriage or in a separate cubicle within the overall circuit breaker assembly.
- They are rated to withstand fault currents for a limited amount of time, but they are not rated to break current.
- They are normally mechanically interlocked with the circuit breaker to prevent them being closed when the circuit breaker is in the closed position.
- They are also operated using anti-reflex handles. The purpose of an Anti-reflex handle is to prevent an operator from trying to immediately open an earth switch after they have inadvertently applied an earth to a live circuit.
- The time taken to remove and re-insert the handle to try and open the earth switch is normally longer than the time taken for the circuit protection system to clear the fault.
We hope this article on the Capabilities of High Voltage Withdrawable Circuit Breakers was helpful. For more information from Horizon Engineering Solutions, please visit their CPD Member Directory page. Alternatively please visit the CPD Industry Hubs for more CPD articles, courses and events relevant to your Continuing Professional Development requirements.
