Engineering Tutorials

Ratings and Standards Vary
Medium voltage power circuit breaker ratings vary in different parts of the world. As a matter of fact, "medium voltage" is not uniformly defined around the world. Although the standard we are using in this training module defines medium voltage as 1000 volts to 72.5 kV (a commonly accepted medium voltage range in the United States, as well as other parts of the world), one foreign country considers 1000 volts and above to be "high voltage."
Medium voltage power circuit breaker ratings charts specify many different types of ratings. Usually, a chart indicates the ratings required by the governing standards where the circuit breaker is applied.

ANSI or IEC is normally the governing standard for medium voltage power circuit breakers. ANSI is associated with U.S. standards, and IEC is associated with international standards. However, neither the standards nor the ratings charts are identical. Charts also vary from manufacturer to manufacturer (Figures 17 and 18). To give an idea of the differences, consider the following example.
Figure Partial Sample of a Medium Voltage ANSI Ratings Chart

All of the indicated ratings are important. A number of them are merely calculations. For the sake of this discussion, we will cover only three of them.
These are:
• Maximum Voltage
• Continuous Current
• Short Circuit Current
These three ratings are common to both ANSI- and IEC-rated circuit breakers. In most instances, a grasp of these three ratings will allow you to assist a customer with a medium voltage power circuit breaker selection.

Maximum Voltage
This is the maximum voltage at which the breaker can operate. It is termed Maximum Voltage on ANSI charts and Voltage Class on IEC charts. The operating voltage where the circuit breaker is applied should not exceed the circuit breaker's rated maximum voltage.
Typical maximum voltage ratings encountered with ANSI applications are: 4.76, 8.25, 15, 27 and 38 kV. Typical voltage classes encountered with IEC applications are: 3.6, 7.2, 12, 17.5 and 24 kV.

Continuous Current
This is the amount of current the breaker can carry continuously at 60 cycles without exceeding the temperature rise limit. It is termed Continuous Current on ANSI charts and Normal Current on IEC charts. This maximum rating should always be in excess of the utilization equipment rating to provide for a short-time overload capability.
Typical continuous current currents encountered with ANSI applications are: 600, 1200, 2000 and 3000 amps. Typical normal currents encountered with IEC applications are: 630, 1250 and 2000 amps.

Short Circuit Current
This is the level of three-phase short circuit current that the circuit breaker can safely interrupt. The Short Circuit Current is a rating at the circuit breaker's maximum voltage (ANSI) or voltage class (IEC).
Typical short circuit currents encountered with ANSI applications are: 16, 29, 33, 37 and 63 kA. Typical short circuit currents encountered with IEC applications are: 16, 25, 31.5, and 40 kA.

Enclosure and Mounting
The medium voltage power circuit breaker is always placed in a switchgear assembly. The switchgear assembly is usually referred to as a Metal-Enclosed Assembly. The phrase "metal-clad" means that compartments within the switchgear assembly are separated by metal barriers
Figure Typical Medium Voltage Metal-Clad Assembly Structure with
Two Vacuum Circuit Breakers (Side View)

This is different from a metal enclosed assembly, where the equipment is enclosed, but not necessarily separated by barriers. The metal enclosed assembly is typically associated with low voltage equipment.

Figure typical Vacuum Switchgear Assembly with One Circuit Breaker in
Upper Compartment and One Below with Door Closed

Mounting
There are two methods for mounting a circuit breaker in the switchgear. These are:
• Fixed Mount
• Drawout Mount

Fixed Mount
Fixed Mount circuit breakers are usually found in outdoor applications. When installed outdoors, a special type of housing must be provided to protect the equipment from the elements. This outdoor protection is provided in a number of ways.
Fixed medium voltage power circuit breaker designs exist for a limited range of applications and voltages.

In the work place

This outdoor substation utilizes fixed medium voltage power circuit breakers to perform a series of capacitor switching functions.
Fixed Medium Voltage Power Circuit Breaker
Installed in Outdoor Enclosure



Each breaker must be housed in a weatherproof enclosure to protect it from the elements.
Drawout Mount
The Drawout Mount (or removable) type is - by far - the most common medium voltage power circuit breaker in use. It is used almost exclusively through 38 kV. Prior to the introduction of vacuum designs, medium voltage power circuit breakers were much larger (Figure 21). For example, one vertical structure could accommodate one Magnetic Air Type Circuit Breaker. The circuit breaker was removable, typically rolled out of the structure on wheels.
Figure  Typical Medium Voltage Magnetic Air Circuit Breaker (Front Barrier Removed)



Today, medium voltage power circuit breakers are frequently small enough to be stacked two units high in one vertical compartment.
The circuit breaker has three positions: DISCONNECT, TEST and CONNECT. Normally, the circuit breaker is manually pushed to the TEST position, then mechanically levered between the TEST and CONNECT positions (Figure 22). The secondary and primary electrical connections are automatically connected or disconnected as the circuit breaker is levered from one position to another.
Figure 22. Medium Voltage Vacuum Circuit Breaker
Being Mechanically Levered Into Its Structure



Most drawout medium voltage power circuit breakers can be completely removed from their compartments onto integral extension rails (Figure 23). This feature makes the task of inspecting the circuit breaker and the compartment much simpler.
Figure Medium Voltage Vacuum Circuit Breaker
Shown Removed From Compartment on Extension Rails

If the circuit breaker must be lifted from the extension rails onto the floor, integral wheels permit it to be rolled around outside of its structure.