VCB Rating Chart | Explanation and Example

A VCB rating chart is a chart that represents the rating specification of a VCB.

Vacuum Circuit Breaker or VCB has gained a lot of attention and growth in many countries for its capabilities, function, and advantages over the “normal” circuit breaker. Of course, every electrical “things’ needs to be standardized by international standards since it is used widely in many countries. One of the most used standards is IEC standards or International Electrotechnical Commission.

What is IEC? Well, we don’t think we need to deeply learn what IEC is right now. But take note that IEC is an organization that prepares, manages, and publishes EVERY single International Standard that is related to electronic, electrical, and technology.

Moving on.

Every VCB product has to follow this standard to its newer revision. For example, a VCB needs to be installed or uninstalled, inserted/drawn, into/from the outside of a switchgear.

Leaving aside the IEC standard, let us proceed to the VCB rating chart.

There are a few notes to be mentioned below:

• The current rating for a Vacuum Circuit Breaker (VCB) can be up to 3 kA.
• A VCB works by interrupting the arc that exists in a vacuum bottle.
• The voltage rating for a VCB can be up to 35 kV.

This time we will use a VCB from Fuji Electric to learn and grasp the rating chart for a common VCB. We will use their 12kV and 24kV VCBs here.

12 kV VCB Rating Chart

Observe the base specifications for their 12 kV VCB:

1. Rated voltage = 12 kV
2. Rated current = 630 A, 1,250 A
3. Rated breaking current = 25 kA
4. Rated short-time withstand current (3s) = 25 kA
5. Rated short-circuit making current (crest factor) = 63 kA (50 Hz), 65 kA (60 Hz)
6. Rated short-time power-frequency withstand voltage = 28 kV
7. Rated lightning impulse = 75 kV
8. Rated frequency = 50/60 Hz
9. Installation = Draw-out type with cradle, Draw-out type, Fixed type
10. VCB Mass = 94 kg
11. Cradle mass = 67 kg (Draw-out type with cradle)
12. Standard = IEC 62271-100 (2012)

24kV VCB Rating Chart

Observe the base specifications for their 12 kV VCB:

1. Rated voltage = 24 kV
2. Rated current = 630 A, 1,250 A
3. Rated breaking current = 25 kA
4. Rated short-time withstand current (3s) = 25 kA
5. Rated short-circuit making current (crest factor) = 63 kA (50 Hz), 65 kA (60 Hz)
6. Rated short-time power-frequency withstand voltage = 50 kV
7. Rated lightning impulse = 125 kV
8. Rated frequency = 50/60 Hz
9. Installation = Only for the main unit of draw-out type with cradle (F)
10. VCB Mass = 170 kg
11. Cradle mass = 67 kg (Draw-out type with cradle)
12. Standard = IEC 62271-100 (2012)

VCB Rating Chart Amps

Another example of VCB current rating can be observed from one of the ABB’s product:

11 kV VCB Panel

• Capacity (Amps) = 630, 800, 1250, 1600, 2000, or 2500
• Rated voltage = 11 kV
• IP Rating = IP44, IP55
• Frequency = 50/60Hz

VCB Rating Chart Calculation

Calculating VCB rating won’t be very simple and straightforward. But it will not use very complicated formulas. What we need to take care are:

• Functions and applications
• Size of the protective devices (fuses, breakers, etc)
• Electrical codes and standards
• Loads

Now going straight to the example:

Let’s say we have a 500 kVA transformer along with two protective devices just as seen below:

On the High Tension (HT) or High Voltage (HV) side, we have a supply of 11 kV. Here we install a protective device, H to prevent any major faults such as short circuits for our transformer. Of course, overload protection must also be provided with it or extra devices.

On the Low Tension (LT) or Low Voltage (LV) side, we also install a protective device as fault and overload protection for the transformer.

We can calculate the HT full load current, I_H from:

We can calculate the LT full load current, I_L from:

Here we have the full load current of 26 A on the HT side and 722A on the LT side. Continue to observe the equation, you will see M_H and M_L.

M_H is the margin factor of HT side and dictated by electrical codes. For example, in the US the margin factor is 125% of the full load current for rated voltage 600 V or less.

For the rated voltage more than 600 V, the margin will be greater and we need separate overload protection on the LT side.

Back to US electrical codes, the maximum margin commonly used are M_H = 600% and M_L = 125%. Of course these margins are controlled by voltage levels, protection types, and transformer impedance.

This electrical code refers to the NES (National Electrical Code), NFPA 70, Table 450.3(A).

To ease our memory, protective devices are manufactured in standard sizes:

• HT protection devices = 1200, 2000, 3000 A, etc.
• Three phase breakers = 100, 200, 400, 600, and 800A.
• Fuses = even smaller
• Relays = adjustable

Frequently Asked Questions