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How does altitude affect the rating of a vacuum contactor

How does altitude affect the rating of a vacuum contactor?

The force of atmospheric pressure tends to keep the contacts of a vacuum interrupter in the closed position. In a vacuum contactor there is a balance between the forces tending to close the vacuum contacts and the return spring. The function of the contactor magnet is to work with the forces of the bellows in the interrupter and the atmospheric closing force to overcome the force of the return spring so that the contactor can close. Higher altitudes with reduced atmospheric closing force make it more difficult for the magnet to close the device. In a vacuum interrupter, the contact force is the sum of the atmospheric force plus the bellows force plus the contact spring force. Higher altitudes with reduced atmospheric closing force reduce the force holding the contacts closed. At altitudes below sea level, atmospheric pressures can develop that exceed the return spring forces and prevent opening of the contactor.

Higher altitudes have less dense air that results in reduced electrical insulation and thermal conductivity. Voltage, current and ambient temperature de-ratings related to the effect of altitude are specified by ANSI Standard C37.30-1970.

Cutler-Hammer 135, 160, 270, 320, 540 and 610 ampere vacuum contactor de-ratings with altitude

To assure proper pickup performance and adequate contact pressure, the maximum operating altitudes for Cutler-Hammer vacuum contactors are as follows:

Ampere Rating

Maximum Altitude*

Minimum Altitude*

Feet

Meters

Feet

Meters

135, 160 amp

8202

2500**

-6562

-2000

270, 320, 540 amp

13123

4000

-9843

-3000

610 amp

16404

5000

-11483

-3500

* Consult Factory/TRC altitudes beyond the limits given above.

** For 3500 meter applications Kit 9085A57G08 can be installed at the Factory.

The following altitude correction factors for rated voltage, rated current, and ambient temperature should be applied per ANSI Standard C37.30-1970:

Altitude

Altitude Correction Factor

Feet

Meters

Rated Voltage

Rated Current

Ambient Temperature*

3300

1000

1.00

1.000

1.00

4000

1200

0.98

0.995

0.992

5000

1500

0.95

0.990

0.980

6000

1800

0.92

0.985

0.968

7000

2100

0.89

0.980

0.956

8000

2400

0.86

0.970

0.944

9000

2700

0.83

0.965

0.932

10000

3000

0.80

0.960

0.920

12000

3600

0.75

0.950

0.896

14000

4200

0.70

0.935

0.872

16000

4800

0.65

0.925

0.848

* - for maximum ambient of 40° C outside the enclosure