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How to Specify Vacuum Contactors for High Altitude Environments

How to Specify Vacuum Contactors for High Altitude Environments

Motor control applications in high altitude environments pose a unique set of obstacles. As altitude increases, the density of the air decreases. Thinner air leads to fewer molecules to dissipate heat, causing electrical equipment to run hotter. More importantly, higher altitudes also decrease the dielectric strength through air. Hotter operating temperatures derate current-carrying components, while a weaker dielectric strength derates the electrical basic insulation level (BIL) and in some cases rated operating voltage.

Vacuum Contactor's Altitude Derating Guideline

It is best to separate the concept of altitude derating in vacuum contactors into two sections—first, power components, including terminals and vacuum interrupters. Second, the control which consists of the magnetic coils utilized to actuate the vacuum interrupters.

1. Power Components Derating

BIL requires derating at higher altitudes because weaker dielectric strength in the air increases the likelihood of flashover. A lower thermal transfer rate of thinner air at higher altitudes requires operating current to derate, albeit less dramatically than BIL. Below, Table A illustrates the recommended derating by altitude for Joslyn Clark's 7.2kV, 3.6kV, and 1.5kV Vacuum Contactors.

Note: All vacuum contactors are fully qualified at 2000 meters, and current derating starts when altitude exceeds approximately 2000 meters. For further detail, please refer to our datasheets.

Table A. Vacuum Contactor Power Section De-Rating By Altitude

Altitude (ft) Altitude (m) 7.2kV BIL (V) 3.6kV BIL (V) 1.5kV BIL (V) ASNI/IEEE Dielectric Factor Operating Current (A) Operating Current De-Rating Factor
 3300  1000  60000  45000  30000  1.00  600  1.000
 6000  2000  60000  45000  30000  1.00  600  1.000
 10000  3000  48000  36000  24000  0.900  570  0.950
 13000  4000  42000  31000  21000  0.800  555  0.925
 16000  5000  36000  27000  18000  0.700  540  0.900
 20000  6000  30000  23000  15000  0.600  525  0.875

2. Control Module Derating

Coils in the control utilize a generated magnetic field to pull on an adequately sized ferromagnetic contact material. The motion produced by the coils actuates the contactor. The coil circuit has two states. The first state draws the maximum current when energized as the ferromagnetic material is at its largest gap (open state). Once the ferromagnetic material is pulled together (closed state), it activates a switch which changes the coil circuit to its economizing state. This prevents the coils from overheating during continuous operation.

Thinner air at high altitudes causes coils to have less natural convective heat dissipation. In this case, premature failures from frequent cycling can occur. To avoid these premature failures, you will want to reduce the switching frequency of your contactor. For the recommended derating for Joslyn Clark's Vacuum Contactors, see Table B below.

Table B. Cycles/Hr Derating

Altitude (ft) Altitude (m) Cycles / Hour
 3300  1000  300
 6600  2000  300
 10000  3000  225
 13000  4000  150
 16000  5000  100
 20000  6000  50


Vacuum contactors rely on the vacuum in the interrupter to provide some of the force required to close the contacts. As the atmosphere thins external to the interrupter, the force created by the vacuum inside the interrupter which assists closing forces is also reduced. Lower atmospheric pressure requires operational derating such as pick- up and drop- out voltages. Lower air pressure at higher altitudes will reduce the force of the closing contacts. Consequently, contact opening time is faster. However, such benefit is marginal. The data in Table C demonstrates the impact of altitude on pick-up and drop-out voltages.

Table C. Pickup and Drop Out Derating

Altitude (ft) Altitude (m) Rick-Up Rated Voltage Drop-out Rated Voltage
 3300  1000  85%  70%
 6600  2000  85%  70%
 10000  3000  86%  72%
 13000  4000  87%  74%
 16000  5000  88%  76%
 20000  6000  89%  78%
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