Specifications

MedWeld 3000 Catalog Revision 8
July 2002 Page 70 of 72
CASE 4:
If two or more transformers are connected to different poles of the isolation
contactor (three-phase operation), whether the transformers are welding
simultaneously or not, the required isolation contactor current rating is the highest
sum of average currents for the all the transformers on one phase, times 1.7 (the
approximate square root of 3).
For example, four 75 KVA fixture welding transformers are connected to all three
poles of an isolation contactor. Two transformers are connected to poles 1 and 2, one
transformer is connected to poles 2 and 3, and the last transformer is connected to
poles 1 and 3. All transformers weld simultaneously:
The largest load is the two 75 KVA transformers on poles 1 and 2. The computed
average current (from Part 1) for these two transformers is 98 + 98 = 196 amperes.
The required current rating for the isolation contactor is:
196 * 1.7 = 333 amperes
Once the required current rating is computed, select the isolation contactor with the
next-higher current rating value. The circuit breaker above the isolation contactor(s)
and any upstream fuses must be sized appropriately to handle the required current
ratings computed above. The magnetic trip of the circuit breaker should be set no
higher than ten times the current rating of the smallest isolation contactor below it.