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CUTLER-HAMMER

AUTOMATIC TRANSFER SWITCHES

TECHNICAL BULLETIN

#1004

TRANSFER SWITCH SHORT CIRCUIT RATINGS

ONE TUSCARAWAS ROAD BEAVER, PA 15009

800-354-2070

When comparing short circuit ratings (withstand, closing, and interrupting) of different

manufacturers transfer switch equipment, the specifying engineer might find it useful to purchase

a good magnifying glass in order to read all of the footnotes and fine print that seem to dominate

different suppliers short circuit tables. In fact, it may be nearly impossible to find any two

manufacturers who publish such ratings in the same format, stating values achieved under

identical test parameters, by which the specifying engineer can make an accurate comparison.One manufacturer may state three or four different short circuit ratings for the same current

rated product while another supplier may state withstand ratings using any upstream current

limiting fuse which is in itself inconsistent with sound application principles. So why the

confusion? The manufacturer is trying to establish a perceived superior advantage in his product

relative to his competitors product. Certainly it is not improper for a manufacturer to highlight

the benefits of his product over comparable competitive products; however, the specifying

engineer must be able to compare such products on an equal basis.

Underwriters Laboratories, Inc. under Standard 1008 has established uniform testing

procedures and minimum acceptance values for short circuit tests conducted on transfer switch

equipment. The first thing the specifying engineer must confirm is whether the manufacturers

published ratings were achieved under UL recognized testing procedures. For example, one

ATS manufacturer publishes ratings titles Additional Test Data. The values listed state 10

cycle withstand ratings which are not addressed by UL 1008; therefore, it becomes impossible

for an accurate comparison to similar ATS equipment since no uniform testing procedure has

been developed to certify such ratings. Another manufacturer could quite easily publish even

higher 10 cycle ratings based on his own testing criteria to further cloud the issue of short circuit

ratings. Only those test parameters established and acknowledged by UL can serve the

consultant as an accurate basis of comparison.

Cutler-Hammer

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It is also quite important that the specifying engineer ascertain complete short circuit ratings of

different transfer switch equipment. There are three different conditions under which transfer

switches may have to function with short circuit current present: withstand, closing, and

interrupting. Obviously, the ATS will be required to carry or sustain (withstand) short circuit

current for a downstream fault. The ATS may also be required to close in (closing) on such

fault current. All ATS manufacturers publish comparable withstand and closing ratings whichare typically the same value for any specific device. A third short circuit rating consideration is

that of opening (interrupting) a faulted circuit. For example, a fault occurring immediately

downstream of the ATS would normally be cleared by the circuit protective device immediately

upstream of the ATS. Should the upstream device be equipped with a short time delay or

should the fault current be of an intermediate value not high enough to exceed the instantaneous

trip point of the upstream device, the ATS could initiate a transfer operation due to the reduced

line voltage precipitated by the faulted line. The interrupting capability of the ATS now

becomes critical since the main contacts must open the faulted circuit. Unfortunately, the

magnifying glass used to read the different short circuit tables probably wont help since few

manufacturers publish interrupting ratings. This is due to the fact that most such interruptingratings are significantly lower than the corresponding withstand and closing ratings. In an effort

to maximize withstand values, most ATS designs incorporate springs or mechanical assemblies

which significantly reduce the products ability to interrupt fault current which can lead to

contact damage under conditions outlined in the preceding example. Once again, the specifying

engineer must be aware of the interrupting capability of the ATS to make an accurate

comparison of complete short circuit ratings of similar products.

The short circuit ratings published by Cutler-Hammer were achieved under UL recognized test

parameters using representative samples of production quality units. For ease of application, all

Cutler-Hammer Transfer Switches have identical withstand, closing, and interrupting ratings forany specific current rating. Cutler-Hammer does not complicate the issue of short circuit ratings

by publishing multiple values of no practical application benefit. Although Cutler-Hammer ATS

ratings usually equal or exceed those of competitive products, such ratings have been

established using conservative, sound engineering practices so that YOU CAN BE SURE in

specifying Cutler-Hammer Transfer Switch products. You can save the magnifying glass for

enjoying your stamp or coin collection, not for reading the fine print in Cutler-Hammer literature.

Cutler-Hammer