Bracing of cables in switchboards and current transformer compartments

May 27, 1997

Underwriters Laboratories, Inc. Standard for Dead-Front Switchboards 891, Supplement B, contains requirements for the construction of switchboards, current transformer compartments, cable termination cabinets, and similar equipment with a maximum 100,000 ampere short-circuit current rating for which a short-circuit test may be waived.

Electrical equipment which has been tested for a rating of greater than 10,000 RMS symmetrical amperes, and which required conductor bracing to be installed during the test, must be marked in accordance with Section SB5.1 of Supplement B.

This section states in relative part that the electrical equipment shall be marked to indicate the "type of bracing to be added to the cables routed through the switchboard between the point of entry and the incoming terminals" and the marking "shall be located adjacent to the incoming terminals."

The marking shall state the following or the equivalent: "Wrap line cables together and, if provided, tap cables together with nominal 3/8-inch nylon rope or rope having a minimum tensile strength of 2000 pounds at (1) 6 inches and 12 inches from the line terminals with five wraps and (2) every additional 6 inches with five wraps or every 1 inch with one wrap." In addition, it is recommended that the manufacturer include a drawing that shows the required cable wrapping.

The purpose of the bracing is to restrain the conductors from violent movement during fault conditions. Conductors not properly braced may pull out of their terminals and cause severe destruction of the electrical equipment, which in turn could result in fire or personal injury.

Electrical equipment that has been tested with cable bracing for a rating greater than 10,000 amperes must be marked with the cable bracing requirements
Electrical equipment that has been tested without cable bracing for a rating greater than 10,000 amperes is not required to be marked with cable bracing requirements
UL 891 does not require, nor does it prohibit, any marking indicating that the electrical equipment has been tested without cable bracing for use on systems with greater than 10,000 RMS symmetrical amperes available
For all installations, electrical equipment must be marked with a short-circuit current rating that equals or exceeds the short-circuit current available at the equipment
For electrical equipment that is not clearly marked as to whether or not cable bracing is required, contact the manufacturers representative