Collapse to view only § 111.51-3 - Short-circuit calculations for systems below 1500 kilowatts.
- § 111.51-1 - General.
- § 111.51-2 - Short-circuit calculations.
- § 111.51-3 - Short-circuit calculations for systems below 1500 kilowatts.
- § 111.51-4 - Short-circuit calculations for systems 1500 kilowatts or above.
- § 111.51-5 - Protection of vital equipment.
§ 111.51-1 - General.
Electrical installations must be protected against short circuits, by appropriate devices. The selection, arrangement and performance of various protective devices must provide coordinated automatic protection and selective operation in order to provide continuity of service for equipment vital to the propulsion, control, or safety of the vessel under short-circuit conditions through coordination and selective operation of overcurrent protective devices.
§ 111.51-2 - Short-circuit calculations.
(a) The available short-circuit current must be computed—
(1) From the aggregate contribution of all generators that can simultaneously operate in parallel;
(2) From the largest probable motor load; and
(3) With a three-phase fault on the load terminals of the protective device.
(b) The calculated currents must be used to select suitably rated equipment and to allow the selection and setting of protective devices.
§ 111.51-3 - Short-circuit calculations for systems below 1500 kilowatts.
The following short-circuit assumptions must be made for a system with an aggregate generating capacity below 1500 kilowatts, unless detailed computations in accordance with § 111.51-4 are submitted:
(a) The maximum short-circuit current of a direct current system must be assumed to be 10 times the aggregate normal rated generator currents plus 6 times the aggregate normal rated currents of all motors that may be in operation.
(b) The maximum asymmetrical short-circuit current for an alternating current system must be assumed to be 10 times the aggregate normal rated generator currents plus 4 times the aggregate normal rated currents of all motors that may be in operation.
(c) The average asymmetrical short circuit current for an alternating-current system must be assumed to be 8 1/2 times the aggregate normal rated generator currents plus 3 1/2 times the aggregate normal rated currents of all motors that may be in operation.
§ 111.51-4 - Short-circuit calculations for systems 1500 kilowatts or above.
Short-circuit calculations must be submitted for systems with an aggregate generating capacity of 1500 kilowatts or more by utilizing one of the following methods:
(a) Exact calculations using actual impedance and reactance values of system components.
(b) Estimated calculations using IEC 61363-1:1998 for AC systems and IEC 61660-1:1997 for DC systems (both incorporated by reference; see § 110.10-1 of this subchapter).
(c) The estimated calculations using a commercially established analysis procedure for utility or industrial applications.
§ 111.51-5 - Protection of vital equipment.
(a) The coordination of overcurrent protective devices must be demonstrated for all potential plant configurations.
(b) Protective relays and overcurrent protective devices must be installed so that:
(1) A short-circuit on a circuit that is not vital to the propulsion, control, or safety of the vessel does not trip equipment that is vital; and
(2) A short-circuit on a circuit that is vital to the propulsion, control, or safety of the vessel is cleared only by the protective device that is closest to the point of the short-circuit.