5.1.1.1 Alternating Current (ac) Power

Equipment shall be suitable for installation under the conditions specified for type I electrical power in MIL-STD-1399, section 300 and the additional requirements specified herein. Electrical tests shall be as specified in MIL-STD-1399, section 300 and Appendix D.

5.1.1.1.1 Additional requirements. The duration for the emergency line voltage conditions of plus 35 percent specified in MIL-STD-1399, section 300 shall be changed from "2 minutes to indefinitely". The emergency condition of minus 100 percent (frequency or voltage) will be treated as a power interruption. Momentary power interruptions will be limited in duration to 100 milliseconds. Short term power interruptions will be limited in duration to 5 minutes.

5.1.1.1.2 Fully hardened. Equipment shall remain fully operational through the conditions specified for 440 Volt (V), type I power in accordance with MIL-STD-1399, section 300 and as modified by 5.1.1.1.1; equipment shall remain operational through momentary power interruptions, and rapidly (within 1 second) restart following short term power interruptions.

5.1.1.1.3 Minimal acceptance. Equipment shall be fully operational within the normal tolerance limits specified for type I power in MIL-STD-1399, section 300. The preferred equipment service is 440 V, 60 Hz then 115 V, 60 Hz. Equipment may shutdown or go into a standby condition, but not be damaged, when operating outside these limits.

5.1.1.1.4 Ruggedization techniques. The ship's electrical power system is significantly different from commercial standards. For example, fault current of public utilities will typically be limited to 65,000 Amperes (A) at 13,000 V, where shipboard fault current is limited to 100,000 A at 450 V (with two of three generators paralleled). With proper care, COTS equipment can be ruggedized to be powered from the ship's electrical service. The primary considerations for ruggedization are as follows:

(a). MIL-STD-1399, section 300 specifies a harmonic current limit of 3 percent of the fundamental for equipment rated 1 kilovolt-ampere (kVA) or more. The typical approach has been to get a waiver, which is unsatisfactory.The shipboard electrical system has a lower reactance/resistance ratio than the commercial world, and harmonic currents have a higher impact on voltage distortion. Studies that have been performed by Naval Sea Systems Command (NAVSEA) indicate that harmonic current limits should be lowered below present standards.

(b). The voltage variations specified in MIL-STD-1399, section 300 are more severe than those in the commercial environment. The peak value may be as high as 220 V for up to 2 minutes for 115 V equipment. This may be sufficient to damage commercial equipment. Voltage spikes are particularly severe, specified at 1000 V for 115 Vac equipment where commercial equipment is typically rated for 600V.

(c). The line and return leads on the 115-V, 60-Hz service are both "hot", that is, both line and return spade connectors on convenience outlets have a potential to the ground, and the ground (safety) spade connector is grounded to the ship's hull. Commercial equipment will connect the return lead to ground at a power panel, and the return lead on commercial equipment may not be fully insulated. If the return lead is not disconnected when the equipment is shut-off, the equipment will remain energized presenting a safety hazard when assumed to be shut-off.

(d). Equipment which is permanently located and is energized more than 50 percent of the working day, such as copiers, personal computers and peripherals, soda machines, and money machines, are not to be connected to the ship's isolated receptacle circuits. Convenience outlet services are rated 15A, and present a fire hazard when overloaded. Equipment of this type are to be connected to a separate dedicated, hard wired circuit supplied by the lighting distribution system.

(e). To ruggedize for the forgoing conditions, equipment may adopt the approach indicated in Appendix A. In effect, a power conditioner is placed between the equipment and the electrical system. Since harmonic currents are minimized, such types of power conditioners are referred to as "unity power factor power supplies". Navy Standard Power Supplies do not meet the 3 percent current harmonic limits in accordance with MIL-STD-1399, section 300. Appendix A recommends a polyphase transformer since transformer technologies are mature, and transformers are both reliable and robust. Solid state unity power factor power supplies are being developed; however, the specifier should be aware that this technology is not mature and the risk of failing to meet specified performance and reliability criteria is greater.

(f). An alternative for 115-V equipment rated at less than 1 kVA is to use a double-pole switch to disconnect both power and return lines. The equipment shall be tested to verify that the equipment return is fully insulated from the equipment case. Voltage arrestors shall be applied to protect against voltage spikes; voltage arrestors that conduct at less than 220V shall be removed and replaced with voltage arrestors of a higher rating. The equipment shall be tested for operation at plus 35 percent voltage, or over voltage protection applied to remove the equipment from the line under such conditions.

(g). An Electrical Power Interface Compatibility (EPIC) mobile test facility is available for testing equipment under the electrical power conditions specified in MIL-STD-1399, section 300. Further information on this facility may be obtained from NAVSEA, Code 56Z14.