MIL-STD-1275 is a military standard that deals with characteristics of 28-volt DC input power to utilize equipment in military ground vehicles. Utilization equipment means electronic device, equipment, or system.  This standard applies to utilization equipment connected to 28 VDC distribution systems of military ground vehicles. The scope of this standard is to define several requirements and testing procedures for electronic devices, equipment, or systems connected to 28 VDC distribution systems of military ground vehicles. The requirements and testing procedures deal with steady-state voltage conditions, starting disturbances, spikes, surges, and ripples of military ground vehicle power sources with 28 VDC. Specifically, it addresses the electrical compatibility between the vehicle's 28V DC power system and the electronic equipment installed within the military vehicles. 

The standard is titled "Characteristics of 28 VDC Electrical Systems in Military Vehicles" and is used by the U.S. Department of Defense (DoD).  MIL-STD-1275 is neither a power source nor a power system standard. It focuses on the utilization of equipment in military ground vehicles. Currently, the new version of this standard is MIL-STD-1275F. Key aspects covered by the standard include voltage characteristics, transient voltages, voltage spikes, and other electrical parameters that can affect the performance of electronic equipment.

MIL-STD-1275F standard benefits:

• The MIL-STD-1275F-compliant electronic devices, equipment, or systems offer some key benefits, including the following:
• Operating during initial engagement and cranking surges, which occur during vehicle engine start-up
• Operation with input steady-state voltage level of 20V to 33V
• Will tolerate injected spikes and not emit spikes voltage levels/limits greater than those permitted by this standard
• Will tolerate injected surges and not emit surge voltage levels/limits greater than those permitted by the standard
• Offers reverse polarity protection
• Ensures robust electrical performance for use in highly demanding applications
• Superior EMC compatibility, i.e., EMI/RFI management and control (as per MIL-STD-461)

What are the requirements specified in the MIL-STD-1275F standard?

Following are the requirements specified in the MIL-STD-1275F document for the utilization of equipment connected to 28 VDC distribution systems on military ground vehicles. 

Voltage compatibility requirements: 

• Utilization equipment or EUT (Equipment under test) steady-state operational voltage range is 20 to 33 VDC, including ripple. The EUT shall operate without damage or degradation when subjected to the operational voltage range specified in this document. 
• EUT shall operate without damage or degradation when subjected to ripple voltage up to 2V above and below the steady state voltage (4V peak-to-peak). The frequency components of injected ripple voltage are within the range of 30 Hertz (Hz) to 5 kilohertz (kHz). The EUT shall be tested for the injected ripple voltage test method specified in MIL-STD-461 CS101 at nominal voltages of 22 VDC and 31 VDC.
• Emitted ripple voltage from the EUT must not exceed 1V above and below the steady state voltage (2V peak-to-peak). The frequency components of emitted ripple voltage are within the range of 30Hz to 1kHz.

Starting disturbance requirements:

Utilization equipment shall operate without degradation or damage when subjected to engine starting disturbances limits shown in Figure 1.

Figure 1: Starting disturbance limits on 28 VDC systems

Note: Initial Engagement Surge (IES) and cranking surges are known as engine-starting disturbances. These disturbances occur due to changes in system voltage from the normal operating range caused by the engagement of the engine starter and subsequent engine cranking.

Transient Disturbance requirements:              

Requirements for Voltage spikes :

Figure 2: Envelope of spikes for 28VDC system (image source: MIL-STD-1275F)

Utilization equipment (EUT) shall operate without degradation or damage when subjected to the below-mentioned injected voltage spike levels and not emit spikes over these levels: 

• +250V for 70µs - down to 100V for 1ms (figure 2)
• -250V for 70µs up to 18V for 1ms (figure 2)

Note (For injected voltage spikes): EUT is supplied with 28 VDC operating voltage. An acceptable test circuit is provided in the MIL-STD-1275 standard to perform injected voltage spikes test.  Each injected voltage spike shall have a rise time not exceeding 50 nanoseconds (ns) and a frequency of oscillation of more than 20 kHz and less than 500 kHz. The maximum total energy content of a single injected voltage spike is 200 millijoules (mJ).

Note (For emitted voltage spikes): EUT is supplied with 28 VDC operating voltage. The conducted transient emissions test method specified in ISO 7637-2 is used to measure the spikes emitted. Any emitted voltage spike or combination of emitted voltage spikes shall have a total energy content of within 125 millijoules (mJ).

Requirements for voltage surges:

Figure 3: Limits of operational voltages and surges for 28VDC systems 

Utilization equipment (EUT) shall operate without degradation or damage when subjected to the following injected voltage surge levels and not emit surges over these levels: 

• 100V for 50ms - down to 33V for 500ms (figure 3)
• 18V for 500ms up to 20V for 600ms (figure 3)

Note (for injected voltage surges): Table II of the MIL-STD-1275F document provides the following information about positive voltage surge test parameters.

Reverse Polarity Requirements:

In the reverse polarity test, with appropriate current measurement and protection in place, the EUT is provided with a 33 VDC power supply, with positive and negative power leads reversed, for 5 minutes. During the test, the reverse polarity current is measured. If this measured current exceeds the normal operating current, it is considered a failure of the EUT. 

After the test, the EUT is supplied with the correct polarity, and its operation is verified for normal operation as specified. If any deviation from the normal operation, the EUT shall be recognized as a failure of the EUT.

EMC and ESD requirements, as per MIL-STD-461:

MIL-STD-1275F standard does not cover EMC (Electromagnetic compatibility) testing and Electrostatic discharge (ESD) testing. However, before MIL-STD-1275 testing, the EUT must (must) comply with applicable EMC requirements and ESD immunity requirements as per MIL-STD -461 standard. 

Prior to MIL-STD-1275 testing:

• the EUT shall (must) demonstrate compliance with EMC requirements for control of electromagnetic interference (EMI) and voltage spikes induced by lightning, electromagnetic pulses, and power switching. 
• the EUT shall (must) demonstrate compliance with ESD requirements for  immunity to electrostatic discharge

Environmental conditions: 

• Testing of the EUT according to the MIL-STD-1275F standard shall be performed under the following environmental conditions:
• Temperature: +23°C ± 5°C (+73°F ± 9°F) 
• Relative Humidity: 0% to 90% humidity 
• Atmospheric Pressure: 80kPa to 102kPa (23.62 inHg to 30.12 inHg)