Def Stan 61-5: 12 V and 24 V DC Power in Military Vehicles
Def Stan 61-5 is the UK Ministry of Defence standard for low voltage electrical power supply systems. It is published in parts, and the part that applies to military vehicles is Part 6, which sets the DC supply characteristics of military ground vehicles and what installed equipment must withstand.
For anything connected to a vehicle bus, including lamps, that means steady state voltage limits, engine cranking dips, repetitive transients, load dump and reverse polarity. This guide explains what the standard covers, why each requirement exists and what it means for vehicle lighting.
What Def Stan 61-5 Covers
Def Stan 61-5 is not one document. Each part deals with a different kind of platform or power source, and each has its own issue number and date, so a reference to “Def Stan 61-5” without a part number is incomplete. The parts most often cited are these:
| Part | Subject | Latest issue listed |
|---|---|---|
| Part 2, Section 1 | Ground generating set characteristics: electrical performance characteristics and methods of measurement. Its scope extends to 28 V dc but excludes generators in military vehicles, which belong to Part 6 | 19 May 2006 (issue number not listed) |
| Part 4 | Quality of electrical power systems in HM Ships and Submarines, aligned with NATO STANAG 1008 where practicable | Issue 5, August 2018 |
| Part 6 | Nominal 12 V and 24 V DC electrical systems in military platforms (titled for 28 V dc systems in military vehicles until Issue 6) | Issue 7, 28 March 2020 |
Part 4 is a naval document and is sometimes wrongly quoted as the vehicle part. For land vehicles, and for any lamp, sensor or radio fitted to one, the governing document is Part 6. Equipment run from a ground generating set rather than from a vehicle bus is supplied to the characteristics set in Part 2, Section 1. The rest of this guide is about Part 6.
Part 6: The Vehicle Power Supply Part
Part 6 defines the characteristics of DC power distribution and equipment in the military (ground) vehicle environment. It covers supply sources with a nominal voltage of 12 V or 24 V, whether the power comes from an engine-driven generator, an auxiliary power unit, power import, the batteries or a combination of these. It also defines power export characteristics, and its guidance applies up to 1000 A continuous current.
The purpose is interoperability. A vehicle is assembled from equipment made by many suppliers, and each item has to work on the same bus without damaging it or being damaged by it. Part 6 therefore sets requirements on both sides: on the platform, which must deliver power within defined limits, and on the installed equipment, which must operate through or survive what the platform is allowed to deliver.
The annexes go further than voltage limits. They cover the distribution system at socket outlets, constraints on equipment, solid-state switching devices, cable de-rating factors, example Power Control Plan and evidence review documents, and guidance on one-wire and two-wire systems. Part 6 refers to Def Stan 00-35 Part 3 for environmental test methods, so vibration, temperature and humidity testing is done to Def Stan 00-35, not to 61-5. Other vehicle standards call up Part 6 in turn, including Def Stan 23-09, Generic Vehicle Architecture, and Def Stan 23-006 for logistics vehicles.
Earlier titles. Issue 5 (October 1990) was titled “Electrical Power Supply Systems Below 650 Volts, Part 6: 28 Volt dc Electrical Systems in Military Vehicles”, and an earlier edition dates from December 1984. Issue 6 (February 2009) retitled the part “Nominal 12V and 24V DC Electrical Systems in Military Platforms”.
What Installed Equipment Must Withstand
A military vehicle bus is electrically harsh. Large inductive loads such as turret drives, winches, fans and starter motors switch on and off, the generator regulates under rapidly changing load, and batteries are connected, disconnected and jump-started in the field. Part 6 describes the resulting disturbances so that equipment designers can build for them rather than discover them in service.
| Condition | 24 V system (12 V system) | Why it exists |
|---|---|---|
| Steady state voltage | 18 to 36 V (9 to 19 V). The limits are given for five minutes, but equipment should tolerate them continuously. | Covers a flat battery at one end and a generator regulating high, or a charging fault, at the other. |
| Engine cranking | Loss of function during cranking dips is accepted (clause B.8.1.1); damage is not. | A starter motor pulls the bus down for the duration of the start. |
| Short repetitive transients | +200 / -200 V (+100 / -150 V), 100 ns long, repeating every 100 µs in 10 ms bursts with 90 ms gaps. | Switching of inductive loads such as relays, solenoids and motors. |
| Long transients | Up to +40 V (+20 V) for 50 ms. | Generator regulation recovering after a large load step. |
| Load dump | A high-energy surge peaking at well over 150 V on a 24 V system and lasting hundreds of milliseconds. Function may be lost, but equipment must not be damaged and must be restorable, by manual intervention if necessary. | A heavy load, or the battery, disconnecting while the generator is charging. |
| Ripple and reverse polarity | Immunity to superimposed ripple and to a reversed supply connection is required. | Generator rectification, and batteries or slave leads connected the wrong way round. |
| EMC | Def Stan 59-411 Parts 1, 3 and 4, Service Class A, unless agreed otherwise. | Equipment must not disturb radios and other systems on the same platform. |
Values are as published in independent technical summaries of Issue 7. The standard itself defines the exact waveforms, source impedances and test methods, and these vary between issues. The EMC requirement points to Def Stan 59-411, which is a separate standard with its own test programme.
Load Dump and Centralised Suppression
Load dump is the most demanding requirement in Part 6 and the one most likely to destroy equipment. When a large current is suddenly interrupted while the generator is charging, the generator cannot reduce its output instantly. The stored energy appears on the bus as a surge many times the nominal voltage, and it lasts long enough to deliver real energy into anything connected.
The standard considers two situations: a platform with no control of load dumps, and a platform with a centralised load dump suppression (CLDS) system that clamps the surge before it reaches the equipment. Issue 7 made CLDS mandatory for all platforms. That changes the design problem for equipment suppliers, because what a lamp or radio actually sees depends on the platform it is fitted to, and on whether it may also be fitted to older vehicles without suppression.
The published figures for the uncontrolled case differ between sources and between issues, so this is an area where the standard itself, not a summary, must be the reference for design and test.
What Changed in Issue 7
Issue 7 builds on the design and test requirements of Issue 6. The changes that matter most to equipment suppliers and integrators are these:
- Power Control Plan: the former PCDR becomes a Power Control Plan, a living document that must reach set levels of completeness at design reviews. It is mandatory for platforms only.
- Unique Identifiers: every requirement now has an identifier and a priority, either Mandatory or Priority 1, intended to help MOD accreditors assess compliance.
- Centralised load dump suppression: mandatory for all platforms.
- Circuit protection devices: new design and test requirements.
- New tests: DET09.A Load Balance in Annex A, and DIT10.B Capacitance Isolation (a static measurement).
Tests are referenced by code, for example DET01.A for steady state voltage, DIT07.B for short transients and DIT08.B for load dump. Test houses quote these codes, so asking which codes a product has passed is the quickest way to see what it has actually been tested for.
Why It Matters for Vehicle Lighting
An incandescent lamp is a resistive filament. A surge makes it flash brighter and perhaps shortens its life. An LED lamp is an electronic device: every LED lamp has an LED driver between the bus and the LEDs, built from semiconductors and capacitors that can fail on a single event. That is why the power quality of the vehicle bus is a lighting question.
Load dump. The driver input needs suppression sized for the energy it may see, or the installation must rely on the platform’s CLDS. A lamp that fails open after a load dump leaves a vehicle without position or stop lamps, and in blackout operation without its convoy markers.
Engine start. Part 6 accepts loss of function during cranking. Operators often do not: a convoy marker or blackout lamp that goes dark every time an engine restarts is a tactical problem. A driver that regulates through the cranking dip, rather than merely surviving it, is worth specifying.
Inrush. A driver’s input capacitance draws a current pulse at switch-on. On a vehicle where many lamps switch together, the combined inrush can trip electronic circuit protection. Issue 7 adds requirements for circuit protection devices and a capacitance isolation test, so input capacitance is now part of the integration discussion rather than a detail.
Transients and EMC. The repetitive transients reach the driver at every relay operation, and the driver’s own switching must not disturb the vehicle’s radios. Both have to be designed in from the start.
These considerations apply across vehicle lighting, from military blackout lights to LED rear lights. When requesting a quotation for lighting on a UK programme, by email to info@betalight-tactical.com, include the following so the lamp electronics can be matched to the bus:
- Part and issue: Part 6 for vehicles, and the issue named in the contract.
- System voltage: 12 V, 24 V, or a lamp that must work on both.
- Load dump case: whether the platform has CLDS, and whether the lamp may also be fitted to older vehicles without it.
- Engine start: whether the lamp must stay lit through cranking or may drop out, as Part 6 allows.
- Test evidence: the test codes required, for example DIT08.B for load dump, and whether a test house report is needed.
- Switching: how many lamps switch on together from one protected circuit, so that inrush can be checked.
Def Stan 61-5 and the US Equivalent
The US counterpart of Part 6 is MIL-STD-1275. Both describe the same kind of vehicle power system, but they define it differently, and equipment intended for both markets has to satisfy each on its own terms.
The practical differences lie in the detail. The UK standard covers 12 V platforms, which the US standard does not. Transients are defined by different waveforms, so a pass against one set of tests does not demonstrate a pass against the other. And the UK standard makes suppression of load dump at platform level, through CLDS, an explicit requirement.
Frequently Asked Questions
Yes. Issue 7 of Part 6 is published under the three-digit number Def Stan 61-005 Part 06, while earlier issues carry the number 61-5. Catalogues, test reports and datasheets use both forms, so search for both when looking for a document or a test certificate.
Defence Standards are published by DStan, the Ministry of Defence’s standardization authority, through its extranet at dstan.mod.uk, which sits behind the Defence Gateway. See the UK Defence Standardization guidance on GOV.UK for access. Commercial standards distributors also list the parts and their document histories, which is a quick way to check the latest issue.
No. The two standards use different test waveforms and source conditions, so each needs its own tests. What one design can do is cover both: an input stage that operates continuously from 9 to 36 V spans the normal ranges of 12 V and 24 V UK systems and of 28 V US systems, and protection can then be sized for the harsher transient of each.
The issue named in the contract, which is not always the latest. Issue 6 test reports are still in circulation, and programmes that started before 2020 may remain on Issue 6. Where a contract simply says “Def Stan 61-5”, ask for the part and issue in writing before testing, because the waveforms and the load dump cases differ between issues.