What Is ISO 7637-2?

ISO 7637-2 is part 2 of the ISO 7637 series, “Road vehicles: electrical disturbances from conduction and coupling”. It sets bench tests for the short, high-voltage spikes that travel along a vehicle’s 12 V or 24 V supply wiring whenever loads are switched, and it applies to passenger cars and commercial vehicles whatever drives them, from diesel engines to electric motors.

The standard has two halves. The emissions test measures the transients a device itself creates on its supply line, for example when it switches a relay or motor. The device is connected through an artificial network, which gives the wiring a standard impedance so results from different laboratories can be compared. The immunity test does the opposite: a transient generator injects standard test pulses into the device’s supply input while it runs, and the device’s behavior is recorded.

Tests are run at 23 ± 5 °C with the supply held at 13.5 V for a 12 V system or 27 V for a 24 V system. The current third edition was published in 2011, replacing the 2004 edition and its 2008 amendment.

Part 2 is the best known, but it is one of several parts that share definitions and test methods.

ISO 7637-2 defines how to test, not exactly what every device must survive. The standard itself says some pulses may be left out if a device is not exposed to them, and that choosing the required pulses is the vehicle manufacturer’s job. Many manufacturer standards also modify the waveforms. The table shows the split.

So “ISO 7637-2 compliant” on a datasheet tells you the input was tested with these pulses, but not at which level or to which status. For a vehicle approval, ask for the test report.

The example severity levels in Annex A increase with the system voltage, so 24 V trucks and buses are tested harder than 12 V cars. The fast switching pulses, 3a and 3b, reach a few hundred volts but last well under a microsecond, while pulse 2b is low in voltage but lasts up to a couple of seconds, so it tests a different weakness in the input stage.

Each result is judged against a function performance status. In plain terms, the classification asks whether the device kept working normally through the pulses, recovered by itself afterwards, or needed a reset or power cycle to recover. A navigation display that briefly flickers may be acceptable; a telematics computer that locks up until someone switches it off and on again usually is not.

Load dump, the large surge when a battery is disconnected while the alternator is charging, is no longer part of ISO 7637-2. Pulses 4, 5a and 5b moved to ISO 16750-2 in the 2011 edition, so a vehicle specification normally calls on both standards.

Load dump is the surge on a vehicle supply when the battery is suddenly disconnected while the alternator is charging hard. The battery normally absorbs the alternator’s output, so when it drops off the circuit, through a corroded terminal, a loose cable or a battery isolator opened with the engine running, the alternator’s field takes a fraction of a second to collapse and its voltage rises steeply. Every device still connected sees the surge, which can last a few hundred milliseconds and carries far more energy than the switching pulses above.

The load dump tests left ISO 7637-2 in the 2011 edition and now sit in ISO 16750-2, the standard for electrical loads on vehicle equipment. They come in two forms, and a datasheet that just says “load dump protection” may mean either.

Because the energy is large, a small transient suppressor that copes with pulses 1 to 3 may not survive an unsuppressed load dump. Vehicle power inputs usually combine a transient voltage suppressor sized for the energy, a surge stopper or pre-regulator that limits what reaches the converter, and a component rating above the clamped level. On 24 V heavy vehicles, where the surge is larger and battery isolators are common on tankers and plant, it is worth asking which test and level a datasheet means.

Where the vehicle supply is especially harsh, an isolated DC-DC converter in front of the computer adds a further barrier; our industrial power supplies include rugged DC-DC designs tested to MIL-STD-1275 for 28 V military vehicles.

UN ECE Regulation 10, the basis of the E-mark for electronic sub-assemblies, calls on ISO 7637-2 for its conducted transient tests. Our guide to E-mark certification and UN ECE R10 explains how the approval works and what each pulse simulates. Military vehicles use a different standard for the same job, MIL-STD-1275, which covers 28 V vehicle power.

A computer designed for vehicle power usually handles these pulses in its input stage: a transient voltage suppressor or similar clamp, filtering, a wide-range DC input and enough hold-up to ride through short dips. Several of our in-vehicle computers list ISO 7637-2 pulse protection on their 12 or 24 V DC input together with smart ignition control. Proving a full design still means EMC testing at an accredited laboratory.

We supply in-vehicle computers as part of a complete system, with the power input, ignition settings and mounting matched to the vehicle as standard.