What is the difference between HEMP, IEMI and a standard lightning surge?

The three threats differ by source, waveform and target. A standard lightning surge is the IEC 61000-4-5 reference, with a 1.2/50 us voltage front and an 8/20 us current front, peaking at a few kV on lines and tested on standard equipment. HEMP (High-altitude Electromagnetic Pulse) arises from a high-altitude nuclear detonation and breaks down into three components: E1 with a sub-nanosecond rise reaching tens of kV/m, E2 in the microsecond range similar to lightning indirect, and E3 over seconds inducing slow currents in long power and telecom lines. IEMI (Intentional Electromagnetic Interference) covers hostile narrow-band high-power microwave weapons. The orders of magnitude separating these threats span several decades in time and amplitude, and the mitigations differ.

When is IEC 61000-4-25 mandatory?

The standard is not mandatory by default for commercial CE marking. It applies to equipment intended for HEMP-resilient critical infrastructure, in particular defence command and control (C4I), energy grid hardening programmes, financial data centres designated as critical, and certain government telecommunication networks. Application is generally driven by a customer specification (defence contract, transmission system operator framework, regulated banking requirement) rather than by a horizontal regulation. Outside these use cases, IEC 61000-4-5 surge immunity and the IEC 61000-4 family cover the standard EMC threats.

Is an IEC 61000-4-5 surge test sufficient against the HEMP E1 pulse?

No. The IEC 61000-4-5 surge has a rise time of around 1.2 us, while the HEMP E1 reference pulse has a rise time of about 2.5 ns, three orders of magnitude faster. Surge protection components qualified for lightning indirect (gas discharge tubes, classic MOVs) often respond too slowly to clamp the E1 leading edge. HEMP-rated protection relies on fast components (silicon avalanche, fast TVS), ferrites and filters in series, qualified specifically per IEC 61000-5-5. A product passing IEC 61000-4-5 is not necessarily HEMP-resistant.

What is the role of Faraday cage shielding in HEMP protection?

A Faraday cage is the primary line of defence against the radiated E1 component. The required shielding effectiveness typically reaches 80 to 100 dB over the relevant band (a few kHz to a few GHz), depending on the threat level retained. Performance is governed less by the choice of metal than by penetration discipline: door gaskets, ventilation honeycomb panels (waveguide-below-cutoff), filters on each cable through the shield, equipotential bonding. A single unfiltered cable crossing the shield cancels the effectiveness of the entire cage. Periodic verification of shielding effectiveness is part of the maintenance plan because gasket performance degrades over time.

What does the IEMI part of IEC 61000-4-36 cover?

IEC 61000-4-36 (2020) is the immunity test method dedicated to intentional EM interference. Unlike the HEMP standards which target a broadband transient, IEC 61000-4-36 covers narrow-band high-power sources, typically high-power microwaves (HPM) from a deployed weapon, a covert briefcase HPM or an electromagnetic disruptor. The standard defines test waveforms, frequency ranges, field levels and observation criteria for evaluating the resilience of an equipment under attack. It complements IEC 61000-4-3 (radiated RF immunity) which targets ordinary electromagnetic environment, not hostile threats.

Are HEMP requirements regulated at EU level?

No, there is no horizontal EU regulation imposing HEMP resilience. Application is driven by national or sectoral specifications: NATO programmes (STANAG), national defence procurement, transmission system operator requirements for energy grids, governmental telecommunication network specifications. The United States have a more structured framework, with executive orders (E.O. 13865 in 2019) requesting HEMP and GMD resilience analyses for critical infrastructure, and dedicated military standards (MIL-STD-188-125-1 for ground-based command and control).

What is the difference between HEMP E3 and a geomagnetic disturbance (GMD)?

The two phenomena produce similar effects but originate differently. HEMP E3 is the late-time, low-frequency (sub-Hz) component of a high-altitude nuclear pulse, lasting seconds and inducing slow currents in long power and telecom lines. A geomagnetic disturbance (GMD) results from solar activity (coronal mass ejections, severe geomagnetic storms such as the Carrington event in 1859 or the Quebec blackout in 1989) and induces ground currents (GIC) of the same nature in long conductors. From a hardening standpoint, the mitigations converge: protection of high-voltage transformers against DC bias, sub-Hz filtering on long lines, monitoring of GIC currents at strategic substations.

What does a HEMP testing campaign include?

The campaign combines several methods. For radiated E1, large-volume simulators (parallel plate or bounded wave) reproduce the double-exponential pulse 2.5/25 ns reference, with peaks reaching tens of kV/m on equipment under test. For conducted testing, pulsed current injection (PCI) per IEC 61000-4-25 applies transient currents of several hundred amperes on cable shields and conductors. For HPM/IEMI per IEC 61000-4-36, narrow-band sources sweep the frequency range relevant to the equipment. The campaign sequence typically includes pre-characterisation, simulator testing on the full system, then PCI testing on isolated penetrations.

HEMP and IEMI: IEC 61000-4-25 and hardened electronics

Author Hugues Orgitello Last updated 27 May 2026 ~11 min read

Guide, HEMP and IEMI hardening

HEMP (High-altitude Electromagnetic Pulse) and IEMI (Intentional Electromagnetic Interference) sit at the extreme end of the EMC threat spectrum, well beyond the lightning surge and radiated RF immunity tests of standard product certification. They concern equipment intended for defence, energy grid hardening, government telecommunication networks, financial data centres designated as critical and command and control infrastructure. The standardisation effort is led by IEC subcommittee SC 77C, which has built since the 1990s a coherent family covering environment description (IEC 61000-2-9, IEC 61000-2-11), protection concepts (IEC 61000-5-3, IEC 61000-5-5) and immunity test methods (IEC 61000-4-25 for HEMP, IEC 61000-4-36 for IEMI). On the military side, MIL-STD-188-125-1 frames HEMP protection for US ground-based C4I, and MIL-STD-461 RS105 reproduces the transient E1 field in equipment-level qualification. This guide presents the threat physics, the standards stack, the protection architecture and the recurring pitfalls.

Threat definition

The three threats addressed by this guide differ by source, waveform and operational scenario. Understanding their physics drives the entire hardening architecture.

HEMP, high-altitude electromagnetic pulse

A nuclear detonation at high altitude (typically 30 to 400 km) generates a wide-area electromagnetic pulse covering hundreds to thousands of kilometres of ground. The pulse breaks down into three components separated by orders of magnitude in time and frequency.

Component	Rise time	Duration	Peak field (typical)	Frequency content	Primary effect
E1 (early-time)	A few ns	Tens of ns	Up to tens of kV/m	MHz to GHz	Damages microelectronics, semiconductor breakdown
E2 (intermediate-time)	A few us	Microseconds to milliseconds	Around 100 V/m	kHz	Comparable to lightning indirect, may upset systems
E3 (late-time)	Seconds	Tens of seconds to minutes	Low field, but large induced currents	Sub-Hz	Couples into long power and telecom lines, transformer saturation, slow currents (GIC)

The E1 component is the most demanding for electronics: its sub-nanosecond rise penetrates through cable apertures and resonates on tracks and harnesses, generating fast transients capable of destroying semiconductors. The E2 component is comparable in form to a lightning indirect surge and is generally absorbed by mitigations qualified for IEC 61000-4-5. The E3 component does not directly threaten electronics but induces low-frequency currents in long conductors, saturates high-voltage transformers and can collapse a power grid.

GMD, geomagnetic disturbance

A geomagnetic disturbance is a natural phenomenon caused by severe solar activity (coronal mass ejection, intense geomagnetic storm). Two historical references frame the order of magnitude: the Carrington event of 1859, which set telegraph offices on fire, and the Quebec blackout of March 1989, which took down the Hydro-Quebec grid in 90 seconds. A GMD induces geomagnetically induced currents (GIC) in long conductors, with effects similar to HEMP E3: DC bias on transformer cores, harmonic generation, eventual loss of voltage control. The mitigations against E3 and GMD largely converge, which is why resilience programmes treat them jointly.

IEMI, intentional electromagnetic interference

IEMI groups hostile EM attacks delivered by a deployed source. Typical platforms range from a covert briefcase HPM (high-power microwave) to a vehicle-mounted system, up to specialised military assets. The waveforms are typically narrow-band, in the hundreds of MHz to tens of GHz range, with peak power sufficient to disrupt or destroy targeted electronics at a few metres to a few hundred metres range. Operational effects span from data corruption and system reset to permanent hardware damage. Unlike HEMP, IEMI does not require a nuclear capability and the threat is therefore reachable for non-state actors, which has pushed dedicated standardisation since the mid-2000s.

IEC 61000-2-x and IEC 61000-4-25 standards stack

IEC subcommittee SC 77C maintains the HEMP and HPEM standard family. The structure groups the environment description, the protection concepts, the test methods and the protection device specifications.

Standard	Year	Scope	Type
IEC 61000-2-9	1996	HEMP environment, radiated disturbance description	Environment
IEC 61000-2-10	1998	HEMP environment, conducted disturbance description	Environment
IEC 61000-2-11	1999	Classification of HEMP environments	Environment
IEC 61000-2-13	2005	HPEM environment (radiated and conducted)	Environment
IEC 61000-4-25	2001 + A1 2012	HEMP immunity test methods for equipment and systems	Test method
IEC 61000-4-35	2016 + A1 2024	HPEM simulator compendium	Test method
IEC 61000-4-36	2020	IEMI immunity test methods for equipment and systems	Test method
IEC 61000-5-3	1999	HEMP protection concepts	Protection
IEC 61000-5-5	1996	Specification of HEMP conducted protection devices	Protection

The IEC 61000-2-x family establishes the reference environment. IEC 61000-2-9 in particular fixes the canonical waveform for E1, with a double-exponential pulse of about 2.5 ns rise time and 25 ns duration to half value, reaching peaks of 50 kV/m at the most demanding level of the standard. This waveform is the reference for the radiated tests of IEC 61000-4-25 and for protection device qualification per IEC 61000-5-5.

IEC 61000-4-25 is the central test method document. It specifies the radiated and conducted (pulsed current injection, PCI) test procedures applicable to equipment and systems. It defines several severity classes corresponding to the resilience requirement, with field and current values directly drawn from the environment fixed by IEC 61000-2-9.

IEC 61000-4-36, published in 2020, is the equivalent for IEMI. It is more recent because IEMI was only structured as a distinct threat in the early 2000s, with the development of compact HPM sources and the documented existence of attacks on civilian targets.

IEC 61000-5-3 and 5-5 protection concepts

IEC 61000-5-3 fixes the protection concepts. The reference architecture is layered: an outer shield (Faraday cage), penetration discipline at the shield boundary (filters, waveguide-below-cutoff), internal grounding and bonding, then equipment-level hardening for the most exposed sub-systems.

IEC 61000-5-5 specifies the conducted protection devices, in particular the HEMP-class surge protection components. The qualification criterion is much harder than for ordinary IEC 61643 components: the device must clamp a pulse with a rise time of around 5 ns and let through residual energy compatible with downstream equipment survival. Several technologies coexist (silicon avalanche, fast TVS, ferrites in series with classic MOV), each addressing a specific time window.

Military overlay

The defence sphere uses a parallel set of standards, often older than the IEC family, and with which the IEC standards are partially aligned.

Reference	Origin	Scope
MIL-STD-188-125-1 (2005)	USA	HEMP protection for ground-based C4I facilities (fixed installations)
MIL-STD-188-125-2	USA	HEMP protection for transportable C4I facilities
MIL-STD-461 RS105	USA	Transient EM field qualification at equipment level, E1 reference
MIL-STD-464	USA	EMC requirements for system platforms (aircraft, ship, ground)
STANAG 4145	NATO	EMC compatibility for NATO equipment
STANAG 4435	NATO	HEMP protection for NATO C4I systems

The MIL-STD-188-125-1 framework details the installation-level testing protocol: continuous wave (CW) shielding effectiveness measurement, pulsed current injection on every penetration, validation of internal grounding network. The protocol is demanding and applies to a delivered facility, not an isolated piece of equipment. RS105 in MIL-STD-461 reproduces the E1 transient at equipment level, with a parallel-plate radiator generating up to 50 kV/m in a confined volume.

Civil critical infrastructure

The application of HEMP and GMD requirements to civilian critical infrastructure depends strongly on jurisdiction.

United States

The US framework is the most structured. Executive Order 13865 of March 2019, followed by an update in 2024, requested a coordinated effort across federal agencies on EMP and GMD resilience. NERC (North American Electric Reliability Corporation) has issued GMD reliability standards (TPL-007) applicable to bulk power system operators, requiring GMD vulnerability assessment and reinforcement of high-voltage transformers. DOE and FERC participate in the framework. Some assessment work also targets HEMP, but HEMP resilience for civilian operators remains less prescriptive than for GMD.

European Union

There is no horizontal EU regulation imposing HEMP or IEMI resilience. Application is driven by national defence procurement contracts, by transmission system operator requirements for high-voltage networks (some TSOs include GIC clauses in their grid codes), and by certain government telecommunication network specifications. NATO members may apply the corresponding STANAGs on their defence assets.

United Kingdom

The Civil Contingencies Act establishes the general framework for resilience of critical infrastructure, without specifically prescribing HEMP. Industrial sectoral standards may impose dedicated requirements.

Protection techniques

The hardening architecture combines several lines of defence, each addressing a portion of the threat spectrum. The general principle is layered: never rely on a single component or a single technique.

Faraday cage

The shielded enclosure is the primary line of defence against radiated E1. The required shielding effectiveness typically reaches 80 to 100 dB over a band of a few kHz to a few GHz, depending on the threat level retained. The choice of metal (steel, copper, aluminium) is secondary compared to the discipline of treating boundary discontinuities. The classic measurement procedure follows IEEE 299 (or MIL-STD-188-125-1 for HEMP facilities), with a transmitter inside and an antenna outside (or vice versa) sweeping the frequency.

Penetration discipline

Every cable, pipe, conduit or ventilation duct crossing the shield is a potential point of entry. The discipline rests on a simple rule: any conductor that crosses the shield must be filtered at the crossing point, and any non-conductive opening must be sized as a waveguide-below-cutoff (WBBC). Vent honeycomb panels typically use a hexagonal cell of a few millimetres, generating a cut-off frequency well above the threat band.

A single unfiltered cable cancels the effectiveness of the entire cage. This is the most frequently observed failure mode in field audits.

Filters and surge protection devices

The filters and SPDs deployed at penetration points must be qualified specifically for HEMP, per IEC 61000-5-5. Common pitfalls: reusing a classic IEC 61643 SPD (lightning indirect rated), which responds too slowly to clamp the E1 leading edge; underestimating series ferrite role for fast slope reduction; omitting bandstop filtering on antenna paths (radio frequency penetration of the cage by the antenna itself).

Bonding and grounding

Internal grounding follows single-point earthing logic at the facility scale, with equipotential bonding for sub-systems. The goal is to avoid loops that could re-radiate inside the cage. Conductive surface continuity at door gaskets is treated as a maintenance item: the contact resistance of a gasket increases over years of opening and closing, and regular measurement campaigns are part of the hardening discipline.

Optical isolation

For data crossing the shield, optical fibre is strongly preferred over copper. Fibre is dielectric, does not pick up E1, and breaks any galvanic path that could re-conduct the threat inside the cage. The conversion stages on each side of the shield must themselves be protected, but the inter-shield transport is intrinsically immune.

Test facilities

HEMP and HPEM test resources are concentrated, very few facilities in the world reach the necessary field levels. The list below covers the main public references.

Facility	Location	Scope
ATLAS-I (Trestle) (decommissioned 2007)	Kirtland AFB, USA	Historic large-volume HEMP simulator for aircraft systems
Repetitive Pulse Generators, WIS Munster	Germany	HEMP and HPM testing, defence equipment
HPRF Sirocco	DGA, France	HPM/IEMI testing, French defence framework
TNO HPM facility	Netherlands	IEMI testing, defence and civilian
FOI HPM facility	Sweden	HPM testing, Swedish defence research
DGA-MI	Bruz, France	EM compatibility testing for French defence systems

The test waveforms are framed by the standards: double-exponential 2.5/25 ns for HEMP E1 radiated, pulsed currents of hundreds of amperes for IEC 61000-4-25 PCI testing, narrow-band CW sweeps in the GHz range for IEC 61000-4-36 IEMI testing. Combining these methods on a complete system requires several weeks of campaign and access to specialised facilities.

Test sequence for hardened equipment

The typical sequence for a piece of equipment intended for a HEMP-resilient programme.

Threat specification from the customer (level retained: light commercial protection, IEC 61000-4-25 class, MIL-STD-188-125-1 level).
Architecture analysis: identification of the perimeter to shield, penetration map, filter and SPD plan, internal grounding plan.
Pre-characterisation: shielding effectiveness measurement on prototype enclosure per IEEE 299, SPD characterisation per IEC 61000-5-5.
Equipment-level testing: MIL-STD-461 RS105 transient field if applicable, IEC 61000-4-25 conducted PCI test on each penetration.
System-level testing: full simulator campaign (parallel plate or bounded wave) at a specialised facility.
IEMI evaluation if specified: IEC 61000-4-36 narrow-band sweep over relevant frequency range.
Documentation: report describing the test setup, the levels applied, the observed performance, the residual uncertainty.
Maintenance plan: periodic verification of shielding effectiveness (typically annual), measurement of gasket contact resistance, audit of penetrations after any wiring change.

For the underlying radiated RF immunity test method on standard equipment, see IEC 61000-4-3, radiated RF immunity. For the magnetic field immunity, see IEC 61000-4-8, power-frequency magnetic field immunity. For the defence EMC framework, see MIL-STD-461 and MIL-STD-464. For the test chamber typology, see EMC chamber types.

Frequent pitfalls

Pitfall	Consequence
Treating an IEC 61000-4-5 surge test as sufficient against HEMP E1	E1 leading edge passes through SPD too slow to clamp, destruction of downstream semiconductors
One unfiltered cable crossing the shield	Effectiveness of the entire cage cancelled, dossier non-compliant
Reuse of commercial IEC 61643 SPDs not rated for HEMP	Clamp time too long, residual let-through incompatible with sensitive electronics
Vent honeycomb cell size too large for the threat band	Apparent shielding effectiveness collapses above the cell cut-off
Missing periodic verification of shielding effectiveness	Gasket performance degrades over years, fielded facility no longer meets the spec
Omitting E3 and GMD mitigation on long lines	Transformer saturation, slow currents undetected on the energy grid
Confusion between HEMP and civilian lightning EMP	Peak amplitudes off by three orders of magnitude, mitigation under-sized
No optical isolation for data crossing the shield	Copper data link re-conducts the threat inside the cage
Antenna path without bandstop filter	E1 penetration through the antenna cable directly into the radio front-end
Treating IEMI as covered by IEC 61000-4-3 immunity	The narrow-band high-power threat is not represented in the ordinary EMC immunity envelope

Going further

IEC 61000-4-3, radiated RF immunity: the standard radiated RF immunity reference, the entry-level layer below HEMP
IEC 61000-4-8, power-frequency magnetic field immunity: adjacent low-frequency magnetic threat
MIL-STD-461 and MIL-STD-464, defence EMC: military EMC framework including RS105 transient field
EMC chamber types: chamber typology used for radiated immunity testing
Glossary: definitions of HEMP, IEMI, HPEM, GIC, GMD, SPD, WBBC