The Effect of Slot Openings in a Faraday Cage on EMC

Introduction – The Continuity of a Shield

What made the 300 Spartans powerful at Thermopylae was not individual courage, but the perfect continuity of shields raised side by side. There was also Ephialtes in the story; his body was hunched, his shoulders were crooked, and unlike the Spartans, he could not raise his shield at head level as part of the formation. Despite this, he wanted to join the battle. He wanted to be seen, to be valued, and to feel useful. He asked King Leonidas for only one thing: a shield.

Leonidas rejected him. Because the issue was not Ephialtes’ intention, but the gap that would be created. A single shield not raised properly meant a breach in the entire defense. Ephialtes then went to someone who valued him: the Persian King Xerxes. And that seemingly small opening changed the course of the war.

This story is a powerful metaphor for understanding EMC problems encountered in electronic systems today.

In EMC, the issue is not the existence of noise, but where it leaks from.

1. Electronic Systems and the Reality of EMI

When an electronic product operates, it does not only perform its intended function. It also generates electromagnetic energy.

This energy can be:

Kept under control
Or radiated outward through cables, openings, and metal structures

EMC problems most often arise in the second case. The need for shielding originates precisely here.

2. Currents: The Real Source of EMI

In an electronic system, there are two fundamental types of current.

2.1 Differential-Mode Currents

Differential-mode current is the signal itself. The current flows through one conductor and returns through the other. Since the forward and return paths are close to each other, the fields they generate largely cancel each other out.

For this reason, in a well-designed system, differential-mode currents generally do not create serious EMI problems.

2.2 Common-Mode Currents

Common-mode currents flow in the same direction on multiple conductors. Their return path is not well defined; the reference is most often the chassis, enclosure, and the surrounding environment.

Common-mode currents:

Have large loop areas
Are uncontrolled
Have very high radiation efficiency

The majority of emissions that cause failures in EMC tests originate from common-mode currents. Therefore, shielding is fundamentally about controlling common-mode currents.

3. Where Does Current Flow on a Conductor? – Skin Effect

As frequency increases, current no longer flows through the entire cross-section of a conductor; it becomes increasingly confined to the surface. This phenomenon is called the skin effect.

Its EMC-related consequences are:

The effective cross-section decreases at high frequencies
AC impedance increases
Thin and long connections behave poorly at high frequencies

For this reason, in shielding and chassis connections:

Long, thin wires
Pigtail connections
Narrow contact surfaces

cause serious problems at high frequencies. Short and wide contact surfaces are therefore critical.

4. Time-Varying Magnetic Fields and Eddy Currents

A time-varying magnetic field induces closed current loops within a conductive material. These are called eddy currents.

Eddy currents:

Convert energy into heat
Can attenuate magnetic fields at high frequencies

However, at low frequencies (50/60 Hz):

This effect is weak
Metal plates can hardly block magnetic fields

For this reason, while electric fields are easily suppressed by metal, magnetic fields are much more difficult to control.

5. The Fundamental Difference Between Electric and Magnetic Fields

The electric field (E-field) is related to voltage differences and is formed by the spatial distribution of charges. When it encounters a conductive surface, free charges redistribute and tend to terminate on the surface.

Therefore, electric fields:

Are confined to conductive surfaces
Cause charge accumulation on the surface
Have difficulty penetrating into the enclosed volume

The magnetic field (H-field), on the other hand, is not related to charges but to currents and current loops. Its field lines form closed loops and, especially at low frequencies, tend to pass through materials.

This difference determines why shielding is very effective for electric fields but limited for magnetic fields.

6. Why Are Cables the Biggest Problem?

A cable:

Is long
Is exposed to the external environment
Can create a potential difference relative to the chassis

When these three come together, the cable becomes a very efficient antenna.

There are two main mechanisms:

Common-mode antenna: current flows in the same direction on all conductors, and the return path is the environment
Slot / structural antenna: shielding continuity is broken at the cable exit

This is why the following sentence is often heard during EMC tests: “The device is fine, but when the cable is connected, it fails.”

7. Faraday Cage and the Electric Field

A Faraday cage is an electrically conductive and as continuous a structure as possible. However, what is critical is not that it is a “box,” but how the conductors behave.

When an external electric field is applied:

Free electrons redistribute on the surface
An opposing field is formed on the surface
The net electric field inside the enclosed volume becomes zero

This is not an assumption, but a mandatory result of electrostatic equilibrium.

8. Holes, Slots, and Discontinuities

A Faraday cage works under one condition: the conductive surface must be continuous.

If there are:

Holes
Long slots
Discontinuities

surface charges cannot distribute uniformly, and local field leakage occurs. Especially when the slot dimensions approach the wavelength, the structure behaves like a slot antenna.

9. The Difference Between Chassis, Earth, and GND

These three concepts are often confused.

Earth (PE):

Is for safety
Protects human life
Does not solve EMC problems

Chassis:

Is the metal enclosure of the device
Is electrically conductive and continuous
Is the main EMC reference
Common-mode currents flow here
Shielding works here

GND (Signal Ground):

Is the circuit reference
Is sensitive to noise
Is not suitable for EMI currents

Shielding is connected to the chassis, not to the signal GND.

10. Why “Connecting Everything to Ground” Is Wrong

At high frequencies, there is no ideal “ground point.” Long ground wires are inductive, and HF currents do not prefer this path.

HF current asks one question: “Where is the shortest and widest metal?” The answer is most often: the chassis.

11. Connecting Cable Shields to the Chassis

(Where Does the Shield Continuity End?)

A Faraday cage is not just a metal enclosure. Every cable exiting the enclosure is an opening that tests the continuity of the shield.

If the shield is not connected correctly at this point, no matter how perfect the enclosure is, the shield breaks there.

11.1 Where Should the Cable Shield Be Connected?

Cable shields should not be connected to PCB GND. Cable shields must be connected directly to the chassis.

Because:

Noise on the cable has a common-mode character
The natural circulation path for these currents is the chassis surface
GND is neither low-impedance nor safe for these currents

📌 The rule is clear: Cable shield → Chassis Signal GND → Internal circuit reference

11.2 Why Are Pigtail Connections Wrong?

A pigtail is the connection of the cable shield to the chassis using a thin and long wire. It may appear “connected” at DC or low frequencies, but it is a serious mistake from an EMC perspective.

Because:

Long wire → high inductance
At high frequencies, this path behaves like an open circuit
Common-mode current cannot flow to the chassis and remains on the cable

Result:

The cable becomes an antenna
The enclosure and cable together increase radiation

Therefore: ❌ Pigtail = Shield break at high frequency

11.3 Why Is a 360° Shield Connection Critical?

A correct shield connection is:

Circumferential (360°) around the cable shield
With a wide contact surface
Directly connected to the chassis

This ensures that:

Surface currents flow without interruption
Impedance is minimized
Current finds the “shortest and widest path”

This means that the Faraday cage continues over the cable.

📌 The cable shield must become part of the Faraday cage at the point where it is connected to the enclosure.

11.4 Why Are Connection Elements Important?

Connectors, clamps, and metallic fastening elements are not merely mechanical parts. From an EMC perspective, they are elements that:

Ensure shield continuity
Carry surface currents
Connect the Faraday cage to the outside world

Therefore:

Metal-bodied connectors should be preferred
Paint, oxide, or insulating layers should be removed from contact surfaces
Shield-to-chassis contact should be as short and wide as possible

11.5 The Shield Does Not End Here

When a cable exits the enclosure, the Faraday cage does not automatically end. If the shield is connected correctly, the shield continues along the cable.

But if:

The shield is interrupted
It is connected through a narrow point
It is extended with a pigtail

the shield turns into a slot antenna at that exact point.

Just like a single gap in the Spartans’ shield wall weakens the entire defense.

11.6 Key Message of This Section

The enclosure may be perfect. The PCB may be cleanly designed. But if the cable shield is connected incorrectly:

The Faraday cage ends there. And EMC tests will inevitably reveal it.

Conclusion

What Leonidas saw was not Ephialtes, but the opening that would be created.

In electronics, the strength of shielding lies not in the material, but in continuity.

When cable shields are not connected correctly, the Faraday cage is no longer a whole.

And a single slot wounds the entire shield.

Thank you for reading; let us remember together that in EMC, it is not the details, but the overlooked openings that define the system.