EMC Design Guidelines for Cabling and Wiring

Fundamentals of Cabling and Wiring

EMC Guidelines #1 -

Laying cables along chassis (GND EARTH)

Low radiated emission, conducted immunity, ESD

Whenever possible, laying cables along with chassis:

• Emission: This keeps the electromagnetic field generated by the voltage and current in the cable at a minimum radiation level.

• Immunity: This minimizes induced current in the cable and cable shield (assumed that shield is laid on both ends to ground). Reason: loop area for induced currents is minimized.

EMC Guidelines #2 -

Do Not use Shield as signal conductor for Low Frequency Signals

Increase Immunity

Shield should not be one of the signal conductors for low-frequency signals (e.g sensor signals) Never...!

Because of the noise current induced in the ground loop.

Note #1: This rule does NOT apply for high-frequency signals, where the signal return current and the noise current are separated by the skin-effect within the shield (e.g. in a coaxial cable).

EMC Guidelines #3 -

Laying only one end of the cable shield to ground for Low Frequency Signals

Increase Electric Fields Immunity

When shielding cables, there might be usually pops up this question:

"Should we ground the cable shield on both ends or only on one end?"

It may lead to problems when laying both ends of cable shield to ground for low frequency signals, because balancing current in the shield could lead to interference of signals inside the cable. 

Here the rule of thumb for low-frequency (<20 kHz) signals.

• ​Protection against electrical fields (E): lay only one end of the shield to ground (with low-impedance) in order to avoid noise current through the shield (e.g induced by magnetic fields or ground loop currents).

• Protection against magnetic fields (H): Follow Guideline #5

EMC Guidelines #4 -

Laying both ends of the cable shield to ground for high-frequency signals.

Increase electromagnetic fields immunity, lower magnetic fields emission

When shielding cables, there usually pops up this question:

"Should we ground the cable shield on both ends or only on one end?"

It is a must to lay both ends of a high-frequency signal cable shield to ground with low inductance (no pigtails, use a 360º shield clamp or the like).

Here the rule of thumb for high-frequency (>1MHz) signals,

• The shield can be used as signal return path for high-frequency signals (because the signal return current and the noise current are separated by the skin-effect (e.g. in a coaxial cable , you can find calculated skin-depth below). The induced noise current in the shield helps to cancel out the magnetic field of the noise source.

• To lower the emission of magnetic fields from a signal in a cable, the signal should be shielded and the shield has to be laid on ground on both ends.

EMC Guidelines #5 -

Minimize Loop area of the signals in Cables

Increase Magnetic fields immunity, lower magnetic fields emission

The best way to protect a signal in a cable against magnetic fields is to reduce the loop area: 

• Protection against magnetic fields H

• Minimize loop area between a signal and it’s return path

(e.g. use twisted pair, or use neighbor conductors in flat cables).

Note #2: Cable shields made out of copper or aluminum do NOT protect the signals within the shielded cable from magnetic fields by its nature.

A cable shield does protect against electric fields.

SHIELDING SOLUTIONS By LAMBDA INDIA

For further consultations for Design and Manufacturing of EMC Products

Contact Us LAMBDA INDIA

---

Author -

LAMBDA INDIA