Guidelines for Creating the Ground Connection for Cable Shielding

Guidelines for Creating the Ground Connection for Cable Shielding

Guidelines for selecting the method of connection NOTE: Always avoid cable shielding without a ground connection. This type of connection is practically useless from an EMC point of view and cannot be permitted for safety reasons if contact protection is not provided. The table shows how the cable shield should be connected to ground depending on the application:

Long lines For long shielded lines, several ground connections at intervals of 10 ... 15 m along the length of the cable are recommended:

Application

Shielded analog measuring circuit in the cabinet

Shielded analog measuring circuits outside of cabinets in closed buildings Cable shield ground connection

 Ground connections are usually found on one side of the cabinet outlet  Both ends of the cable shielding ground for extreme levels of disturbance

 If only capacitive electrical interference is to be reckoned with: single cable shielding ground connection  If the signal line is setup with highfrequency influences: double sided cable shielding ground connection  If the signal line is long: in addition to double ground connections along the cable length, further ground connections at intervals from 10 ... 15 m

Characteristics of the connection methods The shielding ground connection is very important for the shielding effectiveness. The following ground connection options have differing effectiveness:

Cable shield ground connection

Ground connection on both ends of the cable Extremely effective

Ground connection on only one end of the cable Average shielding effectiveness

Shielding without ground connection Not recommended Effectiveness and advantages Restrictions

 Very effective against external disturbances (high and low-frequency)  Very good shielding effectiveness also against resonance frequency on the cable  No potential difference between cable and ground  Enables common laying of cables that feed different class signals  Very good suppression of high-frequency disturbances

 Enables protection of isolated lines (instrument transformer, ...) against lowfrequency electric fields  Enables buzz to be avoided (= low-frequency disturbance)  Ground-fault current can be induced in high-frequency signals with high interferencefield strength for long cables (>50 m).

 Ineffective against external disturbances caused by highfrequency electric fields  The shielding can cause resonance due to the antenna effect. This means the disturbance is greater than when shielding is present!  Potential difference between the shielding and the ground connection at the unearthed end; danger in the event of contact!

 Limits the capacitive coupling  Ineffective against external disturbances (all frequencies)  Ineffective against magnetic fields  Potential difference between the shielding and the ground connection; Danger in the event of contact!