ElectroMagnetic Compatibility in Cable Tray Systems

Ed Cronin, RCDD, President, Cronin Consulting

Claes Rosenberg, Export Manager, Defem System/Schneider Electric

EMC Research Consortium

Definition of ElectoMagnetic Compatibility

”The ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetical disturbances to anything in that environment”

Source: 2008 International Electrical Committee (IEC)

EMC in the Data Room

• The metal cable tray is part of the electrical structure

• The metal cable tray is part of the metallic structure

• Any metallic structure is known as a “generalized shield”

Definition of generalized shield

• To function as a generalized shield, the metal does not have to be enclosing the cable. Any continous metal along the cable is a shield.

• A shield is any metal structure that is continous and parallel along the cable path

Metal Cable TrayQuick Facts

• Properly bonded and grounded = ”earth”

• Poorly bonded and grounded = ”antenna”

Background

• July 2007 - EU introduces new EMC directive

• It states that any electrical installation shall be made according to ”good engineering EMC practice”

EU Directive

...”installer must document that the EMC

performance of complete installation is made

according to ”good engineering practice” ...

Source: EMC Directive 2004/108/EC

Common EMC Problems

• “Come and go problems” – Lightning strike– on/off transients– radio transmission

• “Slow down problems”– Disturbances may force packaged signals to be

re-packaged and re-sent

Metal structure

Equipment

Cables Differential mode

Principles of Generalized Shield

Various Shield Structures

Principles of Earth Loop Area Reduction

Principles of Earth Loop Area Reduction

Principals of Earth Loop Area Reduction

Principles of Earth Loop Area Reduction

Tray Design

• Minor Importance– wire mesh – solid metal tray – perforated metal tray– cable ladder

…are all “shield” when properly installed

Schneider Electric tests performed with Defem cable tray

Ultimate installationTray directly connected to top of cabinets

Floor is Ground Reference Plane.

Realistic installationTray connected with short ground wire to top of both cabinets

Possible installationTray connected with long ground wire to low point of both cabinets

Poor installationTray connected once with a single ground wire to one cabinet

Schneider Electric Results

Test Conclusions

• Single ground connection – Tray connected to one cabinet only – worse than no metal tray at all – creates a disturbance

• Continous tray installation – Tray from cabinet to cabinet– best option– not realistic for practical reasons

Test Conclusions

• Realistic installation – Sort ground wire between tray and all cabinets – clear improvement

• Critical Parameter – connection between cable trays and cabinets

• continuity of generalized shield

Other Conclusions

• Source and victim cables at bigger separation distance does not compensate for a badly connected cable tray. It is still contagious…

Good Engineering Practices

• IEC Standards– To use gasfree and vibration safe joinings of tray,

ensuring max 50mΩ– To mark tray joints with a bonding sign

• To connect metal cable trays to all cabinets with ground wire, preferrably at cable entrance

Good Engineering Practices

Place cables as close as possible to parallel earth conductor

Good Engineering Practices

Additional cables will enhance external shielding effect

Good Engineering Practice

With multi level cabling, ground wire connection between tray levels

Take Away

• Properly bonded and grounded = ”earth”

• Poorly bonded and grounded = ”antenna”

Thank You

• Claes Rosenberg – Defem

• Ed Cronin RCDD – Cronin Consulting

Thank You