Transcript EMC and cable trays - what is the ”good engineering practice?”

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 Tray
Quick 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
Principles of Generalized Shield
Equipment
Metal structure
Cables Differential mode
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 installation
Tray directly connected to top of cabinets
Floor is Ground Reference Plane.
Realistic installation
Tray connected with short ground wire to top of
both cabinets
Possible installation
Tray connected with long ground wire to low point
of both cabinets
Poor installation
Tray 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