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Most machines are made of metal, the motors are also made of metal and generally a lot of the signal cables routed between the controller and motors are screened cables. This, unless correctly managed, can lead to serious noise issues in the sceened cable and instability in the control of one or all of the axes on the machine.

 

If current is allowed to flow in any of the screen conductor this can transfer noise onto the signal it is there to protect. There is an EU doodaa on the supply of product and requirements to screen it. This can cause problems on machines when there is no easy way to disconnect the screen, therefore ordering the part with the screen specified as disconnected is allowed and should be considered when specifying machine components.

 

Generally throughout industry the practice has been to terminate the screen to ground at one end of the conductor only, this normally being at the control box end. This ensures that the only current flowing is capacitively coupled and the signal being protected is kept clean. If the screen is terminated at a motor, there is no guarantee that the motor itself is correctly earthed back to the main machine star ground. You also then have the potential for screens at different potentials at the control end and this is best avoided if possible.

 

A star topology should be used for ground distribution, this avoids loops and the electrical earth circuit can be clearly traced.

 

Quotation from a supplier when asked about disconnecting the screen on their product for us:

 

"The old problem raises its ugly head again! Our ‘friends’ in Brussels, in their infinite lack of wisdom, have decided that the responsibility for EMC rest firmly with the USER when integrating a system containing a number of ‘components’. Therefore, the statement shown on our attached PDF regarding a ‘UNI-POTENTIAL BONDING CONDUCTOR’, (an earth bond to you and me!), puts this final responsibility WITH the user. TOTALLY different from what I was taught during my ancient B.Eng! That being….ONLY terminate ONE END of the screen!

 

However, WE ARE SYMPATHETIC TO YOUR SITUATION, and are well used to such issues. We can therefore create a ‘Custom Number’ for such a modification. We KNOW what goes on in the REAL and NASTY REAL WORLD, having been dragged through it in our long and varied journey through the field of industrial automation."

 

We therefore ensure our complete systems are correctly wired and screened so that when installed on a machine there is no possibility of earth loops causing noise and control issues.

 

If you have any questions on this topic prior or during installation please do not hesitate to contact us.

 

Contact

TRM International Ltd.
86 Moss Road
Southport, PR8 4JQ, UK.

Tel: +44 (0) 1704 563777

sales@trminternational.com

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