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Author Topic: Shielded Cable  (Read 1893 times)

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Shielded Cable
« on: October 15, 2015, 08:03:31 PM »
Hi there, just a quick question regarding using shielded cable for homing switches, Do you connect  the outside shield (braided covering) to anything or just let it float, I am using a two core shielded cable.



Offline Tweakie.CNC

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Re: Shielded Cable
« Reply #1 on: October 16, 2015, 02:08:13 AM »
Hi Nick,

My recommendation is to just connect the shields to Earth (a star type earth tag) at the controller end only.

Re: Shielded Cable
« Reply #2 on: October 16, 2015, 03:29:26 PM »
You should connect it at the sensor end to the sensor body only if the sensor body is NOT connected to ground. If you happen to get a sensor where the shield is already connected to the sensor body and the sensor is body is connected to ground then insulate the shield in the control panel and do NOT connect it to ground.
Re: Shielded Cable
« Reply #3 on: October 16, 2015, 11:59:10 PM »
Thanks you Tweakie and Garylucas , the sensor is not grounded and I will ground it at the bob end, thanks again for the quick replies.

Re: Shielded Cable
« Reply #4 on: October 17, 2015, 04:22:28 AM »
Hi Naijin

The reason for not connecting both ends of the shield is that slight resistances in the earth path can cause circulating currents to flow in the earth usually at mains frequency. In Audio circles it is known as a hum loop and they are a menace. This is why In Pro audio we use what is called balanced or differential inputs which only respond to the difference between two signal wires and rejects such interference as it is the same on both wires. Some sensors can operate this way too. It is an especially useful technique for very low level signals and long cables but this should not be a problem in your application unless your CNC machine is absolutely massive.

Regards Mark
Re: Shielded Cable
« Reply #5 on: October 17, 2015, 08:00:41 AM »
On machinery the electrical noise actually comes from leakage from all powered devices to ground, not to mention the occasional enormous ground fault current when a motor or other device fails.  The National Electrical Code requirements are all about CREATING as many ground loops as possible!  This is so that no two points that a person could touch would ever have a potential difference large enough to do harm, even when a piece of equipment experiences a catastrophic failure. For some reason they don't seem to teach electronics engineers about this. So you often find them wandering around searching for the mythical 'Clean Ground' to cure noise problems.  You can always tell when someone doesn't really understand when they suggest you drive a ground rod next to your CNC!
Re: Shielded Cable
« Reply #6 on: October 17, 2015, 03:38:36 PM »
Hi Gary

Yes spot on, It's the same in the U.K. We have very strict requirements for earth bonding for very good reasons as you point out. I once had to drive an earth rod in whilst doing P.A. at a church because the electrical system was so shoddy there wasn't an effective earth. This not only cured all the P.A. noises but provided some increase in safety. We left it there afterwards. with the proviso that they get the electrics checked.