Do you find Earthing problematic? Are you getting Earth Loops?
Authored by Wayne Allen (Independent Consultant and Fluke Networks Alumni)
As a structured cabling contractor - Should you install shielded or unshielded fly/patch leads to a shielded permanent link?
The IEC 61935-1 ED:5 standard indicates that for Channel certification - Shielding must be contiguous through the complete channel, including the cable, jacks, and patch leads. But will that create an Earth loop? What is an Earth loop? And will an earth loop kill your network?
Earth Loops and How to avoid them
Earth loops (also known as ground loops) are created when there are two or more earth systems with an equipotential voltage difference. This voltage difference creates a current that can back feed into the active equipment through the shielding. This has been known to cause intermittent network failures and potential equipment damage or even personal injury.
With your shielded installations of data cabling, standards require the shield to be complete end to end for the channel, meaning that shielded patch cords are required at both ends. On a Fluke tester, if you select a Channel Test and use an unshielded patch cable at one end, the tester will fail the link based on the shield was not complete, end to end.
The shielding is supposed to form a Faraday cage around the twisted pairs. When in doubt, the best practice is to go back to what the standards have to say.
The best place to start is in IEC 61935-1 ED:5, take a look at Clause 5.3.3.2 for the correct wire map. The correct wire map is shown in Figure 17 of the clause, below is a representation of what is shown.
In the above diagram, Screen (shield) depicted by the “S” is shown as being a requirement of the correct pair mapping, therefore if the contractor is conducting Channel Testing, the screen MUST be complete end to end.
With that said, earth loops do come up and can be problematic. So problematic, that the only way a system will work is by using an unshielded cord at one end. This is caused by telecommunications earths, power system earths and ground bonding. If the earthing is problematic, earth loops can happen. There is a standard for earthing and measuring earth loop potentials. (My old alma mater, Fluke makes gear to do these measurements.) If telecommunications earths are done correctly end to end, you are talking milliohms between earth points, therefore there should not be earth loops created. Unfortunately, that is often not the case in the real world, especially in Industrial Ethernet situations.
If earth loops are evident, the general suggested guidance is to use unshielded cords at one end of the link. You must be consistent; the guidance has lent towards using an unshielded cord at the field equipment end of the link.
However, most (if not all) structured cabling warranties are based on permanent links rather than channels. As patch cords are not part of a permanent link warranty certification, it becomes an issue between the network owner, installer, OEM supplier, and consultant to determine what patch cords will be used on the project. Just remember, for a compliant channel, you need a compliant permanent link AND compliant patch cords.
The great news is that there are practices you can follow to avoid running into earth loop complications. When in doubt always go back to the standards to see what advice they give. Get your supplier involved, they know the best methods to use when installing their products.