Don't Miss the Drain Wire

Author: Brian Rhodes, Published on Jun 05, 2013

An easy-to-miss cabling specification plays a vital role in access control, yet it is commonly ignored. The drain wire offers protection for readers and controllers, ensuring proper cable performance. In this note, we look at the drain wire, the role it plays, and why it should not be ignored.

Why Is It Needed?

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What ** **?

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Which *** ********?

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Comments (9)

Unless specifically called for otherwise, I always ground the drain wire at only one end: typically the headend. Grounding both ends on a long run is a recipe for ground loops and it's typically easier to find a suitable grounding point where everything comes together than in the field.

What Carl said +1.

This is also very common with addressable fire alarm systems.

We use UTP with additional ground wire for POE door locks. This lets the static electricity from users to go to ground, and not fry the internal locks. It is "required" by the manufacturer (though it may work for a while without it).

You have to seriously question any manufacturer that suggests landing the drain to earth ground at both ends. That is absolutely wrong. One source for a ground at the panel or controller end. Period. Be sure to leave the drain at the field side but do insulate it with tape In that very rare case where you may need to land the ground at the door side and lift it from the panel side. Difference in ground potential or ground loops are to be avoided at all costs.

Hello, Michael:

I agree! Grounding the drain at both ends = most likely two different grounds, which means you're not 'grounded' at all!

In my experience troubleshooting ground loops, many times it is difficult to establish root cause. Like you point out, ensuring that fundamentals are followed go a long way in preventing problems.

Brian,

In most cases, technical support is finished with you and your attempt to troubleshoot until you fix the fundamentals.

Connecting a shield at both ends creates a bond, the exact opposite of shielding. Shielding should be connected at the source end of current allowing dissipation as needed throughout the length of the cable and any stray current noise to be carried out to the most sufficient ground point. You can see/hear the difference a shield makes when it comes to speakers, connect at both end and you'll Have noise on your speakers when you don't want it. A shield acts like an antenna picking up and carrying stray current away from the main conductors. Connecting it at an ungrounded device ends puts the current on the device, connecting it on both ends holds the current in the conductor as its now a signal path, plus introduces new noise from other devices grounded to the same bonded grounding conductor. Now I have seen very few devices which have an isolated terminating position on the connecting block to act simply as a holding spot for the shield. But I still like to wrap the shield around the end of the cable to allow a greater exposed area for picking up stray current, as long as I'm not allowing the conductor to come in contact with metal of which could act as a bond.

I think that the "inconsistencies" from the different manufacturers are not so much real inconsistencies

Within the same enclosure, will the same devices powered by the source, the drain doesn't really have an impact between the devices in that enclosures.

Where it does make a difference is when the linked devices are spread across different enclosures or areas and in that case although my preference is for the source device at the beginning of the serial loop, the ground can be connected to any devices as long as it is connected to only one end. However the drain should follow the rest the cabling even if it is not connected to the device connector.

Why do you call it a "drain" wire. It is connected to ground - it is ground.

Beware when working with ground. "Ground" level voltage is not the same everywhere, you can end up with a few to hundreds of volts of potential difference between locations. Ground only at the source per the NEC. If you connect the ground at both ends and there is a difference, then you now have voltage on your ground, which can lead to current, and then problems.

Also, don't connect the ground wire at one end, and use local power at the other - this too can create a ground voltage difference.

Call it what it is, ground, and you'll have less confusion about what it is and what it does.

Michael Peele, EE, PE

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