If you’ve ever purchased Kris-Tech’s shielded tray cable wire, maybe you’ve noticed the thin, stranded, uninsulated wire running alongside the aluminum shielding.
That small wire is called a drain wire, and it has several important jobs that make it a valuable part of the cable’s construction. Its purpose is to help reduce the amount of electrical noise impacting the copper conductors inside the cabling while creating a ground to offload small electrical surges. A drain wire is made from tinned copper, allowing it to prevent interactions between the copper conductors and the aluminum shield from occurring.
It’s a lot of work for such a small wire, and it does its job well. But does it mean a drain wire could take the place of a standard ground wire in tray cable installations?
No – not by a long shot.
Sure, drain wires help direct small electrical surges away from the tray cable and serve as a ground, but its primary role is to provide a complete circuit for the shielding inside insulated electrical wiring. They don’t have the size or strength to replace a standard ground wire in an electrical installation.
To explain what the drain wire does, we have to take a step back and understand the science behind what’s going on.
If you stripped back the shielded cable’s insulation material and found the drain wire, you would see that it’s attached to the electrical cable shielding. The reason is that those two materials work together to create a Faraday cage.
The Faraday cage was invented by Michael Faraday in the 1830s and is meant to block static and non-static fields from interfering with things inside. It does this by redirecting those charges somewhere else. In the case of tray cable, the shielding and drain wire combine to protect the inner conductors from much of the electrical noise around them.
It seems complex, but we’re surrounded by modern-day Faraday cages and likely use a version of one every single day. Real-world examples include cars, airplanes, microwaves, MRI rooms, protective suits worn by high-voltage power line workers, and even elevators. In every single case, a protective area is created that prevents outside noise and rogue electricity from getting in and impacting the things inside them.
In tray cable, the drain wire’s bond with the shielding creates a one-way ground that moves electricity away from the electrical system, giving it a place to safely exit. If there isn’t a shield included, there won’t be a drain wire because there is nothing for it to connect to block external noise from reaching the internal wire or cable.
So, if the drain wire can move excess electrical surges from a tray cable, it could work as a ground wire, right?
Unfortunately, that isn’t the case. The drain wire is typically smaller than the other conductors in a cable, and it’s only grounded on one end. This is done to prevent current from flowing through the wire.
Ground wires are different because they’re meant to provide a safe and efficient way for excess electricity to reach the earth, which is why it’s sometimes called a grounding or earthing wire. In the case of a power surge, the ground wire provides an outlet that prevents the other conductors from overheating, creating arcs, or short-circuiting.
Pro tip: Do NOT confuse the ground wire with the white neutral wire found in most electrical systems. Neutral wires offer a return path for electrical currents and complete the electrical loop for appliances and other electrical systems.
Drain wire and ground wire are not the same and shouldn’t be treated as such. The roles of these wires are distinctly different, despite sharing a couple of similar qualities. Drain wire, as designed, helps maintain grounding for the shield to prevent outside noise and electricity from impacting the internal conductors.
On the other hand, ground wires offer voltage protection and carry the current back to earth to prevent overloading the circuits during a storm or other high-electricity event. If there were a problem and something short-circuited in an industrial or commercial building or home, the electricity wouldn’t start a fire. Instead, it would flow to earth.
To properly ground your electrical system, you’ll want to use a ground wire that’s at least the same size as your other conductors so it can handle any surges that may come through. With its smaller gauge and one-way grounding, the drain wire is simply not capable of doing the job.
So, next time someone asks if they can just use the drain wire to ground their system, show them this article and prevent them from making a costly and dangerous mistake.