Douglas Krantz - Technical Writer - Describing How It Works

Isn't Conduit Ground Good Enough?

By Douglas Krantz | Descriptions

In order to keep the listing on a panel, it must be installed according to how it is shown in the Listed Installation Manual

Isn't Conduit Ground Good Enough?


Isn't Conduit Ground Good Enough?


Greetings Douglas,

Question for you.

I'm having a conversation with a colleague about proper wiring of a power supply.

We were replacing an Altronix supply that's attached to the access control panel, and I noticed the ground terminal between the neutral and the load had no ground wire in it.

The installers wired the supply in a non-power limited fashion, meaning they only grounded the can. The circuit board still has the metal stand offs, and landed wires are the neutral and the hot wires.

Is it good practice to wire up the power supply in that fashion vs landing a ground in the terminal and attaching all ground to that?

Thank You, AH

Listed for Use

Part of the answer to your wiring practice question would fall under the category of "Listed".

I have had to deal with this exact problem before. In my case, the electrician installing the equipment, and using only EMT (Electric Metallic Tubing) conduit ground, said to me, "It has been installed according to the NEC (National Electrical Code, also known as NFPA 70), so it's legal. That's all that has to be done."

What you are describing, and what I had to deal with, is a piece of equipment that actually was not installed the way it was Listed by a Testing and Listing company, like UL, ULC, FM, CE, CCC, etc.

To get an idea of what UL is about, see: History of UL

The listing, here, includes the installation. Because it is not installed the way it is Listed, it is not Listed.

One method to find out how it was listed for its installation is to look in the Installation Manual that came in the same box as the power supply. In there, you can see exactly what wiring the Listing Company used for their test and their subsequent listing. They show a ground wire, and the ground wire is not EMT or conduit.

Another method to find out how it was listed is to contact the technical support team for Altronix. The technical support team is very careful to describe exactly what is required for the listing.

Call them. That's what they're there for.

Safety-Ground versus Signal-Ground

Safety-ground is a grounding system that protects the building from fire, and people from electrocution. That's as far as the electrician went.

EMT conduit, along with it's fittings, protects buildings from fire, and it protects people from electrocution. EMT conduit is safety-ground. However, because safety-ground is used to bleed off unwanted electrical current from many devices like HVAC and lighting, Ohm's Law says there's voltage produced in the safety-ground.

Signal Ground is an extra Green Insulated Wire grounding system. Even though Signal-Ground is connected to the ground in the main electrical entry, Signal-Ground is a separate grounding system from Safety-Ground.

To reduce signal interference, the NFPA 70 allows for this extra grounding system. That's what's needed for low-voltage signaling.

If the equipment isn't installed with its own Signal-Ground, the unwanted voltage produced in Safety-Ground will be induced into the signal lines and low voltage power lines (like the power lines from the Altronix power supply), and then interfere with the signals between the equipment.

Sometimes this induced interference voltage prevents the low-voltage equipment from being used at all, sometimes the interference causes problems only once in a while, sometimes the equipment isn't interfered with at all.

Unfortunately, no one can tell what kind of interference is going to be produced until after the initial installation.

Reason for the Codes

The Code isn't there to make legislators happy, the Code isn't there to keep lawyers employed, the Code isn't there so electrical inspectors can check off the correct boxes on their permits, the Code is there to properly enforce good electrical wiring practices.

Good electrical wiring practices don't include just fire and electrical safety, good electrical practices include getting the electrical power to where it's needed, without interfering with signal transfer.

Because the copper of the green insulated wire does not make contact with the EMT conduit, the green insulated wire does not pick up the interfering voltages that ride on the conduit.

That's why the electrical code allows for a separate grounding system; the electrical code allows the installation of a green insulated ground wire, connected to the main building ground, installed inside perfectly good EMT conduit.

Extra grounding blocks have been added to almost all data closets and data rooms for a good signal ground.

Look inside data rooms and computer rooms in commercial buildings. There, on the wall, is a thick copper plate. Attached to the copper plate is its own, very large, green insulated wire. The wire is connected to the building's main electrical ground. The purpose of the copper plate is to provide a stable ground point for the equipment in the data room.

Good Wiring Practice

Good wiring practice includes more that just making sure the AC power gets to the Altronix power supply safely; good wiring practice includes making sure that the safety-grounding of the AC power itself doesn't interfere with the subsequent power limited wiring from the power supply.



Douglas Krantz

facpdoug@gmail.com
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