Low voltage systems are often ungrounded, which is normal, but non-current-carrying metal parts of equipment associated with low voltage systems are generally required to be grounded if the supply system is grounded. This article reviews NEC requirements for grounding low voltage systems. We will also visit NEC provisions that do not permit these systems to be grounded. Before I get into grounding low voltage systems, it would be helpful to differentiate between system grounding and equipment grounding.

When a system is grounded, one conductor of the supply system is intentionally connected to ground (the earth). This establishes a reference to earth for the other conductors supplied by the system. When equipment is grounded, it is connected to earth or to some conductive body that extends the ground connection. The process of equipment grounding results in equipment being placed at or close to the same potential (voltage) as the earth. A conductive body that extends the ground connection is often the equipment grounding conductor or it could be another grounding conductor.

NEC 2008, Article 100 provides the following definitions:

• The equipment grounding conductor is “The conductive path installed to connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.”

• “Grounded Conductor. A system or circuit conductor that is intentionally grounded.”

• “Grounding Electrode Conductor. A conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.”

• “Grounding Electrode. A conducting object through which a direct connection to earth is established.”

There is a lot of detail in the Code concerning grounding because it is a very important subject, which if not done correctly can cause problems, for low voltage as well as higher voltage circuits.

Let’s look at the NEC rules for grounding low voltage systems. I will use 50 volts as a threshold when referring to low voltage, because that is the level used in the Code. The rules in the NEC dealing with grounding systems less than 50 volts are provided in Section 250.20(A). There are three conditions for which these systems must be grounded:

1. Systems less than 50 volts have to be grounded, where supplied by a transformer that is supplied (on the primary side) by greater than 150 volts.

2. Systems less than 50 volts have to be grounded, where supplied by a transformer, if the transformer is supplied (on the primary side) by an ungrounded electrical system.

3. Systems less than 50 volts supplying conductors that are run outside as overhead conductors also have to be grounded.

The NEC also indicates that are some low voltage systems that are not permitted to be grounded. Those rules are found in Section 250.22(4) and (5). Section 250.22(4) provides a reference to 411.5, dealing with low voltage lighting systems. The secondary circuits supplied by transformers for these lighting systems are not permitted to be grounded. Examples of these systems include low voltage landscape lighting systems and systems used for area lighting within buildings. An example of another system that is not permitted to be grounded is an isolated power system such as is installed in many health care facilities.

What constitutes a grounded system? The answer is very simple, so let’s keep it simple. Grounded systems are those that include one conductor of the system that is intentionally grounded, whereas in an ungrounded system, there is no conductor supplied by the system that is intentionally grounded (connected to ground or earth). See Figures 1 and 2.

The definitions of the terms ground, grounded (grounding) are provided in Article 100 as follows:

“Ground. The earth.”

“Grounded (Grounding). Connected (connecting) to ground or to a conductive body that extends the ground connection.”

It is important to understand the definitions of terms used in NEC grounding rules. This is one of the basics of properly applying the Code to installations and systems in the field or in design. Words and terms defined in the NEC help users understand how the requirements should be applied. Code rules mean what they imply by definition.

Grounding equipment

Section 250.112 provides the grounding rules for specific fixed equipment. The equipment listed in 250.112(A) through (H) and (J) through (M) must be grounded (connected to an equipment grounding conductor, regardless of the voltage. Notice that list item (I) is not included. The requirements in (I) are directly related to whether the low voltage supply system is grounded or not. To simplify, if the equipment (conduit, boxes, devices, and so forth) is installed for a system that is ungrounded, then the equipment grounding requirements do not apply. Where the supply system is grounded, then all this associated equipment also must be grounded. A good example of this in common applications is where conduit sleeves and back boxes are used within interior walls and are for a communications circuit or a fire alarm circuit that is supplied from a power supply that has an ungrounded secondary (see Figure 5).

Summary

Low voltage systems are either grounded or ungrounded. When a low voltage system is grounded, one conductor of the system is intentionally connected to ground (earth). Equipment supplied by electrical systems of any voltage is generally required to be grounded unless the supply system operates at less than 50 volts, or where equipment is supplied by a low voltage system that is grounded in accordance with 250.112(I).

JOHNSTON, is NECA’s executive director of standards and safety. He is former director of education, codes and standards for IAEI; a member of the IBEW; and an active member of the NFPA Electrical Section, Education Section and the UL Electrical Council. Reach him at [email protected].