Grounding

Down to Earth:
Ground Rods and Ground Electrodes
by Jude M. Russell / PowerLines
(Newington, CT)

TEAL Electronics’ customers appreciate the benefits of high quality grounding. Power subsystem units from TEAL have a lot going for them: filtering, transient suppression, small size, low impedance transformers, high efficiency, site prep-in-a-box. But by and large, the biggest reason our customers come to TEAL is to improve their equipment grounding – and a TEAL power subsystem is the #1 grounding solution in the industries that we serve.

So it is quite a surprise to find that many TEAL customers who have purchased and installed power subsystems do not always adopt “best practices” to ensure optimum grounding. And in almost every case – these best practices include a Ground Rod or Ground Electrode.

Required by Code

A ground electrode is, first of all, a legal requirement. In the United States, NEC 250.30 (2002 Edition) requires that a separately derived AC system (such as provided by a TEAL power subsystem) be connected to a ground electrode. Such an electrode may be an effectively grounded structural member of the structure, an effectively grounded metal water pipe within 5 ft. of the building entrance, or another type of electrode (e.g. – a ground rod) where structural steel or a water pipe ground are not available.

The ground electrode à is intended to supplement the incoming ground conductor , and works with the equipment ground conductor À to ensure that the equipment is properly grounded and bonded.

A Dirty Ground ?

We’d love to see this phrase disappear from the power quality lexicon! Dirty grounds imply that there is also a clean ground that is kept separate from the other facility grounds (presumably, the dirty ones). While this sort of approach (separating and isolating grounds) can be used on occasion, it more often results in an improperly insulated or isolated ground system, and a magnification of ground related problems, especially when external ground loops and connections cannot be controlled.

Use of a TEAL power subsystem ensures that the Neutral-Ground bond is physically close to the sensitive load. Use of a ground electrode ensures a good reference to the local ground or earth. Other techniques (insulated grounds, isolated data and communications ports, floating grounds, etc) are often unsafe, expensive to install, difficult to maintain, or all of these!

Physically Close

We see too many sites where a ground electrode conductor à is run to a ground rod or the main building ground that is hundreds of feet from the power subsystem. This is incorrect. The NEC (and good power quality practice) requires the ground electrode to be “...as near as practical to and preferably in the same area...” as the power subsystem where the neutral and ground bond is derived. Ground electrodes that are derived elsewhere can create ground loops and actually reduce the efficacy of the power subsystem grounding.

From a power quality standpoint, a local ground electrode means that the system grounding is referenced tightly to the local environment, ensuring that the inevitable ground loops that occur within the load are kept small and less likely to introduce noise into the system controls. If the ground electrode is more than 25’ from the power subsystem – you need to consider that your grounding is less than optimum.

Ground Rod – Least Preferred Option

It is a common mistake to want to derive a ground from the distant service entrance. Similarly, it is also very common to see a ground rod chosen as the ground electrode. Power quality experts understand that ground rods are often very poor earth references – with impedance to true earth in the 10’s or 100’s of ohms. Building Steel is almost always a better choice – especially for equipment installed within a large building, above grade level. Ground electrode conductors can be bonded to building steel using listed fittings so long as the connection location is clean and dry; welded fittings may be a better option in some cases.

In buildings without accessible steel structure, a ground rod or other electrode may be required. Keep in mind that the effectiveness of this type of ground is highly dependent upon soil conditions (moisture, soil characteristics) and that multiple ground rods, or higher performance ground electrodes (chemical based, ground rings, buried conductors, etc.) may be required to ensure a low impedance to earth.

Summary

Ensuring a well-grounded electrical system is not easy, but a TEAL power subsystem is the place to start. Once you have your power subsystem in place, you can optimize your equipment grounding by: