12 Jun The Facts and Myths of Isolated Grounding – Part 1
It is not uncommon to step into a data center or communications room and spot an orange outlet mounted on the wall. Ask anyone what that is and you’ll generally get a response like ‘that’s our emergency power’ or ‘that’s our noise-free ground’. It is just as common, though, to get a shrug of the shoulders from that person when asked ‘why’ they have one. To this day, I find that many data center and communications personnel are unaware that the telecommunications standards frown upon their use…in fact, Building Industry Consulting Services International (BICSI) specifically instruct a communications distribution designer to stay away from such wiring schemes in their reference manuals.
Why is Isolated Grounding (IG) a concern? Where did it originate? And is there any truth to an IG system helping noise issues?
This two-part blog article addresses these questions, and others, so the reader may be educated about their use and, to some degree, be forewarned if they’re already employed on site. The first part of this article will focus on electrical noise and the theoretical need for IG, as well as the installation requirements and safety violations that could arise with the poor wiring methods of the IG system. The second part of the article will focus on the technical issues with IG and the quick and easy measurement to determine if IG is a problem for you on your site.
Where to Start?
An isolated grounding (IG) system is often specified where there is a concern regarding electrical noise on the equipment grounding system causing operational problems for electronic equipment. By incorporating an IG system as opposed to a solidly grounded (SG) system, one would be inclined to think that something had been done to fix potential problems. Unfortunately, even properly installed IG systems create significant operational problems, even if it is installed according to National Electrical Code requirements and Institute of Electrical and Electronics Engineers (IEEE) recommended practices. To understand how to install IG systems, and whether or not they really work, we need to understand the reasoning for specifying such a system. This requires a close look at the cause and effect of ‘electrical noise’…
What Is ‘Electrical Noise’?
Electrical noise is a general term used by the professional and layperson alike to describe an event that disruptions the operation of electronic equipment. The correct term for it, though, is common-mode voltage. Common-mode voltage is an unwanted signal that occurs between circuit conductors and ground that can mimic intended signals between devices, often at the wrong intervals. Specifically, the common-mode voltage between neutral and ground is of the utmost concern for power supply designers. This is because there is a lack of filtering between these two points within the equipment’s power supply. As a result, a disturbance that is generated on the ac side of the power supply (i.e., the ac grounding system) is common to the dc side (known as chassis ground), hence the term common-mode.
Sources of Common-Mode Voltage
Nearly all equipment, with the exception of incandescent lighting, is a source of common-mode voltage. Any device that contain motors will direct-couple common-mode currents to the equipment grounding conductor. These include vending machines, copiers, laser printers, refrigeration units, UPS’s, etc.
Studies have also shown that loose connections on the equipment grounding or neutral conductors that are subject to mechanical vibration may also cause mid-level electrical noise or could compound the problem where there is an existing common-mode voltage. Other sources can induce a voltage on the grounding circuit via electromagnetic interference (EMI) or radio frequency interference (RFI). Radio/TV antennas, motion detectors, two-way radios, cellular phones, pagers, and fluorescent lighting bring about these types of disturbances.
Regardless of the source of the disturbance, it is accepted by the (IEEE) Standard 1100 (Emerald Book) that any voltage greater than 1 volt between neutral and ground at the input to electronic equipment will likely cause equipment malfunction.
How Does it Affect Equipment?
Electronic equipment communicates both internally, and with other devices, through a digital pulse that is known as a bit or a string of bits known as bytes. A typical bit, shown in Drawing 1, resembles that of a square wave pulse though, realistically, the waveform is less linear than often depicted. The amplitude of this pulse varies with equipment design and application. Typically, transistor-transistor logic (TTL) and complementary metal oxide semiconductor (CMOS) logic operates at 5 volts. When the pulse is active, it is said to be at Logic 1, or high, state. When the pulse is not active, it is referred to as Logic 0, or a low state.
At the leading and trailing edge of the pulse are transition points. Here, for a short time, the pulse is neither at logic 1 or logic 0. If a random common-mode voltage occurs between the neutral and ac equipment ground at the same time that the transition points occur, it is possible that the intended signal could be reversed. As a result, there is one less bit in the stream of information travelling to circuitry within the device or to an external device. When this occurs, the internal circuitry will not function as a result of parity error to the bit structure, resulting in equipment malfunction.
In 1980, with the unveiling of their high-speed data processing products, a reputable equipment manufacturer began specifying a unique grounding design called an isolated ground. The intent was to require an insulated equipment grounding conductor to be run to the grounding terminal of orange IG receptacles, which are designed to separate the grounding terminal for cord-connected equipment from the mounting strap of the outlet itself. Thus, TWO equipment grounds are installed (and, therefore, to be maintained). The objective of the IG system was to extend the zero volt reference (created by the neutral-equipment ground bond at the electrical service entrance) to the neutral-ground input at the equipment location. The desired intent (and NEC-compliant) of the IG should look something like Drawing 2, below.
Over the next few years, other equipment manufacturers specified the same type of grounding system. Unfortunately, most equipment manufacturers were lax in the exact requirements of an IG installation. Some electricians had to rely on their own interpretation of how it should be installed. To make matters worse, the National Electrical Code and the IEEE did not have requirements for the installation of such a grounding system. As a result, many sites experienced serious safety and equipment performance issues due to the improper installation of the IG circuit. In fact, many of the systems still in use today are potential safety (electric shock and fire), lightning, and operational hazards.
The NFPA Gets Involved
In 1981, a company in Chicago, IL experienced considerable fire damage that was be directly attributed to a ground fault on an improperly installed IG system where the physical installation prevented it from allowing sufficient current to trip the breaker. A basic diagram of this type of improper wiring can be seen below, where a separately driven ground rod is used to reference the insulated equipment grounding bus in a sub-panel.
In 1984, responding to this and other incidents, the authors of the National Electrical Code provided installation requirements for an isolated equipment grounding conductor.
In the 2002 version of the NEC®, Section 250.146 (d) provides the installation requirements of the IG system, however it does not guide the installer as to whether or not its use is beneficial. In summary, the Code requires that an insulated equipment grounding conductor (minimum size per Table 250-122) be run with the circuit conductors from the equipment ground terminal at the receptacle to the equipment grounding terminal at the derived system or service. For safety purposes, it is important to note that the IG equipment grounding conductor cannot be run in its own conduit or run outside the branch circuit or feeder conduit. Furthermore, the IG equipment grounding conductor cannot be terminated to a lone ground rod.
For equipment performance purposes, the IEEE recommends that the IG conductor and the circuit conductors be contained in metallic conduit to protect against radiated EMI/RFI. They also recommend a separate hot, neutral, and equipment grounding conductor be provided for each individual branch circuit. However, this IEEE recommended wiring practice is viewed as cost-prohibitive by most electrical contractors and is rarely encountered in the field. But even if the required and recommended design practices are followed, one must ask….
Does an IG System Actually Work?
The short answer to this question is “NO”. And, not only does it not work, studies have shown it can introduce problems where you wouldn’t expect them. In some cases, IG systems have been known to make common-mode voltage problems worse…or, in extreme cases, create common-mode voltage where no should be!
There are many reasons why the IG system is ineffective. But, overall, consider the fact that each site is unique. As such, the electromagnetic compatibility (EMC) between devices, let alone each branch circuit, can change dramatically. For this reason, the variables that cause, prevent, and amplify electrical noise disturbances are equally dynamic. In our next installation, we’ll address the technical issues that prevent IG from helping us. And, if you’re already using it, we’ll discuss how can you test it to see if it’s causing a problem for you…all with a run-of-the-mill VOM?