IG-style wiring is interesting because it's so unpredictable, from the standpoint of what it does and doesn't do. It may or may not be a good thing at any given site, and its performance often varies under particular local conditions at different parts of the same site. For example, it may reduce daily electrical noise problems for an item of equipment, but at the cost of greatly increased risk from lightning damage during electrical storms. On the other hand, sometimes using the IG-style wiring method may result in generally increased levels of electrical noise on a daily basis, an opposite condition than desired. At the same site, the effect may vary according to the length of the IG circuit or what kind of load is connected to the IG circuit. And more frequently than not, IG wiring appears to create many initial design errors and eventual physical wiring errors due to misinterpretation of its requirements.
Let's try to sort out the confusion and clarify the situation.
The real truths
The real truths about the IG-style wiring method are as follows. First, about 50% of the time, it reduces electrical noise on the circuit to which it is applied; about 50% of the time it increases the noise; and the remaining amount of the time, you can't tell the difference. As such, it's typically a gamble.
Second, whatever effect (good or bad) the IG-style wiring method has in any given situation is directly related to the length of the circuit that is configured as IG. Therefore, longer IG circuits have the greatest effect (good or bad), and the shortest ones have little or no effect at all.
So as an example, if you're paying extra to have IG-style receptacles installed directly into the back of your isolation transformer-based or similar type of power conditioner, you are nearly always gaining nothing at all. You may just increase your overall cost.
The above points and more will be detailed and clarified in this series of articles, so don't take them as unsupported statements.
What does "IG" mean?
Nobody seems to know what the notation "IG" means. This is no joke, as even the NEC pretty much ignores the whole IG terminology thing. So, let's get some sort of definition in place, one that will be consistently used throughout this series of articles and one that will make some sense. You can start getting familiar by reviewing Sec. 250-74, Connecting Receptacle Grounding Terminal to Box, and related Ex. 4, along with the FPN.
In this series, we will use the word "insulated," and not "isolated," for the "I." Thus, "IG" stands for Insulated Grounding. This is not done in an arbitrary or capricious fashion, since a real difference between the two terms can be made. Generally speaking, however, they really are interchangeable. Let's try to explain this distinction by looking at receptacle constructions.
SG and IG receptacle configurations
It seems that a simple explanation is best, so, let's start out at the receptacle. First, what does a solid grounding (SG) receptacle look like? As shown in Fig. 1, the equipment grounding pin and greenwire terminating screw are both made common to the metal device mounting yoke by the receptacle's manufacturer. As a result, mounting the SG receptacle into a metal device box or item of equipment makes the receptacle's equipment grounding pin common to the device mounting box or equipment enclosure and, therefore, to whatever else these items are connected to on the building's installed grounding system.
On the other hand, on an IG-style receptacle as shown in Fig. 2 (on page 14), the manufacturer ensures that insulation is introduced between the equipment ground wire pin and the metal device mounting yoke. This breaks the common connection described above for the SG design and, therefore, eliminates the solid-grounding connection thus provided. So we now have insulated, as opposed to solid grounding at the receptacle end of the branch circuit.
Next, on the IG design, an insulated greenwire is connected to the IG equipment ground pin's associated wiring terminal at one end, and at the upstream end, where it's directly and solidly terminated to equipment ground at the panelboard. Hence, the IG circuit is insulated from the panelboard downstream to the receptacle; due to the connection at the panelboard, however, it has not been isolated from equipment ground.
This brief explanation is useful and can be expanded to account for IG designs that are both directly connected to equipment at outlet level, and are upstream routed clear back to the service equipment before the direct and solid equipment grounding termination finally occurs.
However, if we used the term "isolated" in describing the IG design, there may be a tendency to think that the upstream end of the greenwire would be terminated to ground via an isolating impedance, such as a resistor, inductor, or even a capacitor. Or, that it was terminated to an earth grounding electrode that had no connection, except via the earth, to the site's service grounding electrode system. Also, when impedance is inserted in any of these manners into the equipment grounding conductor path, the NEC gets involved in several important ways, all of which are safety-related and ensure the impedance does not adversely limit fault current, create a fire or shock hazard, or both.
In short, "isolation" in the equipment ground path can become a can of worms, where "insulation" does not appear to do so.
What are IG wiring method limits?
Let's look at the limits to the IG wiring method.
IG terminates at the service equipment. The NEC presently limits the range of the premises wiring system into which the IG wiring method may be installed. This is shown in Fig. 3, where the IG wiring method is shown as being NEC restricted to that portion of the premises wiring system extending from the solidly grounded AC system that is a part of the service equipment to the outlet end of the branch circuit.
Importantly, the arrangement in Fig. 3 also logically requires that the voltage level at the service equipment be the same as that intended to appear at the outlet end of the branch circuit.
Intervening electrical distribution equipment such as pull boxes, fused switches and disconnects, switchboards, panelboards, equipment cabinets or enclosures, and similar equipment are all included in the permitted range of the IG wiring system.
Note that, in Fig. 3, the IG wiring method begins at the outlet end of the branch circuit, since this point is NEC-defined in Article 100 Definitions, as being the last part of the premises wiring system. In other words, it's the point at which the premises wiring system is finally interfaced to the branch circuit connected load equipment.
IG terminates at a separably derived AC system. A common variation of the distribution system shown in Fig. 3 is where a separately derived AC system is interposed between the service equipment and the upstream terminating end of the IG wiring method; this is shown in Fig. 4. Here, a separately derived system, such as an isolation transformer, is used to establish another solidly grounded AC system, which is then separately derived from the service equipment. Per Fig. 4, the NEC now requires that the output of the separately derived system, as opposed to the service equipment, becomes the furthest upstream point from which the IG wiring method is run downstream to the electronic load. The entire downstream path to the outlet end of the branch circuit remains unaffected by the introduction of the separately derived system, and it ends as before.
This method permits the use of a higher level of distribution voltage from the service equipment, since the isolation transformer may be of the step-down variety. Also, the isolation transformer may be positioned very closely to the load equipment served by the IG wiring method; this has significant advantages in many cases. More will be said about this later on in our discussion.
IG wiring path involves a solidly interconnected system. When the AC system supplying the IG wiring method consists of more than one AC source, such as when an uninterruptible power supply (UPS) is used along with its associated transfer/bypass switching, things get much more complex from an equipment grounding standpoint.
Arrangements of this type involve AC systems that are classed as being of the solidly interconnected AC system type, as opposed to being of the separately derived AC system type. A simple solidly interconnected arrangement is shown in Fig. 5 (on page 16), where the service equipment is the upstream AC power source actually supplying current to the UPS, its transfer/bypass switching arrangement and to the UPS' rectifer-charger input.
Note that in this arrangement discussed above, there is no AC system grounding shown at either the UPS or at its associated transfer/bypass switch. Instead, all of the AC system grounding occurs at the service equipment. This happens since the neutral ([X.sub.0]) conductor from the UPS' output is routed upstream to the service equipment, where it's solidly interconnected to the neutral bus bar in the service equipment. This conductor is routed within the UPS' input feeder, along with the line conductors supplying the rectifer-charger. This is what makes the two AC systems (service equipment and UPS inverter) solidly interconnected.
The IG circuit as a beginning, middle, and end. The IG-style wiring design has a highly defined beginning at the outlet end of the branch circuit, where the interface is made to the electronic load equipment in one of two specifically NEC-described ways. (Each will be discussed in some detail later on.)
The end of the IG-style circuit occurs at the extreme upstream end of the IG equipment grounding conductor, at the point where it's finally terminated by connecting it to an equipment ground bus bar.
Everything else in between these two points is the middle, where all that needs to be done is to keep the IG path insulated and correctly routed.
As you will see, all of these areas on the IG wiring path (beginning, middle, and end) are commonly miswired in a startling number of ways, some of which are relatively harmless from an electrical safety point of view, while others are very much unsafe.
IG-style circuit beginning
Let's start at the beginning of the IG-style circuit and discuss the interface to the electronic load equipment at the outlet end of the branch circuit.
IG-style receptacles. The first and most common form of IG-style wiring is where a specially designed and constructed receptacle is used, one called an "IG wiring device." These are always product safety listed by a Nationally Recognized (Electrical Safety) Testing Laboratory (NRTL) and are required to be clearly identified as being of the IG style.
The IG-style receptacle is presently identified by the presence of a permanently attached mark on the receptacle's face that is in the shape of a triangle, or delta [ILLUSTRATION FOR FIGURE 2 OMITTED]. Any color may be used for the face of the receptacle. In the past, the IG-style receptacle was identified by its orange-colored face. This was changed as above, so an orange-faced receptacle may or may not be of the IG style, depending upon when it was manufactured. This means that to avoid problems, you had best check any orange-faced receptacle that you encounter, or you may become redfaced because of what you apparently, missed.
Aside from the marking, what makes the receptacle IG? Simple, the manufacturer leaves out the connection between the equipment grounding pin/wiring terminal and the metal device mounting yoke and puts the letters "IG" into the part number somewhere. In review, an IG-style receptacle's electro-mechanical schematic is shown in Fig. 2, where the lack of a connection between the yoke and equipment grounding conductor pin/screw is clearly shown. The figure illustrates the common duplex type of receptacle of the generic NEMA 5-15 type for 120VAC circuits, but other kinds of NEMA receptacle wiring device configurations are available in IG form. An SG-style receptacle of the same type is shown in Fig. 1 for purposes of comparison.
In summary, what has been accomplished? Well, if you have an IG-style receptacle and if you mount it into a grounded metal device box, you do not connect the equipment grounding conductor to the box and thereby ground/bond it to whatever the box is connected to (such as the metal conduit/raceway system). You have an equipment grounding conductor pin, which is insulated from this part of the equipment grounding system, at the box. This is all shown in Fig. 2.
Now, the only equipment grounding that can occur must occur when the IG equipment grounding conductor is attached and then has its upstream end terminated to an NEC-acceptable equipment grounding termination, such as a bus bar (not shown) at some NEC-acceptable point in the wiring system.
Next month, we'll look more deeply into the IG situation, beginning with a description of how a direct connection style of IG branch circuit, as allowed in the exception to Sec. 250-75, Bonding Other Enclosures, is configured. This is a really interesting variation.
This is the first part of a multipart article on IG-style wiring methods. The first part will discuss the basics and some of the National Electrical Code issues; subsequent articles will move into progressively more technical areas. In some cases, useful information will be presented in the series about the IG-style wiring method that has not been published elsewhere. It's hoped that by the time you have finished the series, you will be more competent in the area of IG-style wiring methods.