Power quality experts cringe when they hear the words “isolated ground” because the consequences of a poorly designed or applied isolated ground system can be disastrous. Several facilities with isolated ground systems have power quality problems severe enough to destroy equipment, stop production, and pose danger.
Despite the problems associated with isolated ground systems, you might use one to improve noise reduction in power distribution systems for computers and distributed data networks. If you understand what an isolated ground is and how it works, you can safely and effectively implement one.
An isolated ground is not a grounding wire run to an unbonded electrode. Instead, it is an insulated grounding conductor terminated directly at the building equipment grounding conductor terminal. Sometimes the isolated ground conductor terminates at a derived power system ground terminal, such as a UPS.
Because this ground is not bonded to the receptacle mounting means, or yoke, the isolated ground won't pick up any noise at that point. Per the NEC, you still must ground the raceway system and the outlet box [Sec. 250-146(d)], but this doesn't mean you should bond your equipment ground at the raceway or outlet box. Instead, bond it directly to the main grounding point.
This sounds good in theory, and in most cases it seems to work. However, the bonding — inadvertent or not — of the isolated ground system to the common ground system through printer cables or other computer or network cables presents a problem. The resultant ground loop and circulating currents defeat the purpose of the isolated ground and compromise safety.
When you create a ground loop, you lose all the benefits of an isolated ground system. And circulating ground currents compromise the clean power from a separately derived power, such as a UPS, which is a direct violation of Sec. 250-30. Many people don't realize that voltage differences (steady-state and/or instantaneous) between the building's common ground and isolated ground systems drive unwanted circulating current through data cables.
Fig. 1 illustrates how an inadvertent bond between a common ground and an isolated ground system creates a path in the computer/printer cable for circulating current.
A ground loop compromises the safety of both people and equipment, and it violates Art. 90-1 of the Code. In the event of a fault or short-circuit to ground, ground fault current will likely disable the printer cable through which it flows. Fig. 2 depicts a ground fault situation where about 35A of current may flow through a printer cable for up to 1 min. This will destroy the cable, and it increases the likelihood of a fire.
Circulating ground current can cause network errors and downtime, so it's important to find it before it can cause problems. You can measure steady-state ground current that flows through printer cables by placing an inductive meter probe on the offending computer cable. On the other hand, you'll need an oscilloscope to detect transient current flow.
You can use an oscilloscope between the bonded ground bus and the insulated isolated-ground bus at the first panelboard to see the transient voltages that can drive these circulating ground currents. Field measurements typically reveal transient voltage driving current flow more frequently than steady-state current flow, which requires a second ground system defect such as improper bonding at the equipment grounding conductor terminal or an inadvertent ground/neutral bond within the system.
The author, who works as an engineer for an office furniture company, most often spots incorrect ground bonding in furniture power systems (Fig. 3). In these cases, “clean” computer receptacle circuits access the isolated ground (IGR). In our facility, we use IGRs, which are identified by an orange triangle on the receptacle face (Photo on page 48). Some IGR receptacles use other means of identification, but this was an early standard, and it is the most prevalent way of identifying an IGR. Printer circuits are often on “dirty” circuits and access the common ground — they plug into a receptacle with no orange triangle. Laser printers placed on the utility (non-IGR) circuit and computers placed on the “clean” IGR circuit create a situation with a potential for melted printer cables and circulating ground currents.
Although this problem sounds complex, it's easier to solve than you might think. Start by placing all computer components, with the exception of printers, on the same ground system. The cold-inrush current to the ink fuser or heater can cause laser printers to create a voltage surge, but you can isolate this electronic equipment surge by powering printers on separate branch circuits.
At this point, choose between isolated or regular ground for the entire computer system, but don't mix the two. This preferred power design ensures all computer system devices are on the same equipotential ground and get their power from the same source — UPS or utility power, but not both.
You can make Ohm's law work for you when you use separate branch circuits to isolate the voltage variations in current-carrying conductors. Variations in branch-circuit voltage are often due to power surges from loads, such as laser printers with high-current startup loads. Place these loads on branch circuits separate from those that power computers, which need more stable voltage.
On the other hand, don't let Ohm's law work against you by providing a path for circulating ground current. Use one source of supply, one ground system, and separate branch circuits for problem loads, such as laser printers. In addition, be careful not to let a fault current split between two ground paths inadvertently bonded with a printer cable. However, clean, safe grounding for computer equipment is as simple as using Ohm's law and following the NEC.
Lundgren is a sales engineer with Haworth, Inc., South Hava, Mich.