Consider all site parameters before recommending an IG system. But first, measure existing equipment grounding system's impedance with a ground impedance tester prior to making any changes to the grounding system.
Yes, specs often require an isolated grounding (IG) system when there's a concern for electrical noise on the equipment grounding system. But, don't think an IG system always works. There are many reasons why this system may be ineffective. Remember: Each site is unique. As such, the electromagnetic compatibility for each site (let alone each branch circuit) can change dramatically. For this reason, the variables that cause, prevent, and/or amplify electrical noise disturbances are equally dynamic. Let's look at some key items you should consider when installing an IG system.
Item 1: nonsensitive equipment. Do not use IG circuits to power copiers, laser printers, space heaters, electric fans, or clock/radios. If you do have nonsensitive equipment on an IG system at any point in the wiring configuration, you've compromised the system for all electronic equipment. Most people don't realize power conditioning devices, such as voltage regulators, ferroresonant transformers, and uninterruptible power system (UPS) units, create common-mode voltages on an IG system.
Item 2: distributed network systems. Do not use an IG system in a distributed network (datacom) system. If you make conductive connections (such as data cabling) between sensitive and nonsensitive equipment, then you create a path to the sensitive devices (via the cabling) for the currents creating common-mode voltage. As a result, you corrupt the IG system. A good example of this scenario is the connection between a computer terminal and laser printer.
Remember, with most cabling, interrupting the data shield at one or both ends will not interrupt the conductive connection between the IG and SG (solidly grounded) systems, because the "signal common" (attached to the grounding planes of the respective equipment) is still intact.
Item 3: IG conductor resonance. Parallel resonance occurs when the distributed capacitance of the grounding conductor equals its inductive reactance, due to a high-frequency event. With an IG system, the disturbance waveform will not be dissipated towards the main service entrance. Instead, it will be reflected back to the point of at which the disturbance was injected into the IG system. As a result, the parallel resonant condition can make the IG grounding conductor path to the main service appear as an open circuit. If the frequency of the disturbance is in the megahertz range, it may reflect back to the end-use equipment and circulate in the data cabling.
Two variables determine if an IG equipment grounding conductor will experience this condition: applied frequency of the disturbance and the length of the grounding circuit.
For example, assume you have a stranded, No. 6 AWG grounding conductor for a particular IG branch circuit. A 100-ft-long, No. 6 AWG conductor has a DC resistance of 50 milliohms, which is a relatively small amount. Exposing this same No. 6 conductor to high-frequency currents will sharply increase its impedance (total opposition to AC current flow). If the applied frequency of the disturbance were high enough, it may cause a parallel resonant condition.
Unfortunately, the frequency at which the circuit resonates may vary from 120 kHz to 10 MHz. Since the frequency of the disturbance is unpredictable, you should focus your attention on the length of the IG equipment grounding conductor.
Typically, the peak resonance occurs at quarter-wavelengths of the applied frequency. Applying this to the previous example of a No. 6 AWG grounding conductor, a 100-ft grounding conductor will experience a resonant condition every 25 ft. If the conductor for a branch circuit is a No. 12 AWG, the length at which the conductor will experience resonance is much shorter.
If you expose the IG equipment grounding conductor to EMI/RFI, you cannot assume it will easily dissipate the disturbance waveform to the neutral-ground bond at the main service entrance.
Taking this one step further, the IEEE Green Book recommends the length of any grounding conductor not exceed 1/20 of the disturbance wavelength. This is to effectively equalize the voltages at both ends. This ensures dissipation of the common-mode event to the main service entrance. The Green Book also provides an equation you can use to determine the length of grounding conductors, including cable shields. However, it's only applicable if you know the frequency of the disturbance.
Item 4: capacitive reactance. One of the recommended practices of the IEEE Emerald Book is to verify the IG conductor is contained in metallic conduit. The intent is to shield the IG conductor from radiated EMI/RFI disturbances. However, any high-frequency currents present on the metallic conduit can still be coupled to the IG conductor via the capacitive reactance between the conductor and conduit.
Capacitive reactance (measured in ohms) is inversely proportional to the applied frequency of the disturbance. You can understand this relationship by looking at the equation: XC = 1 Γ· 2 Γ pi Γ fC. Therefore, the higher the frequency of the disturbance on the conduit, the lower the capacitive reactance between the conduit and IG conductor. As a result, the disturbance passes easily between the conduit and the IG grounding conductor. Capacitive reactance between circuit conductors (hot and neutral) and ground can occur just as easily. This scenario is likely where the hot and neutral conductors, contained in the same conduit as the IG conductor, are exposed to spurious (impulses, etc.) or steady-state (harmonics, etc.) transient events.
What's the solution? Measure the equipment grounding conductor impedance of the existing branch circuits. If you've also used conduit as the equipment grounding conductor for a branch circuit, then the impedance of the circuit should be lower than if you used an IG system. According to the IEEE Emerald Book, the maximum allowable impedance for an IG system is 0.25 ohms. Lowering the impedance of the IG even more will further reduce the possibility that a parallel resonant condition will develop if the circuit is exposed to high-frequency disturbances.
Use an isolation transformer on branch circuits where your customer is concerned about common-mode voltages. By virtue of a neutral-ground bond on the secondary side, a plug-in isolation transformer will equalize voltages between the neutral and ground. You can purchase isolation transformers in various sizes and with faraday shielding to keep unwanted high-frequency disturbances from passing between the primary and secondary.