ECN Forum Forums Code Related Discussion NEC & other Code issues ISO-Grnd or ISO-circuit

Sparky,

Sorry it took so long to get back in touch with you on this thread - you've undoubtedly seen what's happening in other posts/forums

In order to keep the amount of info I'd like to throw in here, I'll spread my message across a few replies, to topics that arise.

Here's some basic stuff to get things started:

The usage of IG circuits can help a lot, or they can help create problems. The key point being the way things are installed and terminated.

The most crucial piece of equipment that would warrant an IG circuit in almost all installations would be the server. Work stations in stand alone buildings would most likely be fine without an IG circuit. When there are multi tenant buildings with various types of power consumption, the work stations could benefit from IG circuits.

There are a few methods for IG branch circuits that would be considered the "preferred" way to wire them. These are:

1: Dedicate only one IG ground conductor per circuit. Don't share the IG with any other circuits in a multiwire branch [1 IG per 20 amp circuit],

2: Dedicate the grounded conductor to only the LAN equipment on multiwire IG circuits [a 3 phase 4 wire multiwire branch would only run workstations, no convenience / G.P. receptacles or other types of loads connected to the multiwire circuit],

3: Dedicated circuits with Dedicated grounded conductors [2 wire circuit with IG conductor] for server equipment, or distribution frames,

4: Specialty equipment - consider using circuitry example #3 above.

If there is no way to get back to the grounding electrode for the system that supplies the LAN's power, IG circuits might be of no help - but might actually create problems [this depends on the level of noise and TVSS distributed on the grounding system].

Placing an IG bus in the sub panel that the IG circuits come from [the bus is Isolated like the grounded conductor's bus is]. Feed the IG bus with a subfeed ground that terminates to the grounding electrode system. It can be of minimum size [per largest circuit], or can be larger. We usually install a #4 cu IG subfeeder.

Don't connect the IG feeder to a stand-alone ground rod, or other means that are not bonded to the grounding electrode system. This not only has a poor ground fault connection, but also generates ground loops and noise. This is worse than no IG at all!

When bonding MDF racks and related equipment, bond it to the grounding electrode also. Bonding to a stand alone driven rod generates ground loops. Bonding to a water pipe at any distance away from where it might be bonded to the grounding electrode will generate noise.

For equipment connected to the IG circuits:

1: Supply at least 2 Dedicated IG circuits for server equipment. Each server and it's related / locally connected peripherals [routers, hubs / MAUs, external MODEMs, monitors] should be on the same circuit, unless there is a request for one peripheral to be on a separate circuit. The other circuit would be for future connected equipment. Verify load KVA on all connected items [most servers should be rated at 750 VA]. If a UPS is used with server equipment, it needs to be rated for the KVA load. If there is a local printer at the server / MDF / IDF, put it on a separate IG circuit from the server [this helps out on the voltage fluctuations].

2: For work stations, limit the maximum of work stations that have local laser printers to 3 per circuit, unless load KVA allows more, in that case 4 might be the Max. Keep all equipment per workstation on the same circuit. If using P.O.C. surge protection, use only for that workstation's equipment [this can be nearly impossible, as clients will plug things into any outlet available!]. Including a 4 plex convenience receptacle for G.P. use might help keep people from plugging unrelated equipment into the IG circuits.
Also, you might need to triple check / triple explain to the LAN equipment vendors that they should only use the IG circuits you have painstakingly labored to install [I have to do this over and over and over!!! and still find things plugged into the convenience receptacles].

3: Place LAN printers on Dedicated IG circuits.

4: Only connect LAN copier / printers to IG Dedicated IG circuits. If not connected to the LAN, no need to IG it's circuit.

5: If you do a lot of bank branch installations [like I do], give the ATM at least 2 Dedicated IG circuits.
Give the TCD machines a Dedicated IG circuit for each machine [unless equipment vendor agrees to allow up to 3 per circuit].
Give the TCD server a Dedicated IG circuit if you can [it doesn't normally need one since it is an inexpensive piece of equipment, however, if the location has excessive noise and/or poor power quality, the server might reboot at random, which can be a nuisance].
Install the ATM and TCD machine's circuitry as the LAN server's circuit would be - dedicated grounded conductor and dedicated IG conductor per 20 amp dedicated circuit, running only that piece of equipment.
If the CCTV system is one similar to the "Accu-Trak" system, consider dedicated IG circuit for it.
When connecting the static build up ground connection for the ATM and MDF room floors, bond that to either a driven rod or water line. It's purpose is to drain static [ESD] only.
Supply the Tel equipment controller with a dedicated IG circuit.
Supply the Telco a minimum #6 cu ground, bonded to the grounding electrode system, for their MPOE. Supply them an IG receptacle if the MPOE is at a remote location from the Tel equipment / MDF [not in the same closet].

When working with shielded network cables [coax, triax, 10 base-2, 10 base 5, IBM type 2, etc.], instead of UTP [CAT 5, etc.], be sure there is only one common point that they get ground bonded at. Otherwise there will be loop current flowing on the shields.


In a nutshell, the PC, or workstation needs to be grounded [solidly] not only to conform with the NEC, but also to drive any received RFI to ground level at the machine it's self. Since the power supply's secondary eventually is center tapped and connected to the frame and chassis inside the machine - and the center tap is used for the "Common" of the +5/0/-5 VDC power [this is for the logic gates], it helps to eliminate any interference that could cause data loss [parity checksums incorrect, oscillation of the local bus that's out of sync with the CPU/bus controller, etc.]
Poor power quality, being in the area of sustained sags and surges in the line voltage might cause the power supply to initiate a series of resets to the CPU [reboot]. The level and duration of this depends on how good the power supply is. Cheaper ones will reboot at the drop of a hat, better ones will accept quite a lot of "Line Trash" and voltage variations before resetting the CPU.
BTW and FYI: This is also the way the systems is restarted when the "reset" button is pressed. It applies a logic 0 to the CPU's power OK signal address, which causes the CPU to hold it's self in a reboot state until the signal becomes high again [logic 1].

Thought this would be interesting


If your client really wants IG circuits and is willing to pay for correctly installed IG systems, by all means go for it. Just make sure the person you are discussing this with understands what is really going on, not just walking around quoting "Buzz Words" to appear like they know what they're talking about.

Be a good consultant to your client. Checking here for some input is a great thing to do! Supplement this by contacting equipment suppliers, vendors and manufacturers for their input. That's where you will get the environment specs and load KVA specs needed to design your circuits.

An extra benefit to IG circuitry is the inherent TVSS suppression. This is one of the reasons that 2 wire circuits with dedicated IG conductors are preferred on certain loads, along with why a dedicated IG conductor is preferred per circuit on multiwires and the whole connected load scheme.


How's this for a "short" message??

Hope this sheds light on some basics.

Scott "S.E.T."

Thanks Scott;
I had a funny feeling about those "stand alone" ground rods. I can see how it would cause problems. I'm not sure where that came from, i'm going to say out of the trade, as we all are heavily programmed to bond all applicable together to the Earth's
core...
could you expand on the distant h2o pipe problem??