Isolated grounding triad

[joshtrevino]

I am designing a control room that will be powered by a 120/208v panel. One circuit will power a UPS and UPS distribution panel for computers and lights. The client has requested an isolated ground for the computers tied to a grounding triad. My plan is to specify the UPS panel with two bus bars (one tied to the common building ground and one tied to the grounding triad). I have two questions:

1. My manager says that he thinks the 2011 NEC requires for an "isolated ground" to be tied into the common ground. This does not make sense to me because any noise that you are trying to avoid with an isolated ground would bleed on to your isolated system. Can someone please comment on this with applicable code article?

2. On my branch circuits to the computers am I required to install two grounding wires (one for the conduit/case ground and one for the isolated receptacle ground)? Please comment with article reference.

 

[iwire]

First off forget isolated ground, that is a misnomer because it cannot truly be isolated. It would be better to call it a separate ground.

1. My manager says that he thinks the 2011 NEC requires for an "isolated ground" to be tied into the common ground. This does not make sense to me because any noise that you are trying to avoid with an isolated ground would bleed on to your isolated system. Can someone please comment on this with applicable code article?

Your manager is 100% correct not only is it required to tie the IG to the 'common' ground it would be dangerous not to do so.

2. On my branch circuits to the computers am I required to install two grounding wires (one for the conduit/case ground and one for the isolated receptacle ground)? Please comment with article reference.

You do not have to use a wire if you have metal conduit but you may.

Typically we would run a green wire as the standard equipment ground, this would connect to all the metal junction boxes etc. we would also run a green wire with yellow stripe. This green with yellow would be the IG and it would only connect to the grounding terminal on the IG outlet and would run back to where we chose to tie the IG to the common ground. The ideal place for this connection is where the neutral is bonded to ground, either at the service panel or at the transformer serving these circuits.

I will post the code sections in moment.

 

[iwire]

First there is the fact that the NEC does not allow the earth to be used as a fault clearing path which is what you would be trying to do if you did not tie the IG to the equipment grounding conductor at some point.

250.4 General Requirements for Grounding and Bond-ing. The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance re-quirements of this section.

(A) Grounded Systems.

(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive ma-terial likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

Then we have this.

250.146 Connecting Receptacle Grounding Terminal to Box. An equipment bonding jumper shall be used to con-nect the grounding terminal of a grounding-type receptacle to a grounded box unless grounded as in 250.146(A) through (D). The equipment bonding jumper shall be sized in accordance with Table 250.122 based on the rating of the overcurrent device protecting the circuit conductors.

(D) Isolated Receptacles. Where installed for the reduc-tion of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding ter-minal is purposely insulated from the receptacle mounting means shall be permitted. The receptacle grounding termi-nal shall be connected to an insulated equipment grounding conductor run with the circuit conductors. This equipment grounding conductor shall be permitted to pass through one or more panelboards without a connection to the panel-board grounding terminal bar as permitted in 408.40, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor ter-minal of the applicable derived system or service. Where installed in accordance with the provisions of this section, this equipment grounding conductor shall also be permitted to pass through boxes, wireways, or other enclosures with-out being connected to such enclosures

The wording has changed a bit over the years but it has never been safe or allowed to truly isolate the grounding conductor supplying equipment.

If you do what you propose and if the equipment was to have an internal ground fault the earth would present such a high resistance at 120 volts that the branch circuit over-current device would never trip.

Beyond all that ..... IGs are a total waste of time in most modern computer room. The data interconnections no longer use ground as a reference so that killed the need for IGs

But old habits die hard and a properly installed and bonded IG will do no harm.

 

[joshtrevino]

Thanks and well said. I appreciate the thorough explanation.