Bonding Two Grounding Electrode Systems - Telecommunications Application

[strap89]

I have never been able to get a solid answer on this...

Here's the scenario. You have a 480V service where you establish your grounding electrode system at an external busbar . You also have a TMGB (Telecommunications Main Grounding Busbar) in your main telecom room which is bonded to the service ground busbar. From the service equipment, you are feeding a 480:208Y/120 transformer which has an established separately derived grounding electrode system and feeds a panel in the main telecom room which powers all the IT equipment. BICSI says you bond the TMGB to this panel's enclosure or equipment ground bus.

Is this acceptable? Your equipment ground bus in the panel is derived from the separately derived system on the transformer secondary, but per BICSI you are also required to bond it to the TMGB which is part of the 480V grounding electrode system at the service. I guess in effect, the panel in the telecommunications room is bonded to two grounding systems which seems funky.

I suppose the neutral current will return back to the separately derived system at the transformer, but any ground fault will split between the two systems. Thoughts?

Thanks,

-Cliff

 

[ptonsparky]

The ground fault current will return to the transformer that feeds it. Yes, some current may flow on another systems grounding conductors just because they all should eventually be tied to the common GES. They are parallel paths.

 

[ron]

To add to Tom's response: All current, whether normal neutral return or ground fault current wants to go back to its source's neutral and takes all paths to get there.

 

[strap89]

To add to Tom's response: All current, whether normal neutral return or ground fault current wants to go back to its source's neutral and takes all paths to get there.

So there is no issue bonding this panel's EGB to the TMGB?

 

[ron]

So there is no issue bonding this panel's EGB to the TMGB?

yes

https://s3-us-west-1.amazonaws.com/...lecommunications-grounding-bonding-system.png

 

[msimms]

I have a similar setup as the one presented by strap89, but with the same dry-type XFMR feeding the panel located in the same communication room. Can you also route the GEC from the terminal bar in the XFMR and terminate at the same TMGB? Or are you required to terminate that GEC to another electrode (i.e - building steel)? Since all grounding electrodes are required to be bonded to the electrode system per 250.50 I'm assuming this is fine. However, I've only been able to locate images and applications (like the attached from ron) where the panelboard is shown bonded to a TGB or TMGB, but nothing in regards to the XFMR, even after reviewing TIA-607-C and other online materials to verify.

 

[jaggedben]

In NEC terms, there shall only be one 'grounding electrode system' in a building. Separately derived systems (transformers) must generally be grounded to that system. Using a telecommunications installation to make that grounding connection would not be allowed unless it also met NEC requirements. I'm not an expert on the telecommunications implications but from a powerline fault current and safety point of view, ground is ground.

 

[msimms]

Thanks jaggedben - I should clarify that in my case, the TMGB is connected back to an MGB in another room. This MGB then ties together all electrical system grounds, other GBs, building steel, concrete encased rebar, etc, before connecting to the exterior ground loop using 4/0AWG bare conductors. The MGB thus meets 250.30(A)(6) tying all grounding electrodes together at a single point and meets the minimum size of 1/4" thick, 2" wide (2014 NEC Handbook, exhibit 250.16). By tying the transformer GEC to the TMGB which is then connected to the MGB, I wasn't sure if this was an allowed termination point, or if the GEC is required to connect directly to a grounding electrode (i.e. - closest available building steel or MGB in the other room) in this case. Would you view the latter being required (GEC to steel/MGB), or would the TMGB still be a suitable location? (FYI - I also see that I misstated my reference before to 250.50 since the TMGB would not be a grounding electrode).

 

[jaggedben]

Thanks jaggedben - I should clarify that in my case, the TMGB is connected back to an MGB in another room. This MGB then ties together all electrical system grounds, other GBs, building steel, concrete encased rebar, etc, before connecting to the exterior ground loop using 4/0AWG bare conductors. The MGB thus meets 250.30(A)(6) tying all grounding electrodes together at a single point and meets the minimum size of 1/4" thick, 2" wide (2014 NEC Handbook, exhibit 250.16). By tying the transformer GEC to the TMGB which is then connected to the MGB, I wasn't sure if this was an allowed termination point, or if the GEC is required to connect directly to a grounding electrode (i.e. - closest available building steel or MGB in the other room) in this case. Would you view the latter being required (GEC to steel/MGB), or would the TMGB still be a suitable location? (FYI - I also see that I misstated my reference before to 250.50 since the TMGB would not be a grounding electrode).

I'm doubtful on running the GEC to the TMGB. To my knowledge the NEC only allows running an SDS GEC to a busbar for multiple SDSs. Otherwise it has to be a continuous wire (or another option, like building steel).

If the idea is that your bank of telcom or IT equipment should be grounded to the same place as its dedicated transformer, perhaps a better option is to bond the main TMGB connection to the SDS GEC in a way that leaves the latter unspliced (Kearney bolt?).

To repeat, this isn't my area of expertise. I'm a solar installer who's gotten extra familiar with all the NEC grounding requirements that have gotten applied (rather unnecessarily) to solar over the years.

 

[jaggedben]

See 250.64(C) and 250.30(A)(6).

 

[msimms]

thank you jaggedben. i appreciate the feedback. I'm going to bring this inquiry to the front of the forum to get some other eyes on this if possible to get some technical reasons why.