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Re: Grounding of Neutral at Service Entrance

> Think about an ungrounded system where the neutral is allowed to 'float'. > How many volts exist between the neutral and earth? Now, what if there is a > 'leakage' (dirt, insulation breakdown, capacitive coupling, squirrel ) path > between the primary and secondary and the primary is 2400 Volts to ground? > Would you want the outlets in your home to be 2400 volts different potential > than the water pipes? What voltage would 'jump' from the hot to the ground > pin of a standard outlet? > > Hard grounding neutral on secondary side will 'carry away' any leakage > currents. If the transformer develops a significant fault, the grounded > secondary will carry enough fault current to prevent the home outlets rising > to full line voltage. > > daestrom

Hello. My original posting was about the advantages of connecting the neutral to earth at the service entrance especially given the fact that the neutral is already grounded back at the utility transformer. Although a previous poster commented that the NEC covering this does not talk about fault clearing I thought of a scenario where the entrance-grounded neutral should help:

A metal case appliance that is not poperly grounded (e.g. green wire not connected or outlet not grounded) develops a hot-to-case fault. The appliance (refrigerator, dishwasher, clothes washer, dryer, etc.) is physically connected to a water or gas pipe (metal not PVC) that at some point is in contact with the soil/earth. If the neutral is earthed at the service entrance then the principal path for fault current is from the hot conductor to the appliance case, through the metallic plumbing system, through the soil till it arrives at the earthed service-entrance neutral and then via the outside neutral back to the source (utility step-down transformer). I am assuming that where the gas/water pipes contact the soil outside they are much closer to the service entrance than they are to the distribution transformer's grounded neutral.

If the neutral was not earthed at the service entrance in this scenario then the fault current has to travel through the earth all the way back to the point where the neutral on the distribution transformer is grounded, thus inhibiting the function of the overcurrent protection device (fuse or circuit breaker) in the house. I think this is one, but not the only, reason why a grounded-neutral distribution system is earthed at regular intervals - to ensure reliable action of OPDs (e.g. the fuses on utility poles) under line-to-earth fault conditions. Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

4555 Overlook Avenue, SW Washington, DC 20375-5337
Reply to
J. B. Wood
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The problem is ground shift. When we were investigating LANs getting blown up in multi building networks we found transient differences of potential in the

10s of volts with a Dranitz power monitor in spite of the fact that all of these buildings were connected at the transformers. The intent of grounding at each building is to insure that the building infrastructure remains at "ground" as referenced at each service entrance, not necessarily to the utility. BTW the answer to our LAN problems was found in supplimental bonding with 4ga copper, run along with the data cables and generous use of ferrites. In this case we decided ground loops were not as big a problem as ground shift. A ground loop may toss in a few packet errors (although we didn't see them) but a ground shift blows the LAN card. As has been discussed several times, the ground electrode has nothing to do with clearing faults. That is the path between the equipment grounding conductor, through the main bonding jumper to the neutral and back to the transformer.
Reply to
Gfretwell

I agree but we were working with people on a budget and fiber was very expensive then. In the PC-PC world it was virtually "unobtainium". 4 ga copper bonding the frames of the machines was pretty cheap and it worked. The buildings were virtually touching each other but they were fed from different services. The electrical inspector in me says they should have bonded the grounding electrode sysems together but the "data" guy in me said to just string a bonding jumper along with the CAT5 and let the ground shift be resolved in the Equipment Grounding Conductor. I did advise them to bond the electrode systems but that was beyond my job responsibility at the time so I never checked to see if they did nor did I have a way to make them.

Reply to
Gfretwell

If this earthing was so difficult, then a lightning strike to any building in town would take out the telco's $multi-million switching computer. It does not happen. Such failures are not acceptable. But then the telco has one advantage. They build buildings so that an earth ground is installed very cheaply. It means earthing is installed when the hole is dug and concrete footings are first poured.

However it does not take a telco switching center ground to routinely earth a direct lightning strike without damage. I have demonstrated effective earthing multiple times with only an eight foot ground rod. That means most every building can be protected with earthing equivalent or more robust. However earthing gets difficult with problematic soil conditions such as FL sand or geology compromised by manmade installations such as nearby buried pipelines. IOW fiber often is not necessary if the human learns and uses simple techniques.

no spam wrote:

Reply to
w_tom

If this earthing was so difficult, then a lightning strike to any building in town would take out the telco's $multi-million switching computer. It does not happen. Such failures are not acceptable. But then the telco has one advantage. They build buildings so that an earth ground is installed very cheaply. It means earthing is installed when the hole is dug and concrete footings are first poured.

However it does not take a telco switching center ground to routinely earth a direct lightning strike without damage. I have demonstrated effective earthing multiple times with only an eight foot ground rod. That means many if not most buildings can be protected with earthing equivalent or more robust. However earthing gets difficult with problematic soil conditions such as FL sand or geology compromised by manmade installations such as nearby buried pipelines. IOW fiber often is not necessary if the human learns and uses simple techniques.

no spam wrote:

Reply to
w_tom

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