Insultated Neutrals and Grounding bars 1

[thelaststroke]

Good day,

I'm reallly confused about this issue, hope you can help me solve this and leave my ignorance behind.

I work on a project with insulated neutrals on the small distribution panelboards that also include their respective ground bars.(the neutral is bonded to ground bar only in the main panel, next to the transformer)

Yesterday we had an issue with some equipment (lets say PC's) getting some circuits burned internally and, the main briefing says that it happended becasuse the supposed-to-be insulated neutral was actually in contact with the ground bar in that specific small panelboard.

So I ask, if the neutral and ground are bonded together in the "higher" part of the electrical system, why should joining them together in a "lower" part cause any problems?, dont they have the same potential (voltage)???.

Why would the equipment burn because of this?.

Hope you can help .
Sorry if this is too basic, but I am just really confused.

Thanks.

 

[Skogsgurra]

There are so many myths and misunderstandings about the TN-C and TN-S systems that a "polluted" TN-S system (one where N and PE are connected somewhere else than main panel) sometimes gets the blame when something else happens.

The system, as you describe it, should not cause damage to connected equipment unless there are other problems as well. Have you got a positive confirmation that the damage was due to the N to PE connection? Or is someone trying to hide what really happened?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

It boggles the mind that many data engineers have never understood the grounding systems in common use. Their approach is more witchcraft and dependency on handed down folklore than an understanding of the grounding systems they are misusing.
If a data person screws up the grounding, he may loose some bits of data.
If a power system person screws up the grounding, people may die. Power people are much more motivated to understand grounding than are data people.
The unbreakable rule is "ONLY ONE CONNECTION BETWEEN NEUTRAL AND GROUND"
Then there are the exceptions. Some legal, some not legal, some accidental.
The reasons for the rule have to do with issues such as protection, corrosion, and the ability of a ground system to be unused for years and still function properly in the event of a fault.
If a neutral is properly installed, it will have a low impedance and no connection of a ground system or anything else in parallel will raise that impedance.
The same holds true for the ground connection, it should be low impedance and any conductor in parallel will serve to reduce the impedance further.
You may well have a problem with your wiring, but a connection from neutral to ground in a sub panel in a properly installed system in good repair would tend to mitigate equipment problems rather than cause them.
Now, if a problem had developed with the neutral and rather than finding and fixing the problem, someone chose a "quick fix" of connecting the ground to the neutral in the sub panel, you could have problems. A line to ground fault on the opposite line could generate a considerable voltage rise on the neutral. On a single phase system this voltage rise could approach 200% of normal voltage.
Is this condition caused by the ground connection? I prefer to consider the cause as the original bad connection on the neutral that was remedied with an illegal connection to ground.
The proper solution is to fix the neutral connection. The data solution may be to drive more ground rods.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[thelaststroke]

Thank you all for your posts,

They've been really helpfull, however, I've been thinking about the hole thing and have not been able to find out why the equipment burned during this accidental neutral-ground bonding, as Bill said "a connection from neutral to ground in a sub panel in a properly installed system in good repair would tend to mitigate equipment problems rather than cause them".

The project I'm working on has a lot of particular issues, for starters:

it was two systems a 220V delta connection system and a 208/110V wye system. the 220V system doesn't use a neutral at all, and the 120V since its a wye connection it obviously does have a neutral, but..... they are both connected to independent ground buses that go to the same underground net... could that be a problem?...

And second, new UPS has been installed on the 120V system that by default includes bounded neutral and ground. If i attemp to do a bounding in the 120V main panelboard will that cause problems?

Hope to hear from you guys again.

Thanks