GEC to What? - Page 2 - Electrician Talk - Professional Electrical Contractors Forum

don_resqcapt19 · 04-09-2012, 08:36 PM

Quote:

Originally Posted by acrwc10

What type of work is being done that they have an ungrounded system? In general the only type of electrical system that should be ungrounded is one that a GREATER hazard would occur if the system shut down, like a crane or some other process that needs to be completed to be safe.

As long as the system is not one shown in 250.20, it becomes a design issue as to grounding or not grounding the system. In general I agree that most systems should be grounded systems, but the code does not always require the use of grounded systems.

brian john · 04-09-2012, 09:02 PM

Quote:

Originally Posted by acrwc10

What type of work is being done that they have an ungrounded system? In general the only type of electrical system that should be ungrounded is one that a GREATER hazard would occur if the system shut down, like a crane or some other process that needs to be completed to be safe.

I would prefer a grounded or resistance grounded, but a arcing ground fault on a 480 system can be durn devastating, in lieu of the same fault on an ungrounded system. Which would trigger an alarm and allow a safe shut down (IN THEROY)

acrwc10 · 04-09-2012, 09:52 PM

Quote:

Originally Posted by brian john

I would prefer a grounded or resistance grounded, but a arcing ground fault on a 480 system can be durn devastating, in lieu of the same fault on an ungrounded system. Which would trigger an alarm and allow a safe shut down (IN THEROY)

It's the "IN THEORY" part that I don't like either, it involves human partisipation and that is where almost all faults occur.

RIVETER · 04-09-2012, 10:14 PM

Quote:

Originally Posted by Cletis

Where do you land your GEC from water main to what? of an ungrounded system (say 3 phase delta. no high leg or corner ground) ????

To understand the "what"...and the "why" of a GEC is important. A GEC is not necessary for the system to work correctly as long as the expected voltages are the only ones present. Do you know that or is this a "Devil's Advocate" question?

acrwc10 · 04-10-2012, 11:36 AM

Quote:

Originally Posted by RIVETER

To understand the "what"...and the "why" of a GEC is important. A GEC is not necessary for the system to work correctly as long as the expected voltages are the only ones present. Do you know that or is this a "Devil's Advocate" question?

Well you may as well be the "Devil's Advocate", it is an election year.

For those who missed it, Hit the link that Don posted, it explains why the voltage can rise up to 6 times higher then system voltage, in a fault on an ungrounded system. The first time I heard that, my responce was "how".

Amish Electrician · 04-10-2012, 11:42 AM

When you have an ungrounded system, it is especially important that you bond the heck out of everything, all the way back to the main panel. You also need to have phase monitors.

With an ungrounded system, it is critical that there be a very low-impedence grounding network. This is so that the slightest short from a second leg will instantly trip a breaker. If the breaker doesn't trip, your line voltages will fluctuate all over the place, anywhere (on a 480 system) from 700 volts to 70. This will cause all manner of problems for the rest of the plant.

Likewise, anything with electronics in it - ballasts, frequency drives, induction equipment - will put all sorts of transients on the grounding network.

Case in point: plants with ungrounded systems will usually go to the extreme of cad-welding 4/0 copper jumpers to the structural iron at every other column. It's that important.

don_resqcapt19 · 04-10-2012, 01:21 PM

Quote:

Originally Posted by Amish Electrician

...
Case in point: plants with ungrounded systems will usually go to the extreme of cad-welding 4/0 copper jumpers to the structural iron at every other column. It's that important.

That is a common spec for industrial plants with both grounded and ungrounded distribution systems and it doesn't really accomplish much in either case.

don_resqcapt19 · 04-10-2012, 01:25 PM

Quote:

Originally Posted by Amish Electrician

...With an ungrounded system, it is critical that there be a very low-impedence grounding network. This is so that the slightest short from a second leg will instantly trip a breaker. If the breaker doesn't trip, your line voltages will fluctuate all over the place, anywhere (on a 480 system) from 700 volts to 70. This will cause all manner of problems for the rest of the plant.
...

A "slight" short will not trip the breaker. The tripping of the breaker on the second fault will be a function of the breaker trip curve the same as with a grounded system. In both cases the EGCs are to provide a fault clearing path. They are important in both grounded and ungrounded systems.
Yes there can be an issue with "overvoltages" when you have a ungrounded system, but these overvoltages typically occur with a single re-striking ground fault...not with ground faults on two phases.