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#97245 - 02/07/06 11:39 AM Ground Indicating Lights  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Are you required to install ground indicating lights on an ungrounded system? Seems like it's optional and I can't seem to find anything in the code to indicate (no pun intended) that I need them.


George Little

2017 / 2014 NEC & Related Books and Study Guides

#97246 - 02/07/06 12:45 PM Re: Ground Indicating Lights  
Creighton  Offline
Member
Joined: Jan 2002
Posts: 59
Hayward, CA
George: Check out 250.21 in the 2005 NEC.
Creighton


#97247 - 02/07/06 01:07 PM Re: Ground Indicating Lights  
Dnkldorf  Offline
Member
Joined: Dec 2004
Posts: 1,064
nowhere usa
But it doesn't say ground indicating lights.
just detectors..

Are you refering to 250.21(3)(D)?

Dnk..


#97248 - 02/07/06 01:11 PM Re: Ground Indicating Lights  
Dave T  Offline
Member
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
George Little,
I would question if it is required of not because it does not make any sense not to.
If you draw a simple diagram of an ungrounded delta feeding 2 3p breaker2 that is supply power to motors in different part of a facility 100 feet apart. The motor winding in one of the motors fails and the 'A' phased contacts the frame of the motor which is grounded mechanically. Will there be a ground fault? No, because you have not provided a path for current to flow back to the source. Do you know that it has failed? Most likely not because what indications would you have?
Now, the other motor just happens to have a failing winding also except that it involves the 'B' phase. Now the 'A' and 'B' phase have contact with the ground through the metal frames of each motor. Does the current stop there? No. The current is going to seek a conductive path through the facility form one motor to the other, though anything that is conductive.
You never want that to happen so don't even question is GF indicating lights of any other approved system is optional. It doesn’t make sense not to.
BUT, GF indicating lights are useless unless there is a qualified person present to react to a GF indication. That's when he springs into action to identify and isolated the the grounded line be there is a catastrophic failure.
So, it doesn’t end with the lights but must follow through with personal who are qualified to recognize and respond to the event.
Dave


#97249 - 02/07/06 01:20 PM Re: Ground Indicating Lights  
winnie  Offline
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Joined: Sep 2003
Posts: 649
boston, ma
If I read this correctly, ground fault indicating _lights_ are not required, just some sort of ground fault detector.

IMHO ground fault indicating lights are the _simplest_ and least expensive form of ground fault detector that one could use.

But if a facility had a different sort of ground fault detector, then that would probably meet the requirements of the code.

Ungrounded systems are usually used for continuity of service. But what about an ungrounded system used for the purpose of reducing energy dissipated in a fault. If the breakers were arranged to detect and trip on a ground fault for an _ungrounded_ system, then IMHO this would meet the requirements of having a ground fault detector.

One could imagine a system that used coordinated ground fault detection so that the breaker nearest the fault would trip, at the same time never having large ground fault currents flowing.

-Jon


#97250 - 02/07/06 04:58 PM Re: Ground Indicating Lights  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Jon- I guess I don't follow your scenario. If you have a system that is not grounded then a single fault to ground will not trip the overcurrant device. Now I can buy into the option of having either an audible or visual indication of a fault. Could be a display on a computer screen and it wouldn't have to be a light. Please elaborate on the overcurrent protection option you referenced.


George Little

#97251 - 02/07/06 05:45 PM Re: Ground Indicating Lights  
Dave T  Offline
Member
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
It is also very important to never forget that it is with almost with all certainty that you can get enough current to flow through the ground form one point of contact to another to trip a breaker. The NEC requires that when a single ground rod is driven that there is 25ohms or less resistance. If one applies that to an ungrounded delta where 2 lines go to ground in different locations in a facility us 25 ohms as an example of the L-L resistance to determine if enougn current will flow to trip a breaker, not. The resistance may very likely be much higher.
Remember that GF indicators of any type are worthless unless somebody is notified and that person has the qualifications to locate the failure.


#97252 - 02/08/06 08:33 AM Re: Ground Indicating Lights  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
I understand that in an ungrounded system, a single ground fault would not cause an overcurrent condition that would trip a breaker.

But overcurrent is not the only thing that a breaker can be designed to detect. If a breaker were designed to detect and trip on a ground fault in an ungrounded system, then IMHO this would serve the purpose of ground fault detection.

I can think of several plausible ways to implement such breakers, though I don't know the precise details nor if anyone sells systems that would do this.

For example, something like a GFCI type breaker would detect the capacitive charging current flowing through the ground fault. A system using coordinated ground fault detecting breakers might be sufficient to detect and _trip_ on ground faults. Such a ground fault system would function with an impedance grounded electrical system, I don't know if it would function properly with an ungrounded system.

-Jon


#97253 - 02/08/06 09:52 AM Re: Ground Indicating Lights  
Ryan_J  Offline
Moderator
Joined: Aug 2003
Posts: 1,374
West Jordan, Utah, USA
But if it tripped on the first fault of an ungrounded system, the whole idea of an ungrounded system is destroyed. Not tripping on the first fault is one of the reasons you install an ungrounded system.


Ryan Jackson,
Salt Lake City

#97254 - 02/08/06 11:25 AM Re: Ground Indicating Lights  
Dave T  Offline
Member
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
winnie,
If one elects to use a GFCI, what device did you have in mind? Ungrounded deltas are commonly 480v and in industrial facilities.
Would that device be list for application in such a system at 480v and would it work?
The purpose of and ungrounded delta is the continuity of service. As an example a line that produces glass. If it's shut down with no warning what happens? With an ungrounded system, as soon as there is a ground indication it affords the opportunity to locate and isolate the failure or shut the process down in an orderly fashion.
Dave


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