what really is an ungrounded system?

[unsaint34]

Q 1) What is the purpose of ungrounded system. Is it not to trip the circuit breaker when the first fault occurs? (for more controlled deactivation?)

Q 2) In a grounded system, having one conductor connected to earth provides the reference for steady voltages supplied to loads. How is stable voltage accomplished in an ungrounded system?

Q 3) Actually, there is no such thing as an ungrounded system, right? (Because an ungrounded system is capacitively grounded.. whatever that means)


Thanks for your answers

Sung

 

[bphgravity]

Re: what really is an ungrounded system?

A1: This is probably the primary reason to have an ungrounded system. The first fault is really not a fault at all. The setup essentially becomes a corner-grounded system.

A2: It isn't. That's one of the major drawbacks. Transient overvoltages, switching surges, and issues like lightning cannot be controlled.


A3: Well sort-of. Capacitive grounds or leakage capacitance always exists when insulated materials separate conductive bodies. I guess in theory, a truley ungrounded system can only be created in a laboratory environment, however for the purposes of applying the code, if you don't intentional ground the system, it is not a grounded system.

 

[kiloamp7]

Re: what really is an ungrounded system?

Well put, bph. You are right on the money.

 

[dereckbc]

Re: what really is an ungrounded system?

A1. Primary reason is for applications that cannot tolerate an outage from a L-G fault like process control like glass extrusion. Where an interuption would cause catastophic down time, repairs or threat to life.

A2. It shorts out the capacitive coupling, thereby eliminates the steady-state leakage current current that would flow through the conductor insulation, speeding up deterioration thus decreasing useful life.

A3. I disagree, no lab needed, just aiplanes, boats, trains, automobiles, flashlights, cellphones, any battery powewred device, etc.

[ February 28, 2005, 10:47 PM: Message edited by: dereckbc ]

 

[charlie tuna]

Re: what really is an ungrounded system?

i was on a nuke submarine and they have ungrounded systems---we would monitor voltage readings to ground to determine when there was a problem - like a stove element going to ground, then we would find the ground by isolating circuits until the voltage to ground returned to full voltage. then use ohm meters or meggars to test the device prior to re-energiging. it would be dangerous to have two grounds from different phases at the same time, so we always hunted the first ground down immeadiately!!

 

[charlie]

Re: what really is an ungrounded system?

Charlie, I have the utmost respect for anyone who is or has been in the submarine service. It take a special type of person to handle the pressure of the environment. Thank you for your service.

 

[powerbillnky]

Re: what really is an ungrounded system?

Just a sidebar to ungrounded systems. When working at facilities that utilize an ungrounded system, extreme caution must be exercised when working on any part of the electrical system. Ensuring proper BONDING is absolutely critical. Should a fault in one phase develop in one part of the system, then a fault develops in a different phase somewhere else, there would/could be line potential between points not adequately bonded. Caution is also necessary when simply disconnecting bonds between equipment.

 

[charlie b]

Re: what really is an ungrounded system?

Originally posted by unsaint34: Q 1) Q 3) Actually, there is no such thing as an ungrounded system, right? (Because an ungrounded system is capacitively grounded.. whatever that means)

I would like to echo Dereck?s disagreement with this statement. But let me add that if the system is not intentionally connected to planet Earth (or to the ship?s hull, as another example), then the fact that there is capacitive coupling between the system and the planet does not cause the system to become classified as ?grounded.?

 

[poweringtech]

Re: what really is an ungrounded system?

Ungrounded system are used for service continuity because during a first ground fault there will still be continuity of power.

Line-to-line voltage will still be the same during the first ground fault. As compared with a solidly grounded system where circuit protection will automatically remove the faulted circuit regardless of the type of fault.

To have an advantage of both ungrounded and solidly grounded system, Most industries are using High resistance grounding.

Ground fault are limited to as low as 5amps thereby having enough time to locate and repair the fault without shutdown. (See IPC Resistors)

 

[poweringtech]

Re: what really is an ungrounded system?

There are many advantages of a High Resistance Grounding. Among them are:

1. Reduce burning and melting effects in faulted electric equipment
2. Reduce mechanical stress in circuits and apparatus carrying fault currents
3. Reduce electric shock hazards to personnel caused by stray ground fault currents in the ground return paths
4. Reduce arc blast or flash hazard to pesonnel wo may have accidentally caused or who happen to be in close proximity to the ground fault
5 Reduce the momentary line voltage dip occasioned by the occurence and clearing of a ground fault
6. Secure control of transient overvoltages while at the same time avoiding the shutdown of a faulty circuit on the occurence of the first ground fault
For more information please contact
Sergio Panetta [spanetta@ipc-resistors.com]

 

[poweringtech]

Re: what really is an ungrounded system?

The main problem in using an ungrounded sytem is during an arcing ground fault. Voltage may increase up to 6 to 8 time rated voltage and may cause a second ground fault that will be dangerous.