grounded

[domnic]

WHY do we use a grounded system for power?

 

[GoldDigger]

WHY do we use a grounded system for power?

Several reasons; among them are:

1. To provide a return path for static and lightning induced currents to keep them from damaging equipment.
2. To insure that the voltage relative to ground on any conductors will not exceed the nominal line to line value as a result of charge pumping or other phenomena. Otherwise the insulation in loads and devices might be subjected to voltage above their design limits.
3. To provide a warning (in the form of an overcurrent trip) when a single fault occurs between line circuits and earth ground or incidental metal.

Now some of these concerns are addressed just as well by high or low impedance ground connections rather than the simple solidly bonded ground connection that we use for the most part.

 

[domnic]

Grounding

Grounding

What is charge pumping?

 

[domnic]

grounder

grounder

In a ungrounded system like a delta 3 phase what must I do to get shocked ?

 

[don_resqcapt19]

In a ungrounded system like a delta 3 phase what must I do to get shocked ?

Exactly the same thing as you would do with a grounded system. Unless the system is very small, there is a shock hazard between the circuit conductors of an ungrounded system and the earth.

 

[electrofelon]

WHY do we use a grounded system for power?

In case of accidental contact with higher voltage lines (primary falls on service drop). IMO, there is a fair amount of voodoo with system grounding; the purported benefits are not commensurate with the actual benefits, and the benefits of ungrounded system are typically ignored.

 

[Sahib]

Even in case of ungrounded system, insulation failures would occur in it in case of accidental contact with higher voltage lines (primary falls on service drop).

 

[GoldDigger]

But only because the higher voltage system is grounded!

Tapatalk!

 

[Sahib]

Even that is not required because in such ungrounded systems capacitive coupling to the ground exists, for example, through the cable insulation of cables lying on ground. So when a MV voltage line touches a low voltage line, such capacitor dielectric i.e the cable insulation is liable to breakdown.

 

[GoldDigger]

So you have an uncontrolled high impedance ground.

Tapatalk!

 

[Sahib]

Of course high resistance is connected between neutral and ground to stabilise line to ground potential in ungrounded systems. But still the dielectric breakdown of cables on ground of LV system is liable to take place when there is a contact of MV line on LV.

 

[electrofelon]

Even in case of ungrounded system, insulation failures would occur in it in case of accidental contact with higher voltage lines (primary falls on service drop).

Well the main reason a we would want a grounded residential system in this case is so that a utility ocpd will clear quickly. If the utility system is ungrounded, I am not sure if capacitive coupling gets strong enough to trip their ocpd if there is a ground fault?

 

[Sahib]

Well the main reason a we would want a grounded residential system in this case is so that a utility ocpd will clear quickly. If the utility system is ungrounded, I am not sure if capacitive coupling gets strong enough to trip their ocpd if there is a ground fault?

Sure, capacitive coupling is not strong enough to trip ocpd if there is a ground fault. But still the metal parts of the ungrounded system is grounded with a ground wire not an EGC.

 

[ActionDave]

In case of accidental contact with higher voltage lines (primary falls on service drop). IMO, there is a fair amount of voodoo with system grounding; the purported benefits are not commensurate with the actual benefits, and the benefits of ungrounded system are typically ignored.

There was another thread going recently that had a post about the power system in Europe. In Norway they run ungrounded and based on what I read running ungrounded has it's problems as well.

 

[electrofelon]

Sure, capacitive coupling is not strong enough to trip ocpd if there is a ground fault. But still the metal parts of the ungrounded system is grounded with a ground wire not an EGC.

In the above statement are you referring to the utility side of things? Here in the states for premise wiring we would still call it an EGC even if its in an ungrounded system. In a grounded system, an equipment grounding conductor is serving two purposes: it is providing the low impedence fault path back to the source and it is connecting equipment to the earth which is also required by the NEC. Regarding what I said about the high voltage line falling on a service drop, well its actually a long shot - dont blame me, the NEC says it. The thinking is that if the messenger/neutral is not grounded then the 4160V or whatever would energize parts of the premise wiring system and not necessarily clear the fault. In reality, I have never seen an ungrounded system that used a bare messenger as one of the ungrounded conductors - the messenger terminates at the attachment point and all the phase conductors are insulated. Sure the insulation on the service drop isnt rated for 4160 so who knows what would actually happen. I guess the thinking is the 4160 will pierce the insulation and wreak havoc with the premise electrical system where if it was grounded, the bare messenger would conduct the fault to ground thru the premise grounding electrode system. But for this to work they seem to assume the primary system is grounded which is often not the case.

 

[electrofelon]

There was another thread going recently that had a post about the power system in Europe. In Norway they run ungrounded and based on what I read running ungrounded has it's problems as well.

I would like to know what those problems are and why grounded systems seem to be preferred. I really have never heard any concise explanation. Maybe there is a utility EE on here that can give some specific documented reasons and examples.

 

[Sahib]

In the above statement are you referring to the utility side of things? Here in the states for premise wiring we would still call it an EGC even if its in an ungrounded system. In a grounded system, an equipment grounding conductor is serving two purposes: it is providing the low impedence fault path back to the source and it is connecting equipment to the earth which is also required by the NEC. Regarding what I said about the high voltage line falling on a service drop, well its actually a long shot - dont blame me, the NEC says it. The thinking is that if the messenger/neutral is not grounded then the 4160V or whatever would energize parts of the premise wiring system and not necessarily clear the fault. In reality, I have never seen an ungrounded system that used a bare messenger as one of the ungrounded conductors - the messenger terminates at the attachment point and all the phase conductors are insulated. Sure the insulation on the service drop isnt rated for 4160 so who knows what would actually happen. I guess the thinking is the 4160 will pierce the insulation and wreak havoc with the premise electrical system where if it was grounded, the bare messenger would conduct the fault to ground thru the premise grounding electrode system. But for this to work they seem to assume the primary system is grounded which is often not the case.

When we call an EGC an EGC? When it is connected with neutral at the service equipment. But in an ungrounded system the ground wire is not connected to the neutral. Still it serves its safety purpose: if a MV line, say 4160V, itself ungrounded falls on the exposed metal parts of an equipment so grounded, the resulting voltage on the equipment may not be dangerous to life.

 

[don_resqcapt19]

When we call an EGC an EGC? When it is connected with neutral at the service equipment. But in an ungrounded system the ground wire is not connected to the neutral. ...

Under the rules of the NEC, it is an EGC no matter what type of system it is associated with.

In general, under the NEC, the only difference between the grounding and bonding requirements for grounded and ungrounded systems is the fact that a ungrounded systems do not have main or system bonding jumpers, and grounded systems do have main or system bonding jumpers.

 

[Julius Right]

According to IEEE 141 CH. 7.2 System grounding:
The common classifications of grounding found in industrial plant ac
power distribution systems are as follows:
a) Ungrounded
b) Resistance grounded
c) Reactance grounded
d) Solidly grounded
Solidly grounded systems, in most cases, are arranged so circuit protective
devices will remove a faulted circuit from the system regardless of the type of fault. Any
contact from phase to ground in the solidly grounded system thus results in instantaneous isolation
of the faulted circuit and the associated loads. The experience of many engineers has
been that greater service life of equipment can be obtained with grounded-neutral than with
ungrounded-neutral systems. Furthermore, a very high order of ground-fault protection for
rotating machinery may be acquired by a simple, inexpensive ground overcurrent relay. The
protective qualities of rotating machine differential protection can be enhanced by grounding
the power supply system.?

 

[ActionDave]

I would like to know what those problems are and why grounded systems seem to be preferred. I really have never heard any concise explanation. Maybe there is a utility EE on here that can give some specific documented reasons and examples.

Found the thread I mentioned - dude needed to hit "Enter" a few times, but if you click on reply with quote you can break it up and make it readable.

http://forums.mikeholt.com/showthread.php?t=160215&p=1553218#post1553218