grounded

[don_resqcapt19]

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.

Under the NEC, you use EGCs for both grounded and ungrounded systems. The terms and the installation do not change.

 

[mivey]

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.

GoldDigger said some of the same but here ya go:

Ungrounded systems are weakly coupled to ground through a capacitive reactance. Grounding helps mitigate the problem of high L-G voltages that cause insulation failures and personnel hazards. The effective LC circuit resonance can cause L-G voltages of at least twice normal. Re-striking arc faults can generate L-G voltages of up to six times normal.

So, in general, grounding the utility distribution:
Assists in proper device operation
Increases safety during normal and fault conditions
Helps stabilize voltage during transient conditions
Helps dissipate lightning

Examples of benefits:
Improved Surge protection
Higher relay current (lowers fault path impedance)
May allow a lower equipment BIL rating
May allow one conductor to serve as both primary and secondary neutral conductor.
May allow lower equipment costs (fewer bushings, fewer cutouts, reduced insulation, etc)
May reduce losses when earth is used as a parallel conductor.
Helps maintain neutral stability
Easier to detect ground faults

 

[electrofelon]

Thanks for the detailed response. Ill make some comments:

Ungrounded systems are weakly coupled to ground through a capacitive reactance. Grounding helps mitigate the problem of high L-G voltages that cause insulation failures and personnel hazards. The effective LC circuit resonance can cause L-G voltages of at least twice normal. Re-striking arc faults can generate L-G voltages of up to six times normal.

Intuitively, it seems that there is no free lunch with a grounded system in terms of insulation requirements: One conductor has "low" insulation requirement, and the others have "high" insulation requirements. With an ungrounded system they are all somewhere in between. I guess my electrical theory is not strong enough to understand how and the situation such that the voltage to ground in an ungrounded system can rise above phase to phase voltage.

So, in general, grounding the utility distribution:
Assists in proper device operation
Increases safety during normal and fault conditions
Helps stabilize voltage during transient conditions
Helps dissipate lightning

Ok, I hear this stuff all the time. How about some specific examples using numbers, ohms law, etc? If I started working as a lineman, at coffee time would they be complaining about all the maintenance and hassle of their ungrounded distribution?

Examples of benefits:
Improved Surge protection
Higher relay current (lowers fault path impedance)
May allow a lower equipment BIL rating
May allow one conductor to serve as both primary and secondary neutral conductor.
May allow lower equipment costs (fewer bushings, fewer cutouts, reduced insulation, etc)
May reduce losses when earth is used as a parallel conductor.
Helps maintain neutral stability
Easier to detect ground faults

Well I dont like the practice of tying primary and secondary neutrals together and using the earth as CCC. Im paying for my electrical service, I want it isolated . Also, again, would like specific "step by step" description of the improved surge protection idea with numbers....

ActionDave, thanks for the link, it is an interesting read.

 

[kwired]

Well I dont like the practice of tying primary and secondary neutrals together and using the earth as CCC. Im paying for my electrical service, I want it isolated . Also, again, would like specific "step by step" description of the improved surge protection idea with numbers....

ActionDave, thanks for the link, it is an interesting read.

There advantages and disadvantages to both systems.

I myself still think the grounded primary and secondary in the distribution probably still wins out, but I also think we should only use the ungrounded conductors for current carrying purposes and leave the grounded conductors for fault clearing and equipotential purposes and we would have much less trouble with "stray voltages" that all come from current on grounded conductors.

 

[dereckbc]

I would like to know what those problems are and why grounded systems seem to be preferred.

That is easy to answer, ECONOMICS and SIMPLICITY.

In a grounded system over current protection is simple and cheap by just using one simple over current protection device on the ungrounded circuit conductor like a breaker or fuse. Ungrounded systems require a OCDP on each of the two or three circuit conductors, ground fault detection, and trained personal to service the system making it more expensive and complicated

The benefit of a ungrounded system is they are less susceptible to unnecessary outages and used where outage and/or interruption cannot be tolerated. Good example is a glass extruder where a power outage would lead to very costly repairs and restart cost to get all the hardened cool glass cleaned out of the extruder. When and if a fault were to occur nothing happens except the GFD alarm is raised alerting maintenance crews to schedule an orderly shutdown for repairs.

 

[dereckbc]

Well I dont like the practice of tying primary and secondary neutrals together and using the earth as CCC. Im paying for my electrical service, I want it isolated

Nothing stopping you from doing that if you want. They are called isolation transformers.

 

[dereckbc]

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.

care to bet your life on that with a simple test? Voltage at the fault point is a simple voltage divider and will remain there until when and if the OCPD operates to remove it. I know two people who would tell you that had they not been killed with 4160 fault.

 

[Sahib]

care to bet your life on that with a simple test? Voltage at the fault point is a simple voltage divider and will remain there until when and if the OCPD operates to remove it.

There will be capacitive current in each line of ungrounded 4160 V system. If one such line falls on the exposed surface of a grounded equipment, it becomes grounded i.e in a sense it becomes a 'neutral'. The capacitve currents of the other lines flow through this 'neutral' and this neutral current may be a maximum of 10A. So if the ground resistance is, say, 1 ohm, the potential rise of the grounded equipment with respect to remote ground is 10V only, not dangerous to life. So the ground resistance determines the life safety in this case.
But still I won't bet my life on that; I won't even come closer to that equipment.

 

[GoldDigger]

care to bet your life on that with a simple test? Voltage at the fault point is a simple voltage divider and will remain there until when and if the OCPD operates to remove it. I know two people who would tell you that had they not been killed with 4160 fault.

:thumbsup:
I think that the key point here is that if there is an accidental connection between one line of an ungrounded 4160 system and grounded or ungrounded conductors of a lower voltage grounded system and there is no other concurrent fault there is no guarantee that there will be lethal voltage and current exposure at any particular point, but there is also no guarantee that there will not be that is strong enough for me to bet my life on.
Ideally there will be no current flow at all except maybe for capacitive current, but no OCPD will open either. A ground detector might shut the high voltage down, but in the absence of that there will continue to be the risk of a second fault which would be lethal but might also eventually open an OCPD somewhere.

If the triggering circumstance is a 4160 line which is part of an ungrounded system falling, there is a pretty good chance that both (or all three) of the ungrounded 4160 lines actually fell and one of them may have hit ground!

Sahib: You cannot base your relative safety calculations on a single type of hypothetical fault and ignore other possibilities.
What you describe is just one part of an overall relative safety argument.