HRG or NRG Resistance grounding

[D!NNy]

Hello,

This question might be too silly or too odd please don't mind....

When i look at the pictures of neutral grounding resistor it is all series winding inside how can a thin wire can limit fault current to as low as 5A or 10A or 100A so on with out melting the conductor.......and these are always listed to limit the fault current to very low as listed above but how long this can hold the fault current to that low value? and how long can you keep the ground fault in your system?

Any suggestions are appreciated.
thanks

 

[jim dungar]

Hello,

This question might be too silly or too odd please don't mind....

When i look at the pictures of neutral grounding resistor it is all series winding inside how can a thin wire can limit fault current to as low as 5A or 10A or 100A so on with out melting the conductor.......and these are always listed to limit the fault current to very low as listed above but how long this can hold the fault current to that low value? and how long can you keep the ground fault in your system?

The wire has a very high resistance similar to the wire inside of a toaster or wall heater. High resistance equal lower current, based on the fixed L-G voltage.

The resistor is sized with a heat rating, just like a wall heater or electric furnace. They can get very hot so they usually have large enclosures.

It is not unheard of for these resistors to carry ground fault current for many days or months, maybe even years. But the NEC, and resistor manufacturers, intend for them to carry current for as short of a time as possible. I am sure most people would consider a time frame of 'weeks' to be excessive

 

[kingpb]

The devices typically have ratings for either 10sec, 60sec, or continuous. You can always specify something else.

The time duration in which you specify the device will have to do with the application, and the desired trip setting of the protective device.

 

[kwired]

They are not designed to clear fault current, they are designed to limit fault current, it is essentially a load that is only energized when there is a fault on the system. The other thing that must go with it is an indication system that tells the operator there is a fault. Without that it will carry current indefinitely or until it fails or the fault clears in some way, or until an operator acknowledges there is a problem and has something done about it.

 

[iceworm]

I've worked with quite a few HRG systems.

All the 480V systems were either 5A, 55ohm, or 10A, 28ohm. All were continuous rated. All the systems were set to alarm on GF. None were set to trip. One might look at these as being either a 1500W or 3000W heater elements - nothing special.

All the 13.8kV systems were 50A, 160ohm, 30sec rating. (I'm not sure about the 30sec rating. It might have been only 10sec.)All were set to trip on GF in 1sec. Now this is a 400KW electric heater - definitely non-trivial.

I'm sure there are plenty of others out there - these are the only ones I've seen

ice

 

[kwired]

I've worked with quite a few HRG systems.

All the 480V systems were either 5A, 55ohm, or 10A, 28ohm. All were continuous rated. All the systems were set to alarm on GF. None were set to trip. One might look at these as being either a 1500W or 3000W heater elements - nothing special.

All the 13.8kV systems were 50A, 160ohm, 30sec rating. (I'm not sure about the 30sec rating. It might have been only 10sec.)All were set to trip on GF in 1sec. Now this is a 400KW electric heater - definitely non-trivial.

I'm sure there are plenty of others out there - these are the only ones I've seen

ice

If they were set to trip what would be the reason for using such a system over a solidly grounded system? One of the main reasons for such a system is the ability to orderly shut down a process after fault indication instead of having it suddenly shut down because power was cut off. Now if users decide to ignore the fault status, they will have unexpected shut down should a fault happen on another phase.

 

[Sahib]

If they were set to trip what would be the reason for using such a system over a solidly grounded system? One of the main reasons for such a system is the ability to orderly shut down a process after fault indication instead of having it suddenly shut down because power was cut off. Now if users decide to ignore the fault status, they will have unexpected shut down should a fault happen on another phase.

A generator protection is involved here and that is the meaning of ''set to trip''.

 

[iceworm]

A generator protection is involved here and that is the meaning of ''set to trip''.

Ahhhh ..... No - that's not it.

ice

 

[JDBrown]

This question might be too silly or too odd please don't mind....

I don't think such a thing exists. (I'm sure somebody will prove me wrong later )

When i look at the pictures of neutral grounding resistor it is all series winding inside how can a thin wire can limit fault current to as low as 5A or 10A or 100A so on with out melting the conductor.......
[emphasis added]

The thin wire doesn't melt because of what it's made out of. If the wire in those resistors was lead, tin, zinc, etc. (like the element in a fuse), it would melt. I'm not sure what material is used for making these wire-wound resistors, but it's probably something similar to the wire used in heating elements -- alloys of nickel-chromium, or nickel-iron, or copper-nickel, or iron-chromium-aluminum, etc. These alloys have a much higher melting point than the metals used for fuse elements, so they can get much hotter without melting. As jim dungar mentioned, that extra heat is the reason these resistors usually have large enclosures.

 

[iceworm]

... One of the main reasons for such a system is the ability to orderly shut down a process after fault indication instead of having it suddenly shut down because power was cut off. ...

Yes

... Now if users decide to ignore the fault status, they will have unexpected shut down should a fault happen on another phase.

Yes

If they were set to trip what would be the reason for using such a system over a solidly grounded system? ....

I'm pretty sure you, Jim, and kb already know this - so, just for the rest of us:
Two different design specs between 480V HRG and 13.8kV HRG. OP did not differentiate. I work with both.

Solidly grounded systems generally fail phase to ground first and often don't trip until the fault goes phase to phase. Lots of damage occurs during the phase to ground fault time. Ungrounded systems are subject to arcing, re-striking ground faults. These faults subject the system to severe overvoltage transients.

So, one selecst an HRG to limit the damage of solidly grounded system GF faults and not be subject to the OV transients of ungrounded system GF faults. This is the normal reasoning for using an HRG on 13.8KV systems. And they are set to trip on GF.

And, as you said, continuity of service is the reasoning for setting the 480V HRG systems to not trip.

To clarify the for the OP:
480V HRG resistors are continuous. 13.8kV HRG resistors are very short time rated

ice

 

[SG-1]

To clarify the for the OP:
480V HRG resistors are continuous. 13.8kV HRG resistors are very short time rated

ice

I am with you except for the last sentence.
I think the 13.8kv NGR was being used in a low resistance grounded system. This type system allows 400 or more amperes to flow during a ground fault so protective coordination can be used to clear the fault.

HRGs are seldom used above 5kv because system capacitance is too great. In other words there is too much let through current.

 

[iceworm]

SG - Perhaps you could read post 5 again.

ice

 

[SG-1]

SG - Perhaps you could read post 5 again.

ice

My only point is that HRG systems are continuous rated & usually limited to 10A max of fault current including the pulser. Low resistance grounding usually has a 10 second rating. Not all neutral grounded systems are HRGs.

Are we in agreement ?

 

[kwired]

A generator protection is involved here and that is the meaning of ''set to trip''.

Again (other than for over 600 volts as iceworm has brought up) what would be the purpose of having a high resistance grounding system if it is going to trip on a fault? We already get that from a solidly grounded system.

Yes


Yes


I'm pretty sure you, Jim, and kb already know this - so, just for the rest of us:
Two different design specs between 480V HRG and 13.8kV HRG. OP did not differentiate. I work with both.

Solidly grounded systems generally fail phase to ground first and often don't trip until the fault goes phase to phase. Lots of damage occurs during the phase to ground fault time. Ungrounded systems are subject to arcing, re-striking ground faults. These faults subject the system to severe overvoltage transients.

So, one selecst an HRG to limit the damage of solidly grounded system GF faults and not be subject to the OV transients of ungrounded system GF faults. This is the normal reasoning for using an HRG on 13.8KV systems. And they are set to trip on GF.

And, as you said, continuity of service is the reasoning for setting the 480V HRG systems to not trip.

To clarify the for the OP:
480V HRG resistors are continuous. 13.8kV HRG resistors are very short time rated

ice

I was not aware of high impedance grounding practices over 600 volts, but thanks for the input about it, and yes I am sure it is a little different reasoning to use it at these higher voltages.

 

[iceworm]

My only point is that HRG systems are continuous rated & usually limited to 10A max of fault current including the pulser. Low resistance grounding usually has a 10 second rating. Not all neutral grounded systems are HRGs.

Are we in agreement ?

Following is my paraphrase\interpretation of the IEEE standards:

IEEE 100 definition agrees with your cutoff of 10A. So I figured you were correct and the 50 A of the 13.8kv systems were, as you said, considered Low Resistance Grounded.

IEEE 141 (green) defines LRG as 100A with 200 - 1000 more usual. And it defines HRG using the 10A cutoff. But, it also states some 15KV high resistance grounded systems may have higher currents than the usual 10A cutoff - to account for the higher capacitive charging durrent of the MV system. And it says the cutoffs are not hard limits.

So, I argee that I won't refer to the 13.8kV 160ohm, 50A GF systems as HRG anymore. I'll call them resistance grounded.

Thanks for your comments. They got me to look up the standards

...I was not aware of high impedance grounding practices over 600 volts, but thanks for the input about it, and yes I am sure it is a little different reasoning to use it at these higher voltages.

You're welcome. Unfortunately, MV system design is not in my area of expertise. This next is a guess:
I suspect the newer relays (within the last 20years) allow better detection of the lower fault currents. And lower is better - less damage. Which may account for why I have not seen any 13.8kV Low resistance grounded systems (GF currents 100A and greater. But, I don't know this for a fact.

ice

 

[SG-1]

There is a lot of reading material here on HRGs:
http://www.postglover.com/highresistance.html

Just scroll down the page about half way.

Iceworm, you are correct about modern relays being able to detect ground faults better than the electro-mechanical type. In medium voltage a ground fault on an ungrounded system will produce about 3 amperes of current, or so I am told. Modern relays are accurate enough to measure this.

The 50A NGR system does seem to fall between HRG & LRG. Maybe it should be called MRG for medium resistance grounding.
How bout it ??

 

[Sahib]

Again (other than for over 600 volts as iceworm has brought up) what would be the purpose of having a high resistance grounding system if it is going to trip on a fault? We already get that from a solidly grounded system.

A high ground fault can be more evil than a phase to phase fault inside a generator and in such cases high resistance grounding of the neutral of the generator may help. A ground fault current of 10A can be sustained indefinitely by a generator but any high ground fault current such as caused by any preexisting inter-turn short in the resistor should trip the generator.

 

[kwired]

A high ground fault can be more evil than a phase to phase fault inside a generator and in such cases high resistance grounding of the neutral of the generator may help. A ground fault current of 10A can be sustained indefinitely by a generator but any high ground fault current such as caused by any preexisting inter-turn short in the resistor should trip the generator.

And in the meantime you have 10 amps flowing through unintended current paths. In another thread you wanted everything grounded/bonded including isolated metal objects. You can't have it all in every installation.

Is said generator a stand alone system? I don't think it would be a good idea to back up a solidly grounded system with a high resistance grounded system

 

[Sahib]

And in the meantime you have 10 amps flowing through unintended current paths.

You can change the settings so that the generator is tripped instantaneously on ground fault.

In another thread you wanted everything grounded/bonded including isolated metal objects.

This is a false idea you have.

Is said generator a stand alone system? I don't think it would be a good idea to back up a solidly grounded system with a high resistance grounded system

It is not a good idea to operate a generator with its neutral solidly grounded either.

 

[kwired]

It is not a good idea to operate a generator with its neutral solidly grounded either.

Other than ungrounded generators all that I have ever seen is solidly grounded systems. Doesn't matter if it is big or small single or three phase, and the units that are ungrounded are usually portable units and they become solidly grounded if connected to a premises wiring system.

I guess maybe all those generator manufacturers maybe need to have a talk with you and change their ways of doing things.

If you are primarily talking about medium voltage here then maybe there is some value in HRG, I certainly am not much of an expert in medium voltage applications, but I am talking 600 volts or less with my replies.