High Resistive Ground System

[yukonjack_99674]

Does it matter where you land your neutrals (neutral bar or ground bar) when there is a "High Resistive Ground" system on the main transformer?

 

[weressl]

Does it matter where you land your neutrals (neutral bar or ground bar) when there is a "High Resistive Ground" system on the main transformer?

In an HRG(High Resistance Grounding) system there is NO neutral bar. The Xo terminal of the transformer secondary winding is connected to the grounding resistor and NOWHERE else. The other terminal of the grounding resistor is connected to your system ground point.

The connection and subsequent use of the Xo to a neutral - 4th wire - bus would defeat the purpose and function of the HRG system.

 

[eric9822]

Does it matter where you land your neutrals (neutral bar or ground bar) when there is a "High Resistive Ground" system on the main transformer?

Usually if you have a HRG system you have separately derived systems for lighting and receptacles. Are you referring to that?

 

[winnie]

The connection and subsequent use of the Xo to a neutral - 4th wire - bus would defeat the purpose and function of the HRG system.

I disagree slightly.

Under NEC code, an HRG system is _not permitted_ to serve L-N loads. There are good safety reasons for this, because systems with a neutral are generally designed with the expectation that the neutral will be near ground potential.

However, I see no reason that, with proper design, an HRG system could not function with L-N loads. How does having a neutral circuit conductor defeat an HRG system? As long as it is isolated from ground, it would not prevent continuity of service in the event of a ground fault.

-Jon

 

[Cold Fusion]

...I see no reason that, with proper design, an HRG system could not function with L-N loads. How does having a neutral circuit conductor defeat an HRG system? As long as it is isolated from ground, it would not prevent continuity of service in the event of a ground fault. ...

That is a new thought.

I'll start with what is (would be) "proper design"?

All of the 480V HRG systems I have seen have either 28 ohm or 55 ohm resistors - giving 5A to 10A neutral/resistor current for a line to ground fault. As you know, an L-G fault raises the neutral to 277V and the unfaulted lines to 480V above ground. I don't know of any single phase 277 equipment speced to run at the elevated voltage.

Hence my question:
What would you consider "proper design"?

cf

 

[StephenSDH]

As you know, an L-G fault raises the neutral to 277V and the unfaulted lines to 480V above ground. I don't know of any single phase 277 equipment speced to run at the elevated voltage.

A L-G fault raises the ground to 277v from the x0 terminal, but the neutral would remain at 0v and single phase loads would be unaffected. Although you would have a potential between neutral and ground of 277v.

 

[winnie]

If you call ground '0V', then a L-G fault raises the neutral to 277V. If you call the neutral '0V' then a L-G fault raises ground to 277V. In both cases you end up with a N-G voltage of 277V, with all of your L-L and L-N voltages essentially unchanged.

My thoughts on proper design of an HRG system that permits L-N loads are as follows. There are two situations where it could be appropriate.

1) is where the equipment is suitably rated for 277V L-L connection, and you treat the neutral as an ungrounded conductor. This would mean that two pole OCPD and switches for all single phase 277V loads, and I think that this setup is unlikely to be useful unless you have very small 277V loading. I suspect that there is quite a bit of equipment that would be safe and functional in such a system, but that no equipment is actually evaluated for this use. (Well, I am sure that there is equipment that is 'multi-voltage' capable, and evaluated for both 277V L-N use as well as 480V L-L use, which would imply that it is suitable for 480VL to 277VN use, but if you had such equipment you would just use it L-L.)

2) actually changes the purpose and function of using an HRG system. In a conventional solidly grounded system, you still see _transient_ N-G voltages that approach the L-N voltage. So it seems to me that equipment is currently designed to deal with these sort of N-G voltages when of short duration. Consider an HRG system where all of the OCPD includes ground fault protection, and opens the circuit in the event of a ground fault. This would eliminate the continuity of service benefit of and HRG system. Instead you would have the same sort of (lack of) continuity of service that you expect from a normal solidly grounded system, but with greatly reduced ground fault energy. With 5A of ground fault current available, you can easily detect ground faults and you could easily provide breaker coordination.

-Jon

 

[weressl]

I disagree slightly.

Under NEC code, an HRG system is _not permitted_ to serve L-N loads. There are good safety reasons for this, because systems with a neutral are generally designed with the expectation that the neutral will be near ground potential.

However, I see no reason that, with proper design, an HRG system could not function with L-N loads. How does having a neutral circuit conductor defeat an HRG system? As long as it is isolated from ground, it would not prevent continuity of service in the event of a ground fault.

-Jon

It's already difficult to design a relaying system with three phase unbalanced ground fault leakage and account for variations and still be able to determine if it is a fault or not, or to locate the fault. The neutral further biases your relay settings into the 'guesstimate' area, you would need to have relays that sums zero-sequence currents, ground resistor current and neutral current. Withe the event of non-linear loads there are additional difficulties are encountered with the harmonic restrain filtering to RMS data in the ground fault. Neutral use multiplies these.

Of course you can go ahead and install it and make an 'empirical determination' if it is OK or not. Then, years from now when somebody encounters a faulty relaying you can go ahead and build the math model to try to figure out what happened. (Read my signature motto....)

 

[Cold Fusion]

jon -
It's an interesting exercise in "what if we had ...."

Just because there is no application today, does not mean there won't be tomorrow.

cf

 

[Cold Fusion]

A L-G fault raises the ground to 277v from the x0 terminal, but the neutral would remain at 0v and single phase loads would be unaffected. Although you would have a potential between neutral and ground of 277v.

I admit that thought did not occur to me today - probably won't tomorrow either.:roll:

cf

 

[yukonjack_99674]

Let me see if I have this, I have a HRG system on a main transformer. That transformer feeds several other transformers which feed panels and sub-panels. Some of these sub-panels mix their neutrals (white common) with grounds on the same bar. Other panels are separated, neutrals on the neutral bar and grounds on the ground bar.
Nope by golly, I don't have this, lol and I haven't even got to the grounding the X0 on transformers that are down stream of the main transformer. Confused Thanks for everyones help on this, I'm trying to make our system as safe as possible.

 

[winnie]

It's already difficult to design a relaying system with three phase unbalanced ground fault leakage and account for variations and still be able to determine if it is a fault or not, or to locate the fault. [...]

Oh come on! If we can reliably detect and differentiate between series arc faults and normal arcing in usage equipment..... *grin*

I see what you are saying; I have to admit to seeing things on much smaller scale that you look, and I have no idea what adding a neutral would do to the complexity of protective relaying on a large system. The only HRG systems that I've had direct exposure to are small (<100KVA) lab systems with very simple protective relaying.

Seriously, getting back to the original poster:
In a properly implemented HRG system under current rules, there will be _no_ neutral conductors, and thus no mixing of neutral and ground conductors. If you see a neutral conductor on an HRG system, you have a violation and it needs to be fixed.

_However_ if you have a transformer that is supplied by an HRG system, then the secondary of that transformer is a new 'separately derived' system. Ignore the HRG aspect of the primary, and provide grounding on the secondary just like any other grounded transformer.

-Jon


-Jon

 

[dbuckley]

The problem I see with HRG with a neutral is detecting a neutral/ground fault.

 

[Cold Fusion]

Let me see if I have this, I have a HRG system on a main transformer. That transformer feeds several other transformers which feed panels and sub-panels. Some of these sub-panels mix their neutrals (white common) with grounds on the same bar. Other panels are separated, neutrals on the neutral bar and grounds on the ground bar.
Nope by golly, I don't have this, lol and I haven't even got to the grounding the X0 on transformers that are down stream of the main transformer. Confused Thanks for everyones help on this, I'm trying to make our system as safe as possible.

yj -
As others have mentioned, you have several different concepts mixed together. Here is a start for you.

First transformer is HGR. All of the panelboards/switchboards fed from this xfm do not use a neutral for any loads. No single phase Line-Neutral loads - only 3phase or single phase line-line.

All transformers downstream from the first transformer are likely solidly grounded, "separately derived systems". You need to look that term up and get a clear understanding of what it means - this is really a key point.

Panels fed from solidly grounded transformers downstream from the first transformer. These panels can feed line-neutral loads, Line-line single phase, 3phase loads. For these panels, mixing the neutrals and grounds on the same bar is not wrong - the ground and neutral are bonded, but it certainly looks sloppy.

Panels fed from "Panels fed from solidly grounded transformers" These are 5 wire (for three phase) panels. The neutral and ground are separated. Mixing neutrals and grounds on the same bar is wrong.

Read through this and pick out the parts that don't make sense and ask about them. And yes, you are going to have to undeerstand the concept of "SDS".

cf

 

[zog]

The problem I see with HRG with a neutral is detecting a neutral/ground fault.

That is what the ammeter and alarm on the resistor bank is for.

 

[yukonjack_99674]

Thanks I think I'm getting on track. So if I take a 480/480Y transformer, the neutral I pick up is a true neutral and should be seporated from the grounds.
Also, if a transformer is powered from a solidly grounded transformer (though fed from a panel that is HRG), a separately derived system is formed and should be treated with "normal" grounding procedures; neutrals and grounds on their own bars. Is this a fair statement?

 

[rcwilson]

You got it!

HRG is a great for maintenance and safety, IMHO.

One side benefit of the isolation transfomer on larger systems is that it knocks down the short circuit level for the 480/277 downstream loads. You can usually save money on the 277 panelboard breakers.