High Resistance Grounding for 1000 volts and below

[wirenut1980]

The only thing I can think of is that it excludes 240 V because it is a system meant for industrial situations that have trained authorized access only. If 240 V is included, I suppose someone could have an impedance grounded system installed at their house.

Now I too would like to hear the real reason for this

 

[coulter]

...What if the customer decides to use HRG because he wants continuity of service even with a single line to ground fault at 240 volts three phase? ...

pt -
I need some clues about how you are defining "240 volts three phase" and the application.

1. Is this 240/120 Delta? This has a centertap on one of the phases.

2. Is this 240 Delta? No center tap.

3. Is this 240/139 Y? This one has a neutral that can't be used for single phase loads (possible oddball exceptions noted).

4. Some other 240V, 3 phase system currently unknown to me.

The way you phrased your question leaves open a lot of possibilities. For some the physics are plenty safe, but legally unsafe. For some systems and some applications, I would ask the customer, "What is the perceived benefit?" Just because, "...the customer decides to use HRG...", doesn't give any information on the science or physics involved.

carl

 

[asnow21]

EE

EE

Do we need route the ground conductor in PVC conduit if the ground conductor is burried in the concrete slab. Does the PVC conduit will protect the concrete in the case the short-circuit is happenned?

Anh

 

[poweringtech]

"The way you phrased your question leaves open a lot of possibilities. For some the physics are plenty safe, but legally unsafe."

What do you mean?

For some systems and some applications, I would ask the customer, "What is the perceived benefit?" Just because, "...the customer decides to use HRG...", doesn't give any information on the science or physics involved."

It is very clear that I said what if customer wants continuity of service at 240 volts system? HRG is a solution when there is a ground fault. Code does not allow it to be used at voltage lower than 480 volts.

 

[rcallen]

High Resistance Neutral Grounding

High Resistance Neutral Grounding

I have been installing HRNG systems since 1965, so I know a little about them.

In the beginning continuous process facilities like petroleum refineries, that couldn't tolerate outages, used ungrounded systems so that a ground fault would not shut down equipment, which is a catastrophe for them. There are problems with ungrounded system, but they (the refineries) had to live with them. In 1962 Fran Fox (an applications engineer for GE based in San Francisco) came up with the pulser system to locate ground faults in resistance grounded systems. He tested the systems at the Flying A refinery in Martinez, CA, and the Socal refinery in Richmond CA. It was so successful that we converted all 480 volt systems in the Richmond and El Segundo refineries, and later the other Chevron refineries. This system was not acknowledged by the NEC at that time. When the NEC finally acknowledged the HRNG it put some limitations on it, probably out of concern for the skills of "ordinary" electricians. The systems were only to be installed in facilities with trained, on-site electrical staffs who could test them, use the pulser to trace the fault (and then clear the fault at an appropriate time) and understand the system. HRNG avoids system burndowns due to arcing ground faults. For systems rated about 1000 amps the literature was full of burndowns using solidly grounded systems (I have personally seem several of these). The solution was either HRNG or the use of ground fault interrupters (which came into use on low voltage systems in the late 1960s). The HRNG has the advantage of not tripping in the face of the fault. The GFI does trip. Because burndowns almost never happen below 480 volts or in systems with breakers rated below 1000 amps it is not necessary to use HRNG on these systems. I have used HRNG on 2400 and 4160 volts systems, and it works well for these. Above this level they should not be used because the normal leakage current of the system is too high. For systems 4160 volts and below the resisters are sized for approximatley 1 amp (480V) up to 5 amps (2400 and 4160). This level of ground fault can be tolerated for extended periods without damge to the system. Above 4160 the resistor must be set to allow too much fault current so it cannot be used. The fault current must be allowed to exceed the leakage current to maintain system stability, and this is the reason for 1 amp at 480 volts. There are exceptions but by now you are getting MEGO so I won't continue.

The bottom line is simply this: for 240 or 208 volt systems the HRNG offers no advantages. For 480, 2400 or 4180 volts it offers advantages as long as the personnel caring for the system understand it. In 480 volt systems if one wishes to use the neutral for lighting or other loads the HRNG may not be used.

So there you have it. For more info see the IEEE Red Book, the IEEE Green Book, or the IEEE papers published by Fran Fox and Clyde Tipton or Dunke-Jacobs of GE.

 

[winnie]

High resistance grounding is not permitted at any voltage if line-neutral loads are served. There is a real safety issue here, because some line to neutral loads are designed with the understanding that the grounded conductor will be at or near earth potential. For example, the relatively exposed screw shell of a lampholder is supposed to be supplied by the grounded neutral conductor.

In a high resistance grounding system, during a fault, the voltage of the 'grounded' neutral will rise to the line-neutral voltage of the system. This would clearly be unacceptable if the load is designed to operate with the 'grounded' conductor at low voltage.

Perhaps the CMP felt that the chance of line-neutral loads being used below 480V was simply too great. As Carl mentioned, there are several different 240V delta systems, some of which provide normal single phase 120/240V power. In addition to the fact that these systems do not have a true neutral, in the event of a ground fault the single phase 'neutral' would jump to 120V or 208V relative to ground. There are very few 240V delta loads that cannot simply be reconnected as 480V loads.

Or perhaps there is no real reason other than that the CMP simply didn't consider the option, or that the hardware was not out there so no manufacturer tried to push the CMP to allow 240V resistance grounded systems.

Not relevant to the present discussion, but IMHO resistance grounded systems should be permitted to feed even line-neutral loads, but only if fault detection caused the circuit to open immediately. In this case, the resistance grounding would not be used to provide continuity of service, but instead to simply reduce ground fault power levels.

-Jon

 

[coulter]

...It is very clear that I said what if customer wants continuity of service at 240 volts system?...

Yes. you were very clear. I'll try to illustrate my point with an absurdity. If the customer came to me and said, We want to run all of our motors on 24VDC because it is safer." My response would be, "We can do that, but first help me understand what you are trying to accomplish. Perhaps there are better methods."

"The way you phrased your question leaves open a lot of possibilities. For some the physics are plenty safe, but legally unsafe."

What do you mean?...

Jon pretty well explained that one. However, an example is: The code does not allow 1ph, 480V lighting connected to impedance grounded systems. That one completely eludes me as to why this would be physically unsafe. However, per the NEC, it is legally unsafe.

carl

 

[coulter]

Reid -

I 'm not sure why you would limit HRNG to 4.16kV. I'm working on a new 13.8kV system right now that has a 167ohm ground resistor. Passes 47A across a phase to bonded part fault. Trips in 2 sec or less, depending on where the fault is. The impedance grounded system is not for continuity of service, but for limiting of phase to ground fault current to zero damage levels - provided the GF goes out before the fault goes phase to phase. Looks good to me.

carl

 

[rcallen]

The purpose of HRNG is to avoid tripping in the fact of a ground fault. At voltages in excess of 4160 the amount of leakage current required to maintain system stability is too high. Fran Fox tried to get us all to use it anyway and simply trip, and this is OK. We have been using grounding resistors for years to reduce fault current (industrial use), and the usual value was 400 amps with the ground fault relay set to pick up at 200 amps. This is left over from the days of induction disk relays. The intent was to give enough fault current to make tripping certain. With electronic relays this is no longer much of a concern because there is no induction disk to hang up.

As long as the current being passed by the resistor under normal (non-fault) conditions exceeds system charging current I see no reason not to use any value at all, but it isn't really HRNG as this is usually defined because HRNG is normally intended not to trip on the first ground fault.

 

[jim dungar]

Reid -

I 'm not sure why you would limit HRNG to 4.16kV. I'm working on a new 13.8kV system right now that has a 167ohm ground resistor. Passes 47A across a phase to bonded part fault. Trips in 2 sec or less, depending on where the fault is. The impedance grounded system is not for continuity of service, but for limiting of phase to ground fault current to zero damage levels - provided the GF goes out before the fault goes phase to phase. Looks good to me.

carl

Most installations of high resistance ground limit the current to a maximum of 10-20A.

I would call your system of 47A a low resistance ground, even though most of the ones I have seen are over 100A.

 

[RayS]

The issue in this thread is why is it not permitted in the code to use High Resistance grounded in voltages lower than 480 volts?

Any code expert or member of the code who can explain? If it is safe to use HRG at lower than 480 volts, should the code be changed to allow HRG be applied to 240 volts?

Can't claim expert status, but I believe, as Kingpb alluded to in post 6, solidly grounded systems are required below 480V because the phase to ground voltage can be kept below 150V.

In light of the recent increase in the awareness of arc-flash hazards, perhaps we can see a move towards more HRG systems?

 

[coulter]

...IMHO resistance grounded systems should be permitted to feed even line-neutral loads, but only if fault detection caused the circuit to open immediately. ...

Jon -

You would have to have a four pole panel. There would need to be a CB in the neutral. Probably not real feasible.

carl

 

[George Stolz]

I have been installing HRNG systems since 1965, so I know a little about them.

In the beginning...

Reid, thanks for the background. It made for good reading, very informative.

 

[rcwilson]

So there you have it. For more info see the IEEE Red Book, the IEEE Green Book, or the IEEE papers published by Fran Fox and Clyde Tipton or Dunke-Jacobs of GE.

Mr. Allen - When I worked with Clyde Tipton at Richmond in the '70s, the Hi Res grounding systems and Fran Fox's locating system were doing their job quite well. The older 480V delta systems had zig-zag transformers added to create a neutral grounding point. All of the subs had a flashing light that went off anytime a fault occurred. The electric shop trouble truck would come and use the locating clamp-on CT to chase and repair the grounds. The result was a safe and reliable power system that literally saved millions of dollars by not tripping the crude oil units so often.

I was sold on the system and have tried to convince all my clients to use it since then. My present boss won't allow it because in facilities we do not operate, we cannot assure that the conditions of maintenance and supervision will meet the Code requirements . So our clients’ plants trip on ground faults. But we set up the system so the client can install the Hi-Res with little trouble. I find this interesting because we always use resistance grounding (or the equivalent) on the 4.16 kv through 28 kV systems.

We religiously do not allow any 277 V loads on the main 3- wire, 480V systems. We use 480 delta- 480/277 isolating transformers to feed lighting panels. (That also knocks down the short circuit levels on the lighting and reduces arc flash. Panels are cheaper and safer too.) The main ssytem neutral is always bonded/grounded in such a way at the switchgear or transformer to make it easy to insert the grounding resistor later.

I’m sold on the safety of the system when operated by competent people.