ground current

[pbunn]

I was asked to consult on a problem with excessive grounding conductor current on a group of new APC UPS systems. A testing company measured 26 amps on the grounding conductor. When the generator was running, the current increased to over 50 amps.

The systems is fed with a 2000 amp three wire 480 volt switchgear.

On a closer inspection - the APC equipment had both neutral and ground busses and they were jumpered which solved the problem.

The problem is, installation of a neutral will cost many thousand of dollars. APC states that there is no need for a neutral and that that it is normal for the ground to carry the measured current. In fact, in the letter to the customer from Schnieder (Square D , APC) they state that this grounding conductor is totally safe and normal and "harmless". They state in the letter that their equipment is properly installed for a three wire system (tying the neutral bus to the ground bus and using the grounding conductor and ground system to carry unbalanced harmonics current).

This equipment is in a very critical data center in a hospital.

I still maintain that they have a serious problem and do not meet code but I am being overuled.

Any comments?

 

[zbang]

What are the feeds in and out of the UPS (more than one UPS?)? Is there a static-bypass line and built-in switch?

 

[pbunn]

The UPS input is 3 wire 480 volt - three 500 Kcmil plus a grounding conductor which is the code minimum.

The output side is 208/120 volt 4 wire - the secondary side is bonded and grounded as would be done is a seperately derived system.

No bypass as the input and output voltages are not equal

The input of the APC system has both a ground bus and neutral bus and APC has stated that these need to be bonded which is a code violation.

APC engineering states that is is normal for the grounding conductor to carry harmonic unbalance current and that is is safe and meet all codes.

To me, the fact that the unit has a neutral bus, means it needs to be fed with a 4 wire 480/277 volt system. APC says no - that all you need to do is bond the neutral and grounding conductor for three wire operation.

 

[pbunn]

Also,

There are multiple UPS systems wired to the 2000 amp switchgear - The switchgear is fed with five sets of three 600 Kcmil copper - no neutral just the minimal NEC grounding conductor. The switchgear feeds five seperate UPS systems. The 2000 ampere panel is not the service panel.
When I first examined it it had bonding jumpers to the main water line and building steel. These have been removed.

APC "tested" the systems and measured approx 6 amps flowing in each UPS grounding conductor - in their report they sated this current was normal and was not a safety problem - they measured from the cabinet of the ups to the grounding conductor - zero volts and stated in the report - "zero volts is absolutely safe"

I'd challenge them to open one of those conductors and put one in each hand!

 

[brian john]

He is wrong, what vintage is the UPS, APC had some issues with design that resulted in ground current.

There is a bypass they just use the input for the bypass (single source input).

 

[__dan]

I was asked to consult on a problem with excessive grounding conductor current on a group of new APC UPS systems. A testing company measured 26 amps on the grounding conductor. When the generator was running, the current increased to over 50 amps.

The systems is fed with a 2000 amp three wire 480 volt switchgear.

On a closer inspection - the APC equipment had both neutral and ground busses and they were jumpered which solved the problem.

The problem is, installation of a neutral will cost many thousand of dollars. APC states that there is no need for a neutral and that that it is normal for the ground to carry the measured current. In fact, in the letter to the customer from Schnieder (Square D , APC) they state that this grounding conductor is totally safe and normal and "harmless". They state in the letter that their equipment is properly installed for a three wire system (tying the neutral bus to the ground bus and using the grounding conductor and ground system to carry unbalanced harmonics current).

This equipment is in a very critical data center in a hospital.

I still maintain that they have a serious problem and do not meet code but I am being overuled.

Any comments?

Your inquiry needs some additional information.

At the 2000 amp source switchgear, if it is 3 phase 3 wire there is no neutral, so there cannot be a second neutral to ground connection. You would have only an EGC, equipment ground busbar, and if that has redundant connections to the building steel and water main, that may be good. Connecting to the building steel may be more is better. Connecting to the water main may be discretionary, if there is a group point at the main with other EGC'S, GEC's, adding one more may be good. If the water main connection intercepts and adds a circulating ground current on the water piping, that would be bad (250.6).

The UPS has to have some type of bypass scenario to take the unit offline for servicing or for the times it will take itself offline.

Line side 480 volt 3 wire delta source, the source neutral does not connect to the load. There is no adding of the neutral. The secondary neutral at 208 volt Y is seperately derived and "clean" at the transformer output secondary.

250.6 applies to current flow on the ground paths and is a violation that requires addressing or investigation, the discovery of if there is a problem that can be fixed. Your post indicates the objectionable ground current is on the line side equpment ground. One question is if this current flow is there even with the equipment deenergized and the breakers open. It is possible the equipment is intercepting an existing ground loop current flow.

Your post indicates the secondary neutral is grounded and bonded for seperateyly derived sources. I would pay particular attention to the grounding electrode conductor per 250.30 (A) 3 for presence or absence of circulating ground loop current. That wire should be clean and zero. In particular, I have seen some APC documentation where there is no GEC conductor in their diagrams and IMO, a violation of 250.30 (A) 3. That would be the first thing I would look at. Look to see if the GEC is actually installed, to where, and is there a measurable ground current on it.

Measurable ground currents on the line side are objectionable but not surprising in a big system. Deriving the neutral cleanly at the secondary and clean earth grounding per 250.30 (A) 3 is the remedy.

 

[pbunn]

Yes -

I was asked by the hospital for a position letter because of the excessive ground current. I was not the system design engineer.

My position was that 25-50 amperes of grounding conductor current was excessive and potentially dangerous. Initially the ground bus in the 2000 ampere gear was bonded to the water pipe system and building steel. This was a code violation because this gear is not the service gear. I asked that the contractor make a measurement of total ground current with a CT around all the 2000 ampere conductors but this was not done. Ground current may be much more than measured in the "green grounding conductor" as it is likely flowing in the piping and building steel.

The design drawings clearly show a three wire 480 volt system - no neutral.

The APC gear has both a primary (480 volt) neutral bus and a ground bus. This (in my opinion) defines it as a 4 wire system.

When APC started up the system, they placed a bonding jumper between the ground bus and neutral bus. With no neutral conductor, the grounding conductor becomes the neutral with this connection which is a gross violation of the NEC.

APC has responded to my letter, with a comment "that most CEs would understand that when a balanced harmonic load is summed - that the current is not zero and that this harmonic unbalance is allowed by NEMA"

I perfectly understand that principal.

They continue with "this current flows in the ground system and is not objectional or dangerous and is allowed by NEMA." They actually show the measured ground current in the letter and state the safety of the system by claiming that there is no voltage between the ground and the APC cabinet.

I totally disagree with this statement.

The hospital system has taken the APC letter position, and has close the matter, believing that the system is safe and is not a problem. They plan to add more APC systems which will likely result in ground currents rising to levels close to 100 amperes.

The actual design engineer has sided with the hospital. Correcting the system by adding a neutral conductor would likely cost in excess of $100K.

Do I continue to fight or give it up. How dangerous is this. My feeling is it is very dangerous to an unsuspecting person who might interupt the ground current assuming there is no current flow?

I am really surprised that a company the size of APC would be foolish enough to write a letter like this.

 

[__dan]

Yes -

I was asked by the hospital for a position letter because of the excessive ground current. I was not the system design engineer.

My position was that 25-50 amperes of grounding conductor current was excessive and potentially dangerous. Initially the ground bus in the 2000 ampere gear was bonded to the water pipe system and building steel. This was a code violation because this gear is not the service gear. I asked that the contractor make a measurement of total ground current with a CT around all the 2000 ampere conductors but this was not done. Ground current may be much more than measured in the "green grounding conductor" as it is likely flowing in the piping and building steel.

The design drawings clearly show a three wire 480 volt system - no neutral.

The APC gear has both a primary (480 volt) neutral bus and a ground bus. This (in my opinion) defines it as a 4 wire system.

When APC started up the system, they placed a bonding jumper between the ground bus and neutral bus. With no neutral conductor, the grounding conductor becomes the neutral with this connection which is a gross violation of the NEC.

APC has responded to my letter, with a comment "that most CEs would understand that when a balanced harmonic load is summed - that the current is not zero and that this harmonic unbalance is allowed by NEMA"

I perfectly understand that principal.

They continue with "this current flows in the ground system and is not objectional or dangerous and is allowed by NEMA." They actually show the measured ground current in the letter and state the safety of the system by claiming that there is no voltage between the ground and the APC cabinet.

I totally disagree with this statement.

The hospital system has taken the APC letter position, and has close the matter, believing that the system is safe and is not a problem. They plan to add more APC systems which will likely result in ground currents rising to levels close to 100 amperes.

The actual design engineer has sided with the hospital. Correcting the system by adding a neutral conductor would likely cost in excess of $100K.

Do I continue to fight or give it up. How dangerous is this. My feeling is it is very dangerous to an unsuspecting person who might interupt the ground current assuming there is no current flow?

I am really surprised that a company the size of APC would be foolish enough to write a letter like this.

At the UPS input, internally in the APC cabinet, if APC is connecting anything that is a load to the common grounding busbar, like a Y connected cap and inductor filter bank, if the center of the Y is grounded on the busbar, they have a neutral connected load on the EGC busbar and IMO, a violation of 250.6. As the caps age, they will go out of spec, or out of balance with the other units, and the imbalance could be hundreds of amps reactive.

If APC has nothing that is a load connected to the EGC busbar, everything would have to be connected line to line delta, the unit is 3 phase 3 wire, and should be clean of any stray neutral currents on the grounding system.

IMO, APC is great with their marketing but not so on issues like this. I have an unresolved issue with their equipment and system grounding. Saying "most CE's would understand" is a flip blow off answer. I would immediately check to see if the letter was written by a licensed person so liability would attach in the right place.

My primary concern would be the load side grounding. If you have those measureable ground loop circulating currents on the load side grounding conductors, the ground is noisy, and IMO, a problem that cannot be ignored. The load side common grounding busbar is certainly using the primary side EGC conductor connection (which is noisy), in addition to the second, required per 250.30 (A) 3, earth ground. I would want to see that the earth ground is installed per code and that there is no current flow on it.

There is some type of bypass scenario in place and this should be determined. From what you have described, I would expect the bypass to also be 3 phase 3 wire 480 volt, no neutral, with the 480 delta to 120/208 Y transformer after both the UPS and the bypass. This is supposed to make the transformer secondary neutral seperately derived and clean of noise on the grounds, EGC + a GEC. Look for the GEC per 250.30 (A) 3, it may be missing in the APC documentation (I have seen this) and may be missing in the install (I have seen this).

Could you post an APC model #, the manuals should be online.

 

[jim dungar]

Little things are important. It is extremely unlikely the transformer feeding the 2000A board is connected in an ungrounded delta arrangement although it is possible. So, the first thing to do is find out more about the system (not just the number of conductors) feeding the 2000A board.

The presence of a neutral bus bar in the APC does not tell me that the unit requires 480Y/277 to operate.
Many equipment manufacturers include filtering, not actual loads, that is intended to be used at 277V L-N and can fail if they are left floating. So when no neutral conductor exists, it is not uncommon for the neutral and ground to be connected together as you have experienced. Because of this N-G bonding there is the possibility that current will flow when the filters are not balanced (i.e. when harmonics exist).

It is not unusual to find cumulative ground currents that reach into the hundreds, this is one reason service entrance GF systems are rarely set below 200A pickup.

You say that

Initially the ground bus in the 2000 ampere gear was bonded to the water pipe system and building steel. This was a code violation because this gear is not the service gear.

.
What does the bonding look like now?

 

[pbunn]

The system at the service is 4 wire 480/277 volt. The service gear feeds this 2000 amp panel with a 3 wire feed - 5 sets of three 600 Kcmil.

The hospital previously used Leibert UPS systems that used an isolation transformer. APC sold the system as a non transformer system that was better. I work for the maintenance side of the hospital doing studies and NSPA 70E work and small replacement projects. They have a large firm that does the large capital projects who designed this system. The large firm does not agree that the ground current is "OK" but they thing it presents no problem.

I never would accept any system that bonded the neutral and ground downstream from the service and allowed working current to pass through the grounding (green) conductor. The APC engineer says that this system will work on a 3 or 4 wire system - but you must bond the neutral bus and ground bus on a three wire system. Again - That defines it as a 4 wire system to me.

The UPS is treated as a seperately derived system on the 208/120 volt side with the neutral and ground bonded and bonded to building steel. The secondary grounding conductor is common with the primary grounding conductor.

The removed the bonds from the 2000 amp gear to the water system and building steel after receiving my letter.

The APC letter was written and signed by a salesman who obviously has no clue. He called the contractor that got me involved and told him "he had never heard that you could not run normal unbalance current through the ground".

I have been advised to bring the matter the the AHD which likely may end my work at this facility. That is probably what I will do after notifying the hospital that I am going to do so.

 

[ritelec]

Yes -

My feeling is it is very dangerous to an unsuspecting person who might interupt the ground current assuming there is no current flow?

The large firm does not agree that the ground current is "OK" but they thing it presents no problem.

Pbunn.......it would seam like your losing some sleep over this and you'd like to see the system right.

Unfortunately, the powers that be, and the ones that are legal bound to this system have no intension of correcting it.

Should an unsuspecting person really interrupt the path to ground? Would not a "qualified" person only be doing any work on that ground?

Sorry to say, but
You can't save the world.

Get some sleep.

Rich

 

[texie]

Little things are important. It is extremely unlikely the transformer feeding the 2000A board is connected in an ungrounded delta arrangement although it is possible. So, the first thing to do is find out more about the system (not just the number of conductors) feeding the 2000A board.

The presence of a neutral bus bar in the APC does not tell me that the unit requires 480Y/277 to operate.
Many equipment manufacturers include filtering, not actual loads, that is intended to be used at 277V L-N and can fail if they are left floating. So when no neutral conductor exists, it is not uncommon for the neutral and ground to be connected together as you have experienced. Because of this N-G bonding there is the possibility that current will flow when the filters are not balanced (i.e. when harmonics exist).

It is not unusual to find cumulative ground currents that reach into the hundreds, this is one reason service entrance GF systems are rarely set below 200A pickup.

You say that .
What does the bonding look like now?

I find the OP's issue interesting due to the technical merits and also the fact that he is so passionate about the situation.
I'm having trouble grasping the idea that, if in fact, there is normal current flowing from the filtering circuit, that this can intentionally be allowed to flow on the EGC path as opposed to a neutral conductor of at least a size in table 250.122 and remove the neutral ground bond at the UPS supply.
Could you elaborate on this further so we can better understand this?

 

[__dan]

I have been advised to bring the matter the the AHD which likely may end my work at this facility. That is probably what I will do after notifying the hospital that I am going to do so.

The building inspector may likely fall back on the manufacturer's statements and allow whatever APC allows, not necessarily what the hospital should allow. (I have seen this).

Not wishing to advise you, if you fear for your employment you need an employment lawyer, who in the worst case could get the hospital to pay you to go away ...

There are two authorities who may protect you in this matter, you could escalate your concern to the hospital's legal compliance person or department (a good bet, maybe required by your employment contract). Not in the manner of making an accusation, in the manner of bringing your concern to the proper authority for them to investigate (and bear liability for). If the hospital is concerned, holding APC's check will make the issue hot.

Not advising you to go to OSHA, but your lawyer may. If you feel an essential safety feature, the equipment ground, has been compromised by placing a current flow on it, OSHA may have an interest, and filing your complaint with them may get you federal whistleblower protection for negative job consequences (in theory). Probably a worst case option.

If your job duty includes electrical safety, you cannot ignore your concerns for fear of your own non feasance. Liability should properly fall on the PE and the equipment manufacturer. The only person who can hold both parties to account is the hospital's legal compliance person, which is where I suggest escalating your concern to. Likely they will not know electrical, but they will know they want no BS or smoke and mirrors. Your duty to notice employer (hospital) legal compliance will make them liable for finding and fixing any deficiencies.

I feel current flow on the grounding system is truly unpredictable, especially in a hospital. There is no way to say it is benign, or if there is trouble, it could be trouble that is very difficult to find the cause of and mitigate. Most hospitals do not allow cell phone use inside the building for fear of EMF. Current flow on the grounding system may make measurable EMF, and a legitimate concern.

You want it fixed, but you also want to be in the clear if there are any problems found down the road. APC will not blow off the hospital's lawyer. Citing code like NEC 250.6, will place the ball in their court. There may be other citations but you indicated 250.30 (A) 3 looks done.

At the very least, it is an issue the hospital must be given notice of so if they choose to accept this condition, they may give informed consent.

 

[pbunn]

I have a meeting with the hospital staff next week. I will try to educate them more on 260.6 . We will see where this goes.

I may not have mentioned this earlier, but another concern is why the grounding conductor current rises to near double when the emergency generator come on line. May possibly be that the APC unit starts charging batteries at this point.