System Grounding

Re: System Grounding

Two general reasons for grounding...

First would be to prevent metal from being energized and causing a shock/fire hazard. Ideally, all metal (plumbing, cases of electrically powered equipment, etc.) should be grounded. That way, if there is a short (for example inside a metal case) the short circuit will cause excessive current to flow and blow a fuse/circuit breaker.

The second reason is to safely direct lightning strikes and static charges safely to ground instead of bringing energy into your home/work/etc.

The recommendation is to only tie the AC and DC grounds together in one place to prevent multiple ground/return current paths...

As an example... You tie the negative terminal of the battery to the AC ground rod (one place--per "rules"). And say you use a car 12 volt radio or CB unit. The meal case of the radio is -12 VDC (case is the power return in a car). And you have an outside whip antenna that has the ground braid tied to earth ground where it enters the home (for lightning/static charge build up suppression).

And here is the problem... If the antenna ground is tied to AC ground--you now have two electrical paths for your return current... One is through the -12 vdc return lead, the second is through the antenna ground braid. Depending on 12 volt loads, wire gauge, and wire length, the return current may actually go through the antenna braid to AC/DC ground point to negative battery terminal. If there are high 12 VDC currents (say a 1,200 watt inverter, > 100 Amps DC), much of the 100 amps may return through the antenna braid/safety ground--And the antenna braid or safety ground may overheat with the >100 amps if the 12 vdc return has higher resistance or if the -12 VDC connection broke for some reason.

The other issue is lightning. Say you tied the antenna braid to a separate ground rod at the edge of the home (separate DC ground--should be OK?--No)... Now you have a two point DC ground system. One point is at the AC ground rod, and because of the common -12 VDC connection, you have a second ground at the antenna earth ground.

You get a nearby lightning strike and it causes a high voltage "gradient" from the lightning strike through the ground out to your home. You have two ground rods XX feet apart. Once may be thousands of volts above the other because of the lightning strike--and that voltage/current is now injected into your DC ground/return system from the antenna ground rod, through your 12 VDC return, to the battery bank, to the AC/DC ground rod.

The above is one example of a "ground loop" that should be avoided--and can be difficult to avoid with "car electrical equipment" which assumes that 12 volt return and 12 volt ground are the same connection.

There are other possible DC ground loops (several DC powered equipment and audio cables with common shield grounds connecting the equipment together would be another example).

These sort of issues are another reason why I tend to like "AC inverters" better than many direct 12 VDC equipment. AC equipment has a separate AC safety ground separate from the AC electrical power connections (and a single point AC ground to AC neutral connection typically in the main electrical panel).

-Bill

Re: System Grounding

Part 1:

You are very welcome Edward... I try to give the reasons behind the rules since it is almost impossible to have a rule that addresses every situation.

keyturbocars wrote: »

I have the new Midnite Classic controller with built in ground fault protection. In the Classic instructions, it says that with this safety feature built in, then the negative side of the battery bank should not be tied to ground. So, what I was planning to do was change my system so that I have a grounding busbar where I will tie together the following grounds: Classic controller, 2 DC breaker boxes, case ground from inverter. Then I plan to run a 6 gauge ground wire several feet over to my AC ground and tie it into that one place.

Ok, I am going into deep water here... And take what I say with a grain of salt. boB or Ryan would probably a much better source of information regarding this than I.

As I understand people's solutions to the NEC requirement for ground fault protection for DC solar arrays, it really does violate the very definition of safety ground.

Pretty much all DC GF systems put a 1-5 amp fuse/breaker/ or in the case of the Classic a 1 amp PTC Resistor (basically a resistor that when it passes too much current gets hot and dramatically increases its resistance) between DC ground and safety ground (add missing text).

At this point, the popping of a fuse/opening a breaker will open the connection between the + solar array and the DC charge controller (or GT inverter) (the 5 amp breaker trips, then this trips the 80 amp breaker too with the handle bar/rod).

In the case of the Classic and a few others, the "high voltage" across the open fuse or hot PTC signals the charge controller (or GT inverter) to stop conversion.

Sort of sounds OK--but, from my humble point of view, all they have done is stopped the power transfer into/through the DC input of the device.

The have not limited to less than 5 milliamps (~limit that can cause heart failure), and they have now potentially energized the DC grounded section of the circuitry by "floating" the ground (open fuse, open breaker, hot PTC)...

In the "olden days", when something was ground referenced, it was done with at least a 6 gauge piece of wire that could handle >600 amps of fusing current and would guarantee to trip a circuit breaker/fuse in the + or hot leads from the energy source (DC or AC systems).

Now, your DC "ground" is a 1-5 amp fuse, that when popped, it has now un-referenced your ground system. And, depending on what happens, it has de-referenced your otherwise operating battery bank--while just stopping the solar panels from charging the batteries (hope you notice in time the warning LED, beeper, etc.).

This is typically considered very bad form (again from my humble opinion) to have the ground automatically lift... Heck, when I designed/installed AC equipment we had to use a separate ground stud so that when a person was servicing the device they could not unhook the green wire by accident (or double nut the safety ground/other ground connections).

And you don't know where the ground loop problems are going to develop... For example, a very common ground loop problem in large systems and high current systems was for people to burn out the RS-232 ground wire between a computer and a modem/printer/terminal/etc. because of mismatched ground references (RS-232 is a terrible interface because of the common ground signaling system).

Anyway... Either connect the Ground Fault system as defined in the manual and as required by the NEC...

OR, hard ground your DC battery negative bus to your common AC/DC ground rod (one point). And disable the GF detection circuitry on the DC Charge controller.

Again, this is not a hit against the Classic or other brands/models of charge controllers... It is just my personal belief that the NEC does not know what it is doing with DC.

If they want the safety of a floating system (in AC, this is done with an isolation transformer, and can be done with DC switching power supplies with slighting higher costs)--They should mandate that (and the testing/monitoring system required to self test/determine when the isolation has failed)... But, nnnnoooo, they will not do that because the could do the Rube Goldberg Machine with a ganged 5 amp / 80 amp (or whatever) dual circuit breaker setup to protect against one sort of failure (positive lead of array to safety ground short)... Does not protect against any other types of failures and creates a whole new set of problems.

OK--Off my soap box now. And I completely understand if others have a different opinion than I... Always open to new ideas/interpretations.

Re: System Grounding

Part 3:

We have a few threads that talk about lightning protection. Sometimes the worst is mixing different grounding theories together in one installation.

BB. wrote: »

A couple threads about Lightning:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's consolidated FAQ page:

www.windsun.com
Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

2) I wonder if somehow that 3 phase lightning arrestor being connected to my single point AC/DC ground tie will cause any potential problems?

Not by itself... It should not make things worse... Personally, unless somebody has a better idea, I would put the 3 phase arrester fairly close to your main AC/DC ground rod and run a nice heavy "multi-strand" wire from the arrestor ground to the AC/DC ground rod (solid wire does not carry lightning as well as stranded/braided cable--You can read about skin effect).

Again, I am certainly no lightning expert--but the above is how I understand the basics of the problem.

If others here (or you have a good local expert)--certainly listen to them and see who has a good record regarding lightning suppression.

At times, after reading other "stories"--I am not sure if it is very good lightning/grounding practices that make for a rugged/safe system--Or it was simply very poorly done (or none) lightning control systems that are problematic.

westom wrote: »

A professional application note demonstrates single point earthing. Two structures. Each must have a single point ground. Any wire entering but structures must connect to that structures single point ground before entering:
http://www.erico.com/public/library/fep/technotes/tncr002.pdf

Earthing requirement applies to every wire in every cable. Even if that cable is underground.

If two structures violate the concept, then a direct lightning strike to one building may become a lightning rod connecting that surge into appliances inside the second building.

Earthing wires to some 3 meter ground rods means a massive increase in surge protection. But better informed builders start protection when footings are poured. Ufer grounds were pioneered in munitions dumps so that direct lightning does not cause explosions. Technology is that routine when surge damage is not acceptable. Effective because concrete is a good electrical conductor.

Buried ground loop is another solution. Or how one solve earthing atop a rocky mountain:
http://scott-inc.com/html/ufer.htm

A case study of how all surge damage was eliminated in a Nebraska radio station by not wasting money on protectors. By fixing defects - bad earthing. They even fixed the utility pole transformer earth ground. Each layer of protection is only defined by that earthing electrode - each single point ground; not by any protector:
http://www.copper.org/applications/electrical/pq/casestudy/nebraska.html

Welcome to what advertising and salesmen in a big box TV / computer store will not discuss to enrich themselves at your expense.

-Bill

Re: System Grounding

I am not sure... It may not have gone through. The link you posted did not work (appeared to be just a "create new post" link or similar--it was not to a specific thread/post).

And I checked pretty quickly after you posted the link, and did not find any new posts at all (other than an earlier post to an existing thread by you on another subject).

Just send a PM to the admin (Ryan?) and see what he says.

I think they are pretty open and he probably would have notified you somehow if the post was disallowed for some reason.

-Bill

Re: System Grounding

I have just posted this also on the Midnite forum:

After a PM to boB--I will post this statement. This is my personal opinion and has not be blessed by Midnite Solar, this forum, or any others. This is just my opinion/interpretation of the various regulatory codes as they exist today (as I understand them). (boB is not making add this--It is my honest opinion ).

The NEC DC GFP system has little to no safety improvement and in fact violates several NRTL Recognition/Listing requirements for multiple countries.

And because of the fundamental flaws in the NEC DC GFP requirements/system, it should immediately be disabled and permanent DC Return/Earth Grounds be used in place of the sensing fuse in DC / Off Grid Systems as a safety/fire hazard.

This includes Hybrid inverter systems that use DC Solar Charge Controllers as part of their DC power design.

I am not too keen on having NEC DC GFP on Grid Tied (Utility Interactive) Inverter systems (I don't think it adds very much in the way of safety) and it does create a nightmare in terms of a ambiguously grounded solar array. And it creates potential confusion of what is or is not grounded in the DC PV Array side of the circuitry--which is always a potentially dangerous situation. Also, I do not know how the NEC DC GFP system affects the new non-galvanically isolated GT inverters. I don't like NEC DC GFP and believe it makes things less safe for a service person--but I don't know any method of bypassing the NEC DC GFP circuitry--and in any case, it may be different by vendor/model number anyway.

Now the details why:

Per Kent0's nice drawing--Yes, that is one of the two (as far as I can tell) failure modes, that DC GFP, is designed to detect/protect against, it actually makes the system very unsafe and an actual safety/fire hazard if it operates as designed (per NEC requirements--Again, not picking on Midnite--They are giving their customers the option of NEC DC GFP grounding or the traditional hard ground of the return bus/earth ground that does not "meet" current NEC).

If there is a fault between +Vpanel (assuming negative ground system) and frame/earth ground, this causes the whole DC battery wiring to no longer meet ELV ratings (more or less, no longer "touch safe") because of the opening of the Ground Fault Sense Fuse/Breaker. Now all down stream 12-48 VDC components/wiring/devices/etc. have to rated for 150+ VDC and Highpot tested to a minimum of 750 VDC. (It has been decades since I have been at this level of design and test for safety/code issues--so I have probably messed some of the details up--but the basics, I believe, are still correct).

And this causes a whole set of nightmares for device manufacturers. For example, the fuse/breaker/PTC needs now to be rated to 150 VDC (or higher for high voltage array product). And if there are any capacitors from -DC to Frame ground, those now need >150 VDC rating. Also, need to check barrier requirements between -DC (and +DC) and Frame ground now that you can have >150 VDC between DC power and frame ground.

Now this means that the DC side of a PV system cannot be considered to be SELV (Safety Extra Low Voltage).

Assuming Wiki is more current than my knowledge:

http://en.wikipedia.org/wiki/Extra-low_voltage

Separated or safety extra-low voltage (SELV)

IEC defines a SELV system as "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits".

This outright affects and violates UL/NRTL registration/listings...

The only way that a NEC DC GFP system would not violate the SELV clause is if Voc-cold is guaranteed to never exceed 120 VDC. I am not sure how solar panels are rated to ensure that this requirement could be satisfied, especially now that Listed Solar panels are rated to 600-1,000 volt (600 VAC / 1,000 VDC?).

And even if the the SELV rating of 120 VDC is not exceeded, I still believe the inversion of the safety ground reference is unsafe because the normal single pole DC breaker/fuses are now on the return lines when the ground reference is reversed (and the Ground Fault detection fuse/breaker is opened). This is also a violation of multiple safety requirements and create both a serious shock hazard and fire hazard.

Personally, I would recommend that people never install (or configure) a NEC type DC GFP system. It is not safe.

-Bill

Re: System Grounding

For anyone that is keeping track, a member from the Midnite forum contacted Mr. Wiles recently, and this was the answer:

http://midnitesolar.com/smf_forum/index.php?topic=142.msg2467#msg2467

author=laszlo « Reply #34 on: December 21, 2011, 11:33:41 AM »
I emailed Wiles a while back asking for his comments about Bill's paper.

He said no changes are planned in the NEC regarding GFP, and the standard the Bill was referring to is a European standard and does not apply here in the US.

All I know. And thank you Laszlo.

-Bill