5 ohm "recommendation"

[coulter]

Regardless of his test points, exactly what type instrument did Mr Nortel use to determine that you were not within his parameters?

Fluke 87 or equavalent

carl

 

[jghrist]

There is a 5 ohm ground resistance requirement in the Motorola R-56 grounding standard for communications sites.

 

[tryinghard]

If sensitive electronic equipment needs a low ground resistance to operate that must be a bear to achieve in this type of occupancy (Tip of the hat to Bennie )

Would a better description of what sensitive electronic equipment needs to operate properly be 'effective bonding of all non-current carrying metallic objects?'

I'm with Bob on this one, if the bonding is correct more earthing "aint gona" make it better.

I sold my ground tester but a ground test of the equipment grounding conductor at the branch circuit should be the same as the resistance at the electrode, or at least extremely close. If it's not, I recommend they repair the equipment grounding (bonding). They don't need more grounding (earthing), but bonding is extremely important.

 

[tryinghard]

Often equipment manufacturers say they want a low resistance ground. Not necessarily because the equipment itself requires this to operate properly, but what they want is a equipotential ground.
Because NEC requires all equipment to be connected to ground for safety this interconnection can introduce problems between equipments that share grounds.

In the EMC consulting world they refer to ground as the sewer system of electronics. All kinds of higher frequency noise can be dumped onto the grounding system by various equipment. EMI filters dump high frequency to ground. When equipments are tied together through a common ground - high frequency noise currents can cause issues.

Excluding lightning and 60hz safety, what is more important is a zero potential ground plane between interconnected equipments than the actual connection to earth.

A parallel neutral path does exist between the service source and the service disconnect of every building/structure with a grounded system. Any branch circuitry or feeders downstream from the main that have bonding between neutral and ground (intentional/unintentional) will also have parallel current according to its resistance, returning neutral current travels all paths to source not just the path of least resistance. I think this is the biggest contributor to stray current.

Earth is not a path for fault current or a neutral return path but is a path for static, but so can the branch circuitry EGC if the system is installed correctly.

 

[dereckbc]

There is a 5 ohm ground resistance requirement in the Motorola R-56 grounding standard for communications sites.

Every swicthing and transmission equipment manufacture has a 5-ohm requirement. Problem is very few to none of them know why or can explain it.

 

[dereckbc]

OK I now have a little time to go into this. First a little background on myself. I have been in the Telecom Industry for 30 years in various capacities from digital switching/transmission to the last 15 involved with what the industry calls a Power Protection engineer that deals with AC electrical distribution, lightning protection, power quality, DC power design, UPS power design, signal reference grounding, and protective grounding. I have worked extensively with all the equipment manufactures like Nortel, Lucent, Motorola, Alcatel, Ericson, Cisco, etc? They all have slightly different grounding requirements like integrated ground planes and isolated ground planes, but they all share the same 5-Ohm ground electrode impedance requirement.

The darn thing is after 30 years of being in the biz, and asking every one of the manufactures top electrical engineers what, where, and why did the 5-ohm requirement come from? To date very few good answers and mostly lousy excuses like:

LIGHTNING PROTECTION: Hogwash answer conductors exhibit very high impedances at lightning frequencies over a given length. A 10 foot long section of 750 MCM exhibits roughly the same impedance as a #6 AWG of about 2.6 K-ohms per 10 feet. So 2.6 K in series with a 5-ohm ground electrode is the same if the ground electrode was 100 ohm?s, useless. Besides the 5-ohm impedance is a low frequency power measurement and has nothing to do high frequencies.

PROTECTS EQUIPMENT AND PERSONELL: This is a half truth, it is the job of proper bonding and use of equipment grounding conductors that protect the equipment and personnel from over voltages. As we all know 5-ohm earth impedance is not capable of protecting anything at low voltages below say 600 volts.

ELIMINATES or MINIMIZES ELECTRICAL NOISE: Again a hogwash answer, because of the same impedance problems with conductors as described in lightning protection. It is the proper design of using SDS (isolation transformers, UPS, and battery plants) special ground schemes like ?Isolated Ground Plane with Single Point Grounded power sources and frames?, ?Integrated Ground Planes?, ?Mesh Grounding?, and Signal Reference Grounds that are used to control noise and outside faults from interrupting service. Even those these methods require to be bonded to the ground electrode system, the impedance of the ground electrode system has no real importance or meaning.

So what are the good answers I have heard, learned, and believe? The same I referred too earlier in the thread, so the system is designed and planned rather than just happened at the EC discretion.

What is more important is all the various ground systems involved in a communication system (GES, AC, DC, LPS, Surge Protection, Signal Reference, etc?) is designed and integrated to work with each other to offer the maximum protection that can be afforded.

It all starts with the Ground Electrode System, and the best one that any communication building can have is a ground ring and rod combination. The reason is in any communication facility has outside services coming into the building from all four sides, and cable entrance protection needs to be done at the point of entry and have direct shortest access to the GES as physically possible. Think about that for a second and it will make sense. Walk into any properly designed telephone office, data center, microwave/radio transmission facility on earth and you will notice a ground bar installed right on the exterior walls where any cable enters the building. We commonly call then Cable Entrance Ground Bars or CEGB for short, These bars are used to bond the cable shields and cable protector right at the point of entry to divert transient, over voltage, or fault directly to earth and equalize any voltage differences once inside the facility.

So where does the 5-ohm requirement come from? Well any communication facility I have had the pleasure to be involved with the design uses a ground ring and rod GES. And I cannot think of too many that did not meet or exceed the 5-ohm maximum limit by the very nature of the ring itself. What this tells the manufacture is you have a well designed GES.

Hope that helps.

Dereck

 

[Frenetic]

Pretty interesting. I never knew lightning exhibited frequencies; I sort of imagined lightning as a DC type surge. Just goes to show you how much I know. I can only imagine the Motorola standard (while I haven?t read it), would have lightning protection as their primary focus; lightning protection in regards to avoiding equipment damage.

Grounding can sure be confusing at times. I guess because in reality it's actually rather simple, we sometimes make it out to be more difficult than it really is.

But, in regards to safety (around high voltage equipment like at a substation), low impedance is actually very desirable and there is a good reasoning behind this principal. Often times the desired impedance is much lower than 5 Ohms if you can believe that.

The ground grid, in addition to providing an equipotential plane, serves to offer a very low impedance to minimize ground potential rise. There's so much energized equipment of very high voltage within a confined space that step and touch potentials present a significant dangers to personnel who have to work inside them. The IEEE green book offers good guidance in this regards.

Most of the numbers being quoted by people in reference to impedance come from the NEC or IEEE and frankly, as you mentioned, the reasoning is somewhat limited. People tend to regurgitate what they read so that?s why there?s such a prevalence of these numbers floating around and people not really knowing the reasoning behind them. Also, while the IEEE does indeed have listed numbers, they do in fact try and avoid ?firm? numbers in regards to grounding and instead try to emphasis good engineering practice when it comes to grounding.

In any regards, due diligence is still required. If an engineer were to just build a grounding system without doing his due diligence and an accident were to occur causing injury (or worse) or equipment damage, you can bet that his design will be thoroughly scrutinized. Now, I don?t know about you, but building a system based on loose principals compared one that was at least built around known and recognized standards/numbers (legitimate or not) is an easy choice for me.

 

[zog]

Great comments Frenetic, you beat me to the punch, but I want to add...

But, in regards to safety (around high voltage equipment like at a substation), low impedance is actually very desirable and there is a good reasoning behind this principal. Often times the desired impedance is much lower than 5 Ohms if you can believe that..

1 Ohm for Generation stations

Most of the numbers being quoted by people in reference to impedance come from the NEC or IEEE and frankly, as you mentioned, the reasoning is somewhat limited. People tend to regurgitate what they read so that’s why there’s such a prevalence of these numbers floating around and people not really knowing the reasoning behind them. Also, while the IEEE does indeed have listed numbers, they do in fact try and avoid “firm” numbers in regards to grounding and instead try to emphasis good engineering practice when it comes to grounding..

Yep, always followed up by "Unless otherwise requested by the owner", or something like that, so it is really not a requirement, just a recommendation.

 

[ELA]

A parallel neutral path does exist between the service source and the service disconnect of every building/structure with a grounded system. Any branch circuitry or feeders downstream from the main that have bonding between neutral and ground (intentional/unintentional) will also have parallel current according to its resistance, returning neutral current travels all paths to source not just the path of least resistance. I think this is the biggest contributor to stray current.

Earth is not a path for fault current or a neutral return path but is a path for static, but so can the branch circuitry EGC if the system is installed correctly.

Huh?
And you quoted me ... why?

 

[gndrod]

5 ohm

5 ohm

What is more important is all the various ground systems involved in a communication system (GES, AC, DC, LPS, Surge Protection, Signal Reference, etc…) is designed and integrated to work with each other to offer the maximum protection that can be afforded.
Dereck

Russ,

Dereck has pretty well summed up the importance of integrating the various ground systems for communications, IT data centers, etc. that pertain to Data center and power protection. A 5 ohm earth ground may only be an ideal partial segment of the overall system as evidenced by the many technical manuals from major companies as mentioned. Cisco has a good download for Earthing/Bonding requirements that go further into details that includes Bonding Networks, diagrams and definitions design. www.cisco.com/en/US/products/hw/switches/ps3873/prod_installation_guide09186a00800dffee.html

rbj

 

[tryinghard]

Huh?
And you quoted me ... why?

I just expanded with your thoughts that current can/does exist on grounds. You have an interesting post; I would wonder if a zero potential ground could ever be achieved?

 

[tryinghard]

Grounding reason: 250-4(A)(1), "shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines".
250-4(A)(5), "...The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path.

And/But​

If 250-24(A)(5), 250-142(B) are respected current will nearly be eliminated on bonded [grounded] conductive materials. And if the neutral (grounded conductor) was not required to be bonded at the service equipment (250-24(C)), but only at the serving source then isolated thereafter, current on grounded conductive materials would be gone.

 

[dereckbc]

If 250-24(A)(5), 250-142(B) are respected current will nearly be eliminated on bonded [grounded] conductive materials. And if the neutral (grounded conductor) was not required to be bonded at the service equipment (250-24(C)), but only at the serving source then isolated thereafter, current on grounded conductive materials would be gone.

Not necessarily so because most of your modern electronic equipment has components like capacitors, MOV?s etc connected line-to-ground as filters and TVSS which inject current.

Now with that said I mentioned ISOALTED GROUND PLANES as a method telephone company?s use in digital switch equipment that achieves this very thing. But the equipment is specially designed where there are no L-G components installed, the equipment is installed with electrical isolation, tested to ensure it is isolated, and the power supply and equipment frames are single point grounded that prevents any current from flowing in any of the equipment frames and grounding conductors.

 

[tryinghard]

Not necessarily so because most of your modern electronic equipment has components like capacitors, MOV’s etc connected line-to-ground as filters and TVSS which inject current...

That may be true, I'm not inclined to accommodate intentional use of equipment grounding and earth to route current unless it's "fault current". Intentional placement of returning/unused current belongs on the conductor design for this purpose the neutral.

 

[ELA]

I just expanded with your thoughts that current can/does exist on grounds. You have an interesting post; I would wonder if a zero potential ground could ever be achieved?

I was attemting to point out that ground serves different purposes. The main one being for electrical safety. In the electronics world it becomes a convenient place to dump high frequency noise currents. As others have pointed out via MOVs and EMI filter caps.
Now when equipments share a common ground, one piece of equiment can thus contaminate another, especially when the common ground is not low impedance.

In the EMC world they talk of a 5:1 rule. For every 5 inches of length you should have 1" of width to provide a low impedance path for noise currents at high frequencies. The thickness is not that important, it is surface area that counts at the higher frequencies. This is of course difficult to obtain without a large surface area "ground plane".
Most equipment that states it is sensitive and dictates a low impedance ground must be provided, will also require braided wire be used for the high frequency ground. This is in addition to the NEC required EGC.

As you mentioned a true zero potential ground plane is difficult to obtain in a practical sense, especially at high frequencies.

People do like to blame improper grounding for noise issues. I once worked with an EMC consultant (hired by another company) that blamed our equipment's poor grounding for another manufacturers equipments intermittent failures. (This was two large pieces of industrial machinery installed next to each other in a Semiconductor manufacturing facility).

Next thing you know this guy was presenting a 7 page report on how poor our grounds were, as well as how the manufacturing companies facilities grounds were not low impedance enough.

I spent many hours working with this consultant and the other companies engineers proving that neither our equipement, nor the facilites grounds were at fault.

In the end it turned out to be an intermittent open cir. in a wire harness that constantly moved with automation!!! When the wire would open it created a spike that they erroniously thought to be electrical interference.

Boy did they end up looking foolish. Sure did waste a lot of time and money chasing supposed improper grounding on that one:smile:

 

[kid_stevens]

5 ohms is getting off light. In the Navy my electronic and electrical systems had to be .2 ohms to the bulkhead connector and no more than 2 ohms to the Generator ground point.

 

[dereckbc]

5 ohms is getting off light. In the Navy my electronic and electrical systems had to be .2 ohms to the bulkhead connector and no more than 2 ohms to the Generator ground point.

Kid I don't think we are talking the same thing here. We are discussing earth impedance where as I think you miight be talking cable impdeance.

 

[kid_stevens]

Yep ground or ship hull and deck not cable so after the .2 came off the cable we had to get down to 2 ohms to the Genset ground rod or hull ground point. Granted on shore the genset could get moved closer but we had to salt the ground to make the requirement.

 

[gndrod]

Yep ground or ship hull and deck not cable so after the .2 came off the cable we had to get down to 2 ohms to the Genset ground rod or hull ground point. Granted on shore the genset could get moved closer but we had to salt the ground to make the requirement.

Are your shipboard power equipment still operating a 400Hz? I use to help retro old Victory ships for down-range tracking. Just curious. rbj