ground rod, will not open breaker.

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reyamkram

Senior Member
Location
Hanover park, il
is it true that a ground rod is only used for lighting, and will not open a breaker, when there is a ground fault, and I was told that is no
purpose, putting ground rods on industrial machinery, and the ground rod can do more harm then good, if lighting hit close to the building,
it can go up to the ground rod and fry the machine

Thank you, for all and any information, on this subject.
 

jumper

Senior Member
Read this first and then we can continue. Note nothing about tripping breakers or overcurrent.

250.4(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded 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 and that will stabilize the voltage to earth during normal operation.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
is it true that a ground rod is only used for lighting, and will not open a breaker, when there is a ground fault, and I was told that is no
purpose, putting ground rods on industrial machinery, and the ground rod can do more harm then good, if lighting hit close to the building,
it can go up to the ground rod and fry the machine

Thank you, for all and any information, on this subject.
in general there is no operational or safety benefit to having a ground rod next to a machine even if it is installed to code. If it is not installed to code it can be hazardous.

It generally is not going to help in opening a ground fault. If done correctly it won't make it any worse. If done wrong it may cause a ground fault to never clear

As for lightning traveling through the Earth and sneaking up inside a building through a ground rod I suppose it's possible. It's also possible for lightning to strike the building and travel through the building structure and get to the same point.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
is it true that a ground rod is only used for lighting, and will not open a breaker, when there is a ground fault,
Let's just look at the simple math of ohms law, say the rod might have a resistance to earth measuring 25 ohms (very unlikely) now considering I = E / R.

Let's look at a residential circuit faulting at 120V

120 / 25 = 4.8A, won't open a 15 amp breaker.

In reality most rods (even two or more) probably measure in the hundreds or thousands of ohms.

Roger
 

Ingenieur

Senior Member
Location
Earth
the primary function is surge V suppression and voltage stabilization
the egc is your gf sc protection

if a person touches a 'hot' current will flow to where he is standing (assume conductive)
it will seek the path of least R, which is likely the egc from the V source
or the nearest grounded j-box, etc
but it will use ALL paths, other egc's, etc, all those parallel paths result in a very low R
some will travel thru earth, but very little

if the rod is 25 Ohm and the egc system 1, 24/25ths of the current will go thru egc's
if the person is 1000 Ohm and v=120
eff R is 1000 + 25//1 = 1000 + 0.96 = 1000.96
i = 120/1000.96 = 119.99 mA
20 A cb will not trip
gfci will

assume no person, just hot to gnd
120/0.96 = 125 A, cb will trip
no egc
120/25 = 4.8 A no trip on a 20 cb


having said that its effectivness at tripping a cb on an line-earth fault depends on 3 things
the beds R
the system voltage
the trip setting

assume a 4 Ohm bed
standing on earth, no egc
i = 120/4 = 30, a 20 cb will trip but not fast

in any personal shock scenario the person is the limiting factor
typ human 1000 Ohm >>> any bed
hence the need for gfci
 

jumper

Senior Member
Mike Holt link on the subject.

https://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/Grounding-Bonding-Poster~20030914.htm

touch.gif
 

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user 100

Senior Member
Location
texas
putting ground rods on industrial machinery, and the ground rod can do more harm then good, if lighting hit close to the building,
it can go up to the ground rod and fry the machine

Thank you, for all and any information, on this subject.

Looked at the other way----if there is a lightning strike or higher than normal voltages (HV drops on sec going to a house/conductors that supply equipment), a rod will at least HELP provide another path other than the bldg/premises wiring/equipment etc for all that extra current.

I used the phrase "HELP" b/c, bear in mind that current from strikes can spike up into the hundreds of thousands of amps, a rod generally won't defend against that at all----might only remove some of that current, but is by no means a reliable panacea for completely preventing damage, nor is it intended to be.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Looked at the other way----if there is a lightning strike or higher than normal voltages (HV drops on sec going to a house/conductors that supply equipment), a rod will at least HELP provide another path other than the bldg/premises wiring/equipment etc for all that extra current.
Interestingly, I investigated a fire caused by a primary-to-secondary fault a few years ago. No water-pipe ground had been connected when an illegal service upgrade had been done in conjunction with a non-permitted central-A/C installation. The line-side service cable was still sized for 60a, even though the load side was sized for 200. Also, no cable clamps were used in the new panel.

I concluded that the fire was caused by the grossly-overloaded #10 EGC in the NM cable feeding the water heater, as it was the only pathway for the current back to the source neutral. :rant:

Added: No breakers tripped.
 
Last edited:

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Interestingly, I investigated a fire caused by a primary-to-secondary fault a few years ago. No water-pipe ground had been connected when an illegal service upgrade had been done in conjunction with a non-permitted central-A/C installation. The line-side service cable was still sized for 60a, even though the load side was sized for 200. Also, no cable clamps were used in the new panel.

I concluded that the fire was caused by the grossly-overloaded #10 EGC in the NM cable feeding the water heater, as it was the only pathway for the current back to the source neutral. :rant:

Added: No breakers tripped.

And thats why we bond the water pipes. The NEC is not stupid, thats for sure.


Yes- a ground rod will not trip a standard breaker at 120 volts. And even if it did or does, the NEC does not allow a ground rod to act as a fault current path.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
And thats why we bond the water pipes. The NEC is not stupid, thats for sure.


Yes- a ground rod will not trip a standard breaker at 120 volts. And even if it did or does, the NEC does not allow a ground rod to act as a fault current path.
More accurately the NEC requires a *fault-clearing* path.
You can't prevent a rod from being a fault current path, current division by conductance being what it is. But you cannot let the rod be the only fault current path, nor even a parallel path which is necessary for fault clearing through the OCPD.

Sent from my XT1585 using Tapatalk
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
More accurately the NEC requires a *fault-clearing* path.
You can't prevent a rod from being a fault current path, current division by conductance being what it is. But you cannot let the rod be the only fault current path, nor even a parallel path which is necessary for fault clearing through the OCPD.

Sent from my XT1585 using Tapatalk

Good point- wrong choice of words on my part.
 

MTW

Senior Member
Location
SE Michigan
is it true that a ground rod is only used for lighting, and will not open a breaker, when there is a ground fault, and I was told that is no purpose, putting ground rods on industrial machinery, and the ground rod can do more harm then good, if lighting hit close to the building, it can go up to the ground rod and fry the machine

Thank you, for all and any information, on this subject.

For an explanation see this video starting at the 45 minute mark for this specific case example about adding a supplemental earth electrode at a machine location.

https://www.youtube.com/watch?v=mpgAVE4UwFw&feature=youtu.be

It's well worth your time to study the entire thing, but the 45 minute mark addresses the bolded part specifically. It's not against code to add a supplemental earth electrode, but is not best practice in most cases. Best practice is to pull a dedicated isolated equipment grounding conductor with the supply conductors, and connect to the main service electrode bonding point.

The idea is that when a surge pulse comes along and enters the system somewhere, be it from the utility, the sky or the earth, you want everything bonded together at the same exact potential reference. So that EVERYTHING rises and falls in unison, with the disturbance pulse.

It is when you have differing connection points that gives you, differences of potential between those different connection points, that can cause damage, when current flows to dissipate the surge. Small differences in impedance and resistance in the earthing system, can result in large current flows, while the disturbance pulse is dissipating.

MTW
 
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