Detached garage grounding

[Del91574]

You need to consider the AWG of the ground cable and path. The key in the referenced unit is the ohm rating that is given (actually pretty much a quote from the NEC) that would reference the ground rod installed itself.
 
The unit you posted would protect the connected equipment assuming a common building system ground plane and the appropriate resistance. It's not going to protect from a surge coming in on system cabling, just a surge coming in on the HV lines....think about the distance involved and how long it'll take for the unit to shunt the surge in relation to any field connected component and cabling length to that component.

 

[RAL]

With all that we have said in this discussion about the path between a surge device and earth needing to be shorter and have less resistance than the path between that same surge device and the equipment it is protecting I want to know how can this device work when it is grounded to an AC outlet? I understood that grounding to an AC outlet is not sufficient protection.
 
https://www.jmac.com/v/vspfiles/files/PMFiles/ditek/InstallationManual/1F31X-InstallationManual.pdf
 
Mike.

 
I would describe the case where you have a shorter, lower resistance path from the surge protector to earth ground than there is to the device being protected as being the the gold standard, and something to do if possible. Grounding the surge protector to the AC outlet ground is far from ideal, but probably better than nothing. 

 

[drvnbysound]

I'm pretty glad this topic was revived... Can we consider a scenario? I would assume this is probably close to the norm...
 

 
Home has standard meter with 12-space breaker/service disconnect panel connected to Earth Ground. This main panel has (2) spaces are used for a breaker that supply the home sub-panel (ground connected to main panel ground), and another (2) spaces are used to supply a sub-panel in the garage (ground also connected back to main panel ground).
 
The detached garage is roughly 50-ft from the home; due to panel locations wiring between the two is around 100-ft in length from termination to termination.
 
Planning to put a separate transformer and power supply in the detached garage along with a surge device [before the P212S]. What would be the recommended Earth Ground connection/location for the surge device?

 

[mikefamig]

The garage should have two ground stakes in the earth that the sub panel is grounded to in addition to being connected to the earth ground at the house. All grounds are connected together. Just ground your surge protection device in either /both buildings to the ground with as short a path as possible either directly to the earth stake outside or in the sub panel.
 
Mike.
 
EDIT
 
AWG 6 wire is recommended.

 

[mikefamig]

I don't know what NEC specifies as an acceptable ground but Ditek does state that the point of ground that you attach their surge protection device to should have no more than 25 ohms resistance to earth ground and ideally no more than 5 ohms.
 
I know that the resistance between your point of connection and earth can be calculated if the wire gauge, type and length is known but I don't know how you would measure it to prove  it. It seems to me that if you try to measure it with an ohm meter then the length of the test lead to the earth ground would be as long as your existing path to the earth ground and it would add resistance to what the meter reads.
 
Mike.

 

[Del91574]

The garage should have two ground stakes in the earth that the sub panel is grounded to in addition to being connected to the earth ground at the house. All grounds are connected together. Just ground your surge protection device in either /both buildings to the ground with as short a path as possible either directly to the earth stake outside or in the sub panel.
 
Mike.
 
EDIT
 
AWG 6 wire is recommended.

6 AWG would be pretty heavy. You only need to plan for the ampacity of the expected surge or fault. Closest EG would be the connection where practicable.
 
 

I know that the resistance between your point of connection and earth can be calculated if the wire gauge, type and length is known but I don't know how you would measure it to prove  it. It seems to me that if you try to measure it with an ohm meter then the length of the test lead to the earth ground would be as long as your existing path to the earth ground and it would add resistance to what the meter reads.
 
Mike.

Very easy if you know the AWG and material (AL or CU) tables for standard resistance values are known and distance can be ballparked if the footage isn't labelled on the cable. You're not metering out the electrode itself, but the path. Meter between the ground clamp and the opposite lead.

 

[mikefamig]

DEL


Very easy if you know the AWG and material (AL or CU) tables for standard resistance values are known and distance can be ballparked if the footage isn't labelled on the cable. You're not metering out the electrode itself, but the path. Meter between the ground clamp and the opposite lead.
 
-------------------------
Yes I understand that the impedance can be calculated if you know the wire type and length but I still don't see how you can use a meter to measure the impedance when the two points that you are testing are a great distance from each other. If I am installing a surge device in the Elk box that is 30 feet from the outside earth ground and I want to measure resistance from that ground point inside the box to the earth ground outside 30 feet away then my test lead on the meter would have to be 30 feet long in order to reach the rod from teh Elk box. No? Wouldn't that affect the meter reading? Wouldn't any conductor 30 feet long cause impedance that would be measured on the gauge?
 
Mike.

 

[drvnbysound]

The garage should have two ground stakes in the earth that the sub panel is grounded to in addition to being connected to the earth ground at the house. All grounds are connected together. Just ground your surge protection device in either /both buildings to the ground with as short a path as possible either directly to the earth stake outside or in the sub panel.
 
Mike.
 
EDIT
 
AWG 6 wire is recommended.

 
Why two?  
 
Currently the detached garage has no ground stakes; the only ground is back to the main panel which is about 100-ft away (rough total wire length).
 
If I add a ground rod to what's already in place [in the detached garage], does that create an issue where I would have multiple panels (home and garage) connected to multiple Earth ground points [even if they are all tied together]? I thought multiple Earth grounds was a bad thing...

 

[mikefamig]

1 - Two earth ground rods are required by NEC. It is due to the fact that while earth does absorb electrical current harmlessly it takes time and space for that current to dissapate into the earth. NEC code calls for two rods about 8 feet long and about 8 feet apart from each other.. I say "about" because I don't remember the exact code. The two rods should have been installed by the electrician that installed the panel at build time.
 
2 - Multiple ground rods can cause a problem with the LV power supply on the security system because it is not the same animal as the ground on your mains electrical system. The ground in your mains is to capture stray current leading it safely to earth and in doing so protect equipment from harm.
 
The common conductor in a system like the security system is a reference point for the DC current supplied by the power supply and does not need to be attached to earth at all. Imagine that there is no connection to an earth ground and only a path back to the negative terminal at the power source. Voltage measurements taken at your aux power supply should refer back to the same common reference as everything else in the system.
 
That's the best I can do in a few words and with a few beers in me. Maybe someone else will help me out here.
 
Mike.

 

[RAL]

Grounding for a detached building with a common feed is covered under section 250.32 of the NEC.
 
Here is an article that explains what is required, with illustrations.
 
http://iaeimagazine.org/magazine/2008/09/16/two-buildings-common-service-grounding-requirements/

 

[drvnbysound]

1 - Two earth ground rods are required by NEC. It is due to the fact that while earth does absorb electrical current harmlessly it takes time and space for that current to dissapate into the earth. NEC code calls for two rods about 8 feet long and about 8 feet apart from each other.. I say "about" because I don't remember the exact code. The two rods should have been installed by the electrician that installed the panel at build time.
 
2 - Multiple ground rods can cause a problem with the LV power supply on the security system because it is not the same animal as the ground on your mains electrical system. The ground in your mains is to capture stray current leading it safely to earth and in doing so protect equipment from harm.
 
The common conductor in a system like the security system is a reference point for the DC current supplied by the power supply and does not need to be attached to earth at all. Imagine that there is no connection to an earth ground and only a path back to the negative terminal at the power source. Voltage measurements taken at your aux power supply should refer back to the same common reference as everything else in the system.
 
That's the best I can do in a few words and with a few beers in me. Maybe someone else will help me out here.
 
Mike.

 
That's very interesting to me as I inspected all sub panels this morning before making my post above and there is certainly only a single ground rod at my home's primary panel; there is a blank cover plate covering the acorn connection (aka ground rod clamp) inside the attached garage. I've also seen this to be the case in other new construction homes here.
 
Here's an example picture I took at a friend's home during its construction (2014). This is the only ground connection to the main panel on the exterior of the home. My home is wired the same way.
 

 

[RAL]

That's very interesting to me as I inspected all sub panels this morning before making my post above and there is certainly only a single ground rod at my home's primary panel; there is a blank cover plate covering the acorn connection (aka ground rod clamp) inside the attached garage. I've also seen this to be the case in other new construction homes here.
 
Here's an example picture I took at a friend's home during its construction (2014). This is the only ground connection to the main panel on the exterior of the home. My home is wired the same way.
 

 
Does your garage have a subpanel, or just a single circuit with the breaker back in the house?
 
Is that rod in the photo driven into the ground?  Looks like a piece of 1/2" rebar, which wouldn't meet code for a ground rod.
 
The NEC does allow using steel rebar of at least 20 feet in length as a grounding electrode when it is encased in concrete (e.g. a concrete floor).   But the copper wire connecting to it in the photo looks like it is #12 or #14, rather than #8.
 
If I understand you correctly, this ground rod is at the house and not the garage, and is inside?

 

[video321]

1 - Two earth ground rods are required by NEC. It is due to the fact that while earth does absorb electrical current harmlessly it takes time and space for that current to dissapate into the earth. NEC code calls for two rods about 8 feet long and about 8 feet apart from each other.. I say "about" because I don't remember the exact code. The two rods should have been installed by the electrician that installed the panel at build time.

 
No, that's not true.
 
The NEC doesn't require 2 ground rods - it specifies that if a single ground rod doesn't meet the 25 ohms - or whatever it is - then a second rod must be installed. However, after the 2nd rod is connected it doesn't matter what the impedance is at that point (it could be 100 ohms!) as that is all the NEC requires.
 
So... if the electrician can prove the ground is less than 25 ohms then his job is done. If not, drive that other rod!

 

[mikefamig]

No, that's not true.
 
The NEC doesn't require 2 ground rods - it specifies that if a single ground rod doesn't meet the 25 ohms - or whatever it is - then a second rod must be installed. However, after the 2nd rod is connected it doesn't matter what the impedance is at that point (it could be 100 ohms!) as that is all the NEC requires.
 
So... if the electrician can prove the ground is less than 25 ohms then his job is done. If not, drive that other rod!

 
 
I'm not an electrician and I was repeating what my local inspector told me to do when I installed the service to my garage. Just as well though, two rods can't hurt anything. It is also important to isolate the neutral from the garage at the sub panel. The neutral should be grounded in only one point in the entire system which is usually at the load center in the main building (house).
 
At any rate it is fine to ground a surge protector to an existing ground rod or to drive a new rod and ground to that. I can't see any difference as long as you maintain the less than 25 ohms rule.
 
Mike.

 

[RAL]

If I add a ground rod to what's already in place [in the detached garage], does that create an issue where I would have multiple panels (home and garage) connected to multiple Earth ground points [even if they are all tied together]? I thought multiple Earth grounds was a bad thing...

 
The purpose of the earth ground is to carry dangerous currents from surges (usually from lightning) to a point outside the house so the surge can be safely dissipated.
 
When there is a lightning strike, there can be a ground potential rise (GPR) between different points. If your house wiring is grounded to earth at more than one point, say at opposite ends of the house, GPR can cause a very large difference in voltage between the two ground rods.  That difference will cause the surge to enter the house through the wiring as it takes the path of least resistance from one ground point to the other.  That's exactly what you don't want to happen.
 
With two separate buildings, it's different.  There may be a GPR between the ground rod at each building.  But the surge will not enter either of the buildings as it dissipates. It will stay outside as it flows from one building to the other.