Ground rod for stand alone refrigeration units

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MotoRand

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Post Falls, Idaho
Our company is building a 20 foot refrigeration container that will have an on board generator, shore power connections, and the ability to run off of diesel power system, during transport. I have recommended that each system should be shipped with a ground rod, due to the fact that many units may be set up on the ground, running off of shore power, or the on board generator. I have received a fair amount of arguments against including a ground rod. My reasoning would include 250.32 and the following:

A major purpose of “grounding” is toprovide a path so a “short” will trip a breaker. That requires a low resistancepath back to the power source, which is the utility transformer. The pathrelies on a neutral-ground connection required at all [not entirely true]services.
The second "grounding"function is actually a collection of three safety functions:

  2. minimize the voltage between exposed metal and the earth
  3. minimize the voltage between the power wires and the earth
  4. provide a sink for lightning, power line crosses, and similar hazards
The 1st grounding function describedabove is accomplished by connecting the grounding conductors at theservice to an earthing electrode.
The 2nd grounding function describedabove is accomplished by connecting the power neutral (the neutral wireentering from the utility company's service) at the service to an earthingelectrode (ground rod).
Does anyone have any additional thoughts for or against?
 
So the intent is to run a cord to supply a refrigerated container and have an on board generator for shipping and power outage. Similar to any box type refrigerated storage unit
 
david said:
So the intent is to run a cord to supply a refrigerated container and have an on board generator for shipping and power outage. Similar to any box type refrigerated storage unit
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That is correct, it will be set up in the field, running off of a generator mini grid system. It may also be powered by shore power, and as a short term fix, it may run off of an on board generator.
 
MotoRand said:
That is correct, it will be set up in the field, running off of a generator mini grid system. It may also be powered by shore power, and as a short term fix, it may run off of an on board generator.
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I think this would be define as a portable structure, and i would apply criteria in 250.34 Portable and vehicle mounted generators
 
MotoRand said:
Our company is building a 20 foot refrigeration container that will have an on board generator, shore power connections, and the ability to run off of diesel power system, during transport. I have recommended that each system should be shipped with a ground rod, due to the fact that many units may be set up on the ground, running off of shore power, or the on board generator. I have received a fair amount of arguments against including a ground rod. My reasoning would include 250.32 and the following:

A major purpose of “grounding” is toprovide a path so a “short” will trip a breaker. That requires a low resistancepath back to the power source, which is the utility transformer. The pathrelies on a neutral-ground connection required at all [not entirely true]services.
The second "grounding"function is actually a collection of three safety functions:

  1. minimize the voltage between exposed metal and the earth
  2. minimize the voltage between the power wires and the earth
  3. provide a sink for lightning, power line crosses, and similar hazards
The 1st grounding function describedabove is accomplished by connecting the grounding conductors at theservice to an earthing electrode.
The 2nd grounding function describedabove is accomplished by connecting the power neutral (the neutral wireentering from the utility company's service) at the service to an earthingelectrode (ground rod).
Does anyone have any additional thoughts for or against?
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?????????????? To me, the first function above is now highlighted in green. That is not actually grounding as you call it, it is bonding. So replace your word in quotes to bonding and you have it. The second function as you call it is grounding. So if 1 2 or 3 is important to you ,then by all means drive a ground rod, if you have metal to earth contact between the container and earth, I wouldn't worry about it. Many people here and I believe Mike himself aren't overly enamored with the efficacy of grounding, period. I don't really have enough expertise to have an opinion, but I do respect many here who do.
 
Strathead said:
The second function as you call it is grounding. So if 1 2 or 3 is important to you ,then by all means drive a ground rod, if you have metal to earth contact between the container and earth, I wouldn't worry about it. Many people here and I believe Mike himself aren't overly enamored with the efficacy of grounding, period.
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I'm not sure i would make that conclusion about earthing a system, I think the emphasis is on the location of earthing a system and understanding that grounding electrodes to not provide a fault clearing path and that is not the purpose of earthing a system.
 
I would think 250.32(A) applies and this is treated like a separate structure, with a grounding electrode required (at least when it is set at a site and fed from 'shore power').
 
You are confusing grounding and bonding. Grounding is protection against lightning and HV contact. Bonding provides the low impedance path to the source.
Nearly all services - your system would be considered a service and require grounding, as with the generator you are the utility.
 
Might I add that when this unit finds itself parked on a concrete slab somewhere, no one is going to get out there hammer drill, bore a hole in the concrete and drive ground rod(s) for a temporary situation.
Just sayin
 
mopowr steve said:
Might I add that when this unit finds itself parked on a concrete slab somewhere, no one is going to get out there hammer drill, bore a hole in the concrete and drive ground rod(s) for a temporary situation.
Just sayin
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Then the 20 foot container itself (I presume it is metal) on concrete slab may serve as 'ground' if code so permits.
 
tom baker said:
You are confusing grounding and bonding. Grounding is protection against lightning and HV contact. Bonding provides the low impedance path to the source.
Nearly all services - your system would be considered a service and require grounding, as with the generator you are the utility.
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I don't think it's a service. It is a separately distributed system and has similar rules about a grounding electrode system being required. In any case a single ground rod is not likely going to be adequate.
 
Sahib said:
Then the 20 foot container itself (I presume it is metal) on concrete slab may serve as 'ground' if code so permits.
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The situation you are describing probably does not meet the requirements for a grounding electrode.

I think a grounding electrode system is required but I don't think that a single ground rod will meet the requirements in most cases and in most cases the metal the structure is made of is not going to meet the requirements for being a grounding electrode.
 
petersonra said:
The situation you are describing probably does not meet the requirements for a grounding electrode.

I think a grounding electrode system is required but I don't think that a single ground rod will meet the requirements in most cases and in most cases the metal the structure is made of is not going to meet the requirements for being a grounding electrode.
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I tried to circumvent the issue by considering the container as part of ground ie earth which is, unfortunately, still not allowed by present code cycle.
 
petersonra said:
I don't think it's a service. It is a separately distributed system and has similar rules about a grounding electrode system being required. In any case a single ground rod is not likely going to be adequate.
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For the sake of argument, why do you think a grounding electrode system is needed. This is a piece of equipment. If I have a stand alone walk in freezer out back of a restaurant, I don't have to drive a ground rod. It isn't a separate building.
 
Strathead said:
For the sake of argument, why do you think a grounding electrode system is needed. This is a piece of equipment. If I have a stand alone walk in freezer out back of a restaurant, I don't have to drive a ground rod. It isn't a separate building.
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I agree.

When running on the onboard generator, that generator grounded conductor should be bonded to frame and EGC's run as they are for most any other application. If a fault occurs you have a low resistance path to the source.

When running on shore power we run an EGC with supply conductors - same as above, during a fault we have low resistance path back to the source.

A grounding electrode is not going to make a difference in either scenario when it comes to fault clearing, earth resistance is going to be higher than the EGC resistance.

What a grounding electrode can do is provide a path for lightning events or other high voltage incidents.
 
ground verses bonding

ground verses bonding

I understand that the circuit bonding is good.

I think there is a need for a ground rod for possible lightening strikes, or similar overvoltage problems. My concern is that there will be people coming in and out of this loading and unloading several times a day, so I would like to be sure there is a good path to ground, in case of lightning. The container is metal, so that does help with the grounding issues.
 
MotoRand said:
I understand that the circuit bonding is good.

I think there is a need for a ground rod for possible lightening strikes, or similar overvoltage problems. My concern is that there will be people coming in and out of this loading and unloading several times a day, so I would like to be sure there is a good path to ground, in case of lightning. The container is metal, so that does help with the grounding issues.
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But it is a stand alone and presumably portable thing. There are millions of refrigerated trucks/trailers out there that have onboard generator as well as shore power capability and they don't drive ground rods, kind of hard to when on the move. Other than the direct lightning strike, only other time there is an issue is nearby strike and some transient current entering via the shore power connection, which is what you have with other buildings connected to the grid. A lot of lightning damage in permanent buildings comes from nearby strikes. Direct strike is likely to have damage whether there is a grounding electrode or not.

Permanent buildings/structures don't require a grounding electrode is supplied by a branch circuit. They do if feeder or service supplied.
 
Strathead said:
For the sake of argument, why do you think a grounding electrode system is needed. This is a piece of equipment. If I have a stand alone walk in freezer out back of a restaurant, I don't have to drive a ground rod. It isn't a separate building.
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I was viewing it as a separate structure when I said I thought 250.32(A) applied. But if an EGC is run with the shore power I don't think the electrode provides any benefit beyond code compliance (if it is in fact a structure). It wasn't clear to me how 'portable' this is.
 
Strathead said:
For the sake of argument, why do you think a grounding electrode system is needed. This is a piece of equipment. If I have a stand alone walk in freezer out back of a restaurant, I don't have to drive a ground rod. It isn't a separate building.
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It is both a structure and has a generator. Both situations require a grounding electrode system. In my opinion a stand-alone walk-in freezer that is not connected to the building as a separate structure and if it's fed by more than one branch circuit requires a grounding electrode system.
 
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