Storage trailer grounding

[gsurace]

I will admit this is a situation that somebody else had questioned me about so much, that I have to make sure I am not missing something. Here is what I have:

A warehouse facility has material they keep stored in a semi trailer parked at one of their loading docks. The material needs to stay chilled so the trailer is equipped with an A/C unit on the front. It is a 230 volt, 3 phase unit that is cord connected. I installed the branch circuit via a couple of buck/boost transformers and local disconnect. The "facilities guy" is insisting that I drive a ground rod to ground the trailer. My response was absolutely not because any fault current should return on the ground wire in the cord. We have to assume that all metal parts of the trailer that could become energized are properly bonded as well.

Not only am I looking for a quick sanity check, if someone can provide a section in the code book as well, that would be fantastic!

 

[Strathead]

I will admit this is a situation that somebody else had questioned me about so much, that I have to make sure I am not missing something. Here is what I have:

A warehouse facility has material they keep stored in a semi trailer parked at one of their loading docks. The material needs to stay chilled so the trailer is equipped with an A/C unit on the front. It is a 230 volt, 3 phase unit that is cord connected. I installed the branch circuit via a couple of buck/boost transformers and local disconnect. The "facilities guy" is insisting that I drive a ground rod to ground the trailer. My response was absolutely not because any fault current should return on the ground wire in the cord. We have to assume that all metal parts of the trailer that could become energized are properly bonded as well.

Not only am I looking for a quick sanity check, if someone can provide a section in the code book as well, that would be fantastic!

You are not correct. 250.32 Also look up the definition of multiwire branch circuit in article 100. And regarding your supposition, current will take the path of least resistance. That will be the ground wire in the cord, but what if the ground wire in the cord becomes compromised and you get a ground fault to the enclosure. Then you have a huge metal conductor, sitting on insulators (tires). I would not want to be the person who leans against it to have a smoke!

 

[petersonra]

the cord provides the egc for the return path.

pounding a ground rod in won't hurt anything but it also will not likely help anything either, especially with respect to clearing a fault. There just is too much resistance between ground and where ever the earth-neutral bond has been made to form an adequate return path.

if you are worried about the egc in the cord failing, running a separate egc from the egc in the building to the trailer is an option.

 

[Strathead]

the cord provides the egc for the return path.

pounding a ground rod in won't hurt anything but it also will not likely help anything either, especially with respect to clearing a fault. There just is too much resistance between ground and where ever the earth-neutral bond has been made to form an adequate return path.

if you are worried about the egc in the cord failing, running a separate egc from the egc in the building to the trailer is an option.

But it does offer an easier path to ground than through the shirt, skin, blood, skin, soles of the shoes and it is required by the code.

 

[iwire]

You are not correct. 250.32 Also look up the definition of multiwire branch circuit in article 100. And regarding your supposition, current will take the path of least resistance. That will be the ground wire in the cord, but what if the ground wire in the cord becomes compromised and you get a ground fault to the enclosure. Then you have a huge metal conductor, sitting on insulators (tires). I would not want to be the person who leans against it to have a smoke!

It is a single brach circuit it does not require an electrode

Furthermore an electrode would do absolutely nothing to increase safety.

 

[petersonra]

But it does offer an easier path to ground than through the shirt, skin, blood, skin, soles of the shoes and it is required by the code.

I think you are wrong about both of your assertions.

 

[Sahib]

What material is inside the trailer makes the deciding factor as to grounding of the trailer or not. If it is explosive material, grounding of trailer is desirable to pass any generated static electricity safely to ground. But if the material is any flammable liquid, grounding is a must; no exception.

 

[jap]

You are not correct. 250.32 Also look up the definition of multiwire branch circuit in article 100. And regarding your supposition, current will take the path of least resistance. That will be the ground wire in the cord, but what if the ground wire in the cord becomes compromised and you get a ground fault to the enclosure. Then you have a huge metal conductor, sitting on insulators (tires). I would not want to be the person who leans against it to have a smoke!

I Wouldnt want to be the one leaning on the trailer neither, but, driving a ground rod is not going to help.

 

[Strathead]

It is a single brach circuit it does not require an electrode

Furthermore an electrode would do absolutely nothing to increase safety.

First part, that is why I stressed the importance of looking up a multiwire branch circuit. If it is three phase feeding a piece of equipment then it seemed to me that it would then not be a branch circuit. I can see where I could be wrong.

Regarding the second part why would it not increase safety? Why would it somehow increase safety if there were multiple branch circuits, but not a single branch circuit?

 

[Strathead]

I think you are wrong about both of your assertions.

Really? There is more resistance through a copper wire through a rod in direct contact with the earth than through a person's skin and any protective clothing they may be wearing? Or is it that electricity no longer takes the path of least resistance?

 

[Lectricbota]

Really? There is more resistance through a copper wire through a rod in direct contact with the earth than through a person's skin and any protective clothing they may be wearing? Or is it that electricity no longer takes the path of least resistance?

I would think electricity takes all available paths. It would not matter if a ground rod was there or not-you're getting zapped.

 

[eprice]

Really? There is more resistance through a copper wire through a rod in direct contact with the earth than through a person's skin and any protective clothing they may be wearing? Or is it that electricity no longer takes the path of least resistance?

Electricity takes all available paths. The person will be in parallel with the grounding electrode. The fault current will take both paths. Neither path is likely to have low enough resistance to clear the fault. The danger will likely continue to exist regardless of whether a grounding electrode is there or not.

 

[gsurace]

As much as I hesitate to post questions on forums, this one did answer my question, indirectly. This forum and others that are out there have a habit of getting off topic...this one was about to.

The answer is found in 250.32(A) (thank you for that part) in the Exception. This is a single branch circuit. I knew that part was important which is why it was part of the original post. It is a multiwire branch circuit to be exact because it is 3 phase but that has no effect on the application of the code. The exception states that a grounding electrode shall not be required for this situation. Grounding is provided by the equipment grounding conductor.

250.32(B) takes it a step further and provides the requirements for the situation that I have where grounding is supplied through the branch circuit.

Thank you all for your replies. At least I didn't get accused of being an amateur like I have seen on many posts! Just remember that forums like this are the place we come with questions whether others think there are stupid or not. It's not like all of us apply all 800 pages of the code book in our businesses.

 

[Smart $]

Really? There is more resistance through a copper wire through a rod in direct contact with the earth than through a person's skin and any protective clothing they may be wearing? Or is it that electricity no longer takes the path of least resistance?

As stated, electricity takes all available paths... inversely proportional to the resistance of each path.
More below...

I would think electricity takes all available paths. It would not matter if a ground rod was there or not-you're getting zapped.

Electricity takes all available paths. The person will be in parallel with the grounding electrode. The fault current will take both paths. Neither path is likely to have low enough resistance to clear the fault. The danger will likely continue to exist regardless of whether a grounding electrode is there or not.

There's quite a few factors involved. A ground rod would reduce a present danger (ground fault with a compromised supply EGC) but not eliminate it. Without the ground rod, the earth around the trailer is at "earth" potential... essentially the same as the grounding reference of the supply system. Under the fault condition mentioned, a person contacting energized metal and standing on the ground is subject to the full potential of the fault (assuming no other path, though the stabilizing jack stands are likely another path but they could be on non-conductive cribbage). With a local ground rod, the potential between local earth and the fault is reduced by raising the voltage of nearby earth. Consider what would happen if a grounding grid was installed throughout the local area. Would be the potential be reduced between the trailer metal and nearby earth under the faulting condition?

With all that said, a ground rod is not required by Code. As of the 2011 NEC, I have to wonder if installing an electrode is actually a violation. 250.54 permits auxiliary electrodes to be connected to EGC's, but the 2011 NEC added 250.121 which disallows an EGC to be used as a grounding electrode conductor. Then we go back to 250.50 which requires all electrodes present to be bonded to GES. Its a roundy round :slaphead:

 

[Smart $]

...

The answer is found in 250.32(A) (thank you for that part) in the Exception. ...

IMO, that don't apply. The trailer is a Transport Refrigerated Unit under Article 626.

 

[Cavie]

Ground rods are highly overrated. They give you that warm and fuzzy feeling of safety here in Florida until you pull on it and find it is highly corroded and what once was 8' is now only 2' long.

 

[gsurace]

Smart$, you win the prize!

I now agree that the situation I have falls under Article 626 more specifically 626.2 Definitions (Informational Note) and 626 Part IV Transport Refrigerated Units (TRUs).

Just as I was easily influenced that I might be wrong by not having a ground rod, I was just as easily influenced that my situation resembles that of a separate storage facility.

Once again I would like remind certain folks that everyone does not utilize every article in our 800 page code book. I refer you all to Article 650!

Thank you and goodnight!

 

[RichB]

But it does offer an easier path to ground than through the shirt, skin, blood, skin, soles of the shoes and it is required by the code.

Strathead--My question now is as follows;

If what you state is true about raising the potential of the earth to a higher level and making things safer---Why are people getting shocked from luminaire standards and handholes that have RMC installed and have been properly bonded--do you mean to say that a 5/8 inch 8 foot long copper rod is going to do more good than all that steel conduit, Ufer grounds, Concrete encased Electrodes put together? And what about the spheres of influence and different step potential voltages?

A ground rod really does nothing for personal protection--that is the purpose of the EGC.

 

[jap]

Strathead--My question now is as follows;

Why are people getting shocked from luminaire standards and handholes that have RMC installed and have been properly bonded.

This seems to be a contradiction of terms to me.

If people are getting shocked from luminaire standards and handholes that have RMC installed, then these
installations arent properly bonded either.

 

[jap]

People may be getting shocked by a metal light pole if there is a ground fault to the pole and the installer
didnt bond the pole to the rigid conduit feeding it,or, the RMC is not continous all the way back to the panel,
or, they didnt bond at all and just drove a ground rod and ran a GEC directly from the pole to the ground rod only.

Otherwise the circuit should trip.