Container Reefer Plugs and Grounding

[transman2]

I hope I get lots of feed back on this question. At a Container Port I have found what I believe is a serious situation where, just picture a 4 story reefer rack that can accommodate 12 reefers plugging in at 12 outlets. Its a 3 phase 480y system. A worker got injured because the ungrounded conductor and the EGC were swapped accidently in the plug/cord that's attached to the container reefer. The container became energized and the worker went phase to ground when touching two containers. The outlet at the rack has a 32A breaker and there is a 400A upstream branch breaker that feeds 12 outlets at each rack. I believe the Container can be classified as a structure, and 250.32 and 250.50 should apply. I believe maybe a ground plate or something could be put in place so when the containers are set on the ground they will be grounded into the system which will trip the up steam breaker if same thing happens. If someone has seen a better way please let me know and does the code require this. Your thought! Thanks!

 

[petersonra]

I hope I get lots of feed back on this question. At a Container Port I have found what I believe is a serious situation where, just picture a 4 story reefer rack that can accommodate 12 reefers plugging in at 12 outlets. Its a 3 phase 480y system. A worker got injured because the ungrounded conductor and the EGC were swapped accidently in the plug/cord that's attached to the container reefer. The container became energized and the worker went phase to ground when touching two containers. The outlet at the rack has a 32A breaker and there is a 400A upstream branch breaker that feeds 12 outlets at each rack. I believe the Container can be classified as a structure, and 250.32 and 250.50 should apply. I believe maybe a ground plate or something could be put in place so when the containers are set on the ground they will be grounded into the system which will trip the up steam breaker if same thing happens. If someone has seen a better way please let me know and does the code require this. Your thought! Thanks!

One would have thought the single phasing would have tripped the CB if nothing else.

Wouldn't you have something close to a dead short through the motor leads?

 

[Smart $]

One would have thought the single phasing would have tripped the CB if nothing else.

Wouldn't you have something close to a dead short through the motor leads?

It wouldn't be a dead short (bolted fault). For a delta-connected motor compressor, one phase would get 480V and the other two 277V roughly. Likely not enough current to trip SC/GF or overload, at least not right away.

But I'd think the other containers touching this one would be grounded and provide an adequate ground fault pathway... unless this was the first one plugged in.

 

[petersonra]

It wouldn't be a dead short (bolted fault). For a delta-connected motor compressor, one phase would get 480V and the other two 277V roughly. Likely not enough current to trip SC/GF or overload, at least not right away.

But I'd think the other containers touching this one would be grounded and provide an adequate ground fault pathway... unless this was the first one plugged in.

as I understood what the OP said happened, he would have had an EGC on one of the motor phases and 480V on the other two. That would appear to be close to a dead short from either 480V phase to the EGC through the motor winding.

 

[david]

Where is the 480 volt feed coming from? I assume the service is 480 volt system. But I also seen equipment at a higher voltage than the service.

Either way, what I am wondering is the bonding done correctly at the source that supplies the equipment?

 

[david]

Where is the 480 volt feed coming from? I assume the service is 480 volt system. But I also seen equipment at a higher voltage than the service.

Either way, what I am wondering is the bonding done correctly at the source that supplies the equipment?

I went and searched on-line, really didn?t understand that they where powered by on board generation equipment

{On line Search}......................

?These racks represent a new generation of field storage gear,"
"With electrical gear centralized at one end of each rack, access to power supplies and maintenance is simplified, Each cell is equipped with its own hookup. A system of ladders and platforms allows field workers to reach boxes to perform inspections, testing and repairs on containers and their on-board power-generation equipment?

 

[Smart $]

as I understood what the OP said happened, he would have had an EGC on one of the motor phases and 480V on the other two. That would appear to be close to a dead short from either 480V phase to the EGC through the motor winding.

With a 480Y/277V 3? 4W system, the EGC is essentially at the same potential as the neutral. Say ?C and EGC were inadvertently swapped. You'd have 480V A-B and 277V A-C and B-C. Compare to starting current of a wye-start, delta-run motor.

 

[ActionDave]

.... Its a 3 phase 480y system. A worker got injured because the ungrounded conductor and the EGC were swapped accidently in the plug/cord that's attached to the container reefer. ....

That is a bad thing.

I believe maybe a ground plate or something could be put in place so when the containers are set on the ground they will be grounded into the system which will trip the up steam breaker if same thing happens. If someone has seen a better way please let me know and does the code require this. Your thought! Thanks!

I don't think that is the solution. Connections to earth don't trip breakers.

Sounds like a good hard look at the wiring and why there is not an effective fault path is needed.

 

[david]

But I'd think the other containers touching this one would be grounded and provide an adequate ground fault pathway... unless this was the first one plugged in.

This is unclear to me. I am picturing twelve rec. feeding a single rack with one phase connected to all the metal associated with this rack through energized green conductors (equipment grounds)

He got shocked when touching two different containers. I would think that the containers on the same rack would be bonded together through there mounting to the rack.

I am thinking he was touching two different racks ( containers)

edit:

or the rack is not a metal structure

 

[david]

would it be common for one of these racks to be supplied from an ungrounded 480 volt sytem?

 

[GoldDigger]

would it be common for one of these racks to be supplied from an ungrounded 480 volt sytem?

That would explain lack of OCPD trip, but not the unit to unit voltage.

 

[david]

That would explain lack of OCPD trip, but not the unit to unit voltage.

I agree and I?m thinking the units have to be metallically isolated from each other. Perhaps the only thing bonding them together is the equipment grounds and they are now energized.

If the green conductors are bonded together ( say at the supply panel) how did they get a difference of potential between the containers?

 

[GoldDigger]

I agree and I?m thinking the units have to be metallically isolated from each other. Perhaps the only thing bonding them together is the equipment grounds and they are now energized.

If the green conductors are bonded together ( say at the supply panel) how did they get a difference of potential between the containers?

Because the supposed EGC wire of the cord for the offending container is connected to the container frame but not to the source EGC wire.

 

[david]

Because the supposed EGC wire of the cord for the offending container is connected to the container frame but not to the source EGC wire.

Ok, then the containers has to be metallically isolated from each other.

It just wasn?t clear in the ones that I look at on line that was the case.

We do not have shipping docks here and I did not have a picture in mind of refrigeration racks that are loaded and moved from one place to another

 

[transman2]

Sorry it took me so long to get back and answer your questions. Power is fed to these 4 story racks or levels that has also steps that go to each level. The rack is about 10' wide also. Each level has a 3 gang terminal box with buss and each outlet paralleled off the buss. These are right above each other on each level. The 400A Branch Bkr feeds the 4- 3 gang reefer female outlets. 12 total. Each outlet has a 30A breaker and is fed from the main with 3 hots and 1 EGC. So we have an EGC run from the panel to all outlets. The steel structure and the outlet boxes are tied into a grounding grid system. However the reefer container sit 2' away and don't touch in any way. The reefer container has a male plug on SOW cord that connects to the outlet mounted on the structure. The container relies on the EGC in the cord to wired correctly. Therefore if the ungrounded conductor and the EGC in the cord are swapped the container can become energized. I hope this helps.

 

[GoldDigger]

... However the reefer container sit 2' away and don't touch in any way. The reefer container has a male plug on SOW cord that connects to the outlet mounted on the structure. The container relies on the EGC in the cord to wired correctly. Therefore if the ungrounded conductor and the EGC in the cord are swapped the container can become energized. I hope this helps.

OK, the best thing to do for safety, including avoiding dangerous situations caused by miswired cords on the units, is to provide a separate ground/EGC wire which is connected to each unit totally separately from the cord and outlet. It needs to be attached before you plug in the cord.
The only other way I can see to get protection is to install a ground fault circuit interrupter protecting each receptacle/outlet. For a 480V system, that may take some searching.

But do not just do nothing, as 480V can be extremely dangerous, as you have discovered.

 

[david]

I believe maybe a ground plate or something could be put in place so when the containers are set on the ground they will be grounded into the system which will trip the up steam breaker if same thing happens. If someone has seen a better way please let me know and does the code require this. Your thought! Thanks!

However the reefer container sit 2' away and don't touch in any way. The reefer container has a male plug on SOW cord that connects to the outlet mounted on the structure. The container relies on the EGC in the cord to wired correctly. Therefore if the ungrounded conductor and the EGC in the cord are swapped the container can become energized. I hope this helps.

OK, the best thing to do for safety, including avoiding dangerous situations caused by miswired cords on the units, is to provide a separate ground/EGC wire which is connected to each unit totally separately from the cord and outlet. It needs to be attached before you plug in the cord.
The only other way I can see to get protection is to install a ground fault circuit interrupter protecting each receptacle/outlet. For a 480V system, that may take some searching.

But do not just do nothing, as 480V can be extremely dangerous, as you have discovered.

Ground fault waring comes to mind,

Testing
Assured Equipment Grounding Conductor Program.
(1) All equipment grounding conductors shall be tested for continuity and shall be electrically continuous.
(2) Each receptacle and attachment plug shall be tested for correct attachment of the equipment grounding conductor. The equipment grounding conductor shall be connected to its proper terminal.
(3) All required tests shall be performed as follows:
a. Before first use on site
b. When there is evidence of damage
c. Before equipment is returned to service following any repairs
d. At intervals not exceeding 3 months

and i like your idea of a bonded steal plate to set the containers on

 

[transman2]

Quote Originally Posted by transman2 View Post

I believe maybe a ground plate or something could be put in place so when the containers are set on the ground they will be grounded into the system which will trip the up steam breaker if same thing happens. If someone has seen a better way please let me know and does the code require this. Your thought! Thanks!





Quote Originally Posted by transman2 View Post

However the reefer container sit 2' away and don't touch in any way. The reefer container has a male plug on SOW cord that connects to the outlet mounted on the structure. The container relies on the EGC in the cord to wired correctly. Therefore if the ungrounded conductor and the EGC in the cord are swapped the container can become energized. I hope this helps.





Quote Originally Posted by GoldDigger View Post

OK, the best thing to do for safety, including avoiding dangerous situations caused by miswired cords on the units, is to provide a separate ground/EGC wire which is connected to each unit totally separately from the cord and outlet. It needs to be attached before you plug in the cord.
The only other way I can see to get protection is to install a ground fault circuit interrupter protecting each receptacle/outlet. For a 480V system, that may take some searching.

But do not just do nothing, as 480V can be extremely dangerous, as you have discovered.

Ground fault waring comes to mind,

Testing
Assured Equipment Grounding Conductor Program.
(1) All equipment grounding conductors shall be tested for continuity and shall be electrically continuous.
(2) Each receptacle and attachment plug shall be tested for correct attachment of the equipment grounding conductor. The equipment grounding conductor shall be connected to its proper terminal.
(3) All required tests shall be performed as follows:
a. Before first use on site
b. When there is evidence of damage
c. Before equipment is returned to service following any repairs
d. At intervals not exceeding 3 months

and i like your idea of a bonded steal plate to set the containers on

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Yes, the upstream 400 amp branch bkr is GFCI and will take all your comments to my next meeting. I hope most of you are in agreement that I can reference 250.32 and 250.50 to support additional grounding, preferably a grounding pad of some sort that's tied into the grounding system. I will also ask that additional testing on plug and cord before plugging in as well. Thanks for all your comments and hopefully we can make this a safer place to work.



Thanks!!!!

 

[suemarkp]

I may have missed it, but is there a panel at the refer rack? You say it has 12 receptacles, and is fed from a 400A upstream breaker. I hope there are not twelve 40A circuits all hanging off a 400A breaker. There should be a panel somewhere feeding these 12 circuits. Where that panel is may need a ground electrode system per 250.32 if it is on a detached structure from what is feeding it or it is fed from a transformer. You could argue the rack needs a ground electrode system per 250.32, but grounding (earthing) isn't your problem. Although the container is a structure, a ground electrode system is not required when only one branch circuit feeds a structure (its an exception to 250.32).

Bonding is the issue, not grounding. The idea of providing a separate bonding path to the container outside of the cord is a good solution. If this could be automatic in that setting the container onto a piercing point biting into metal that is bonded to the rack which is then bonded to the source equipment ground could work. But I don't know how those containers are designed to know if that could be ensured. Running a separate grounding jumper that a worker attaches to the container before plugging it in seems like a more reliable solution.

I'm surprised that just plugging in a container that is sitting on a metal rack did not trip the breaker when it had an ungrounded conductor connected to its metal frame. Is that rack effectively bonded now, or are there plastic or rubber parts in it providing isolation or heavy paint or coatings? I would verify that the rack is well bonded to the power source, and run bonding jumpers across the rack if there are isolated parts where the containers sit.

 

[transman2]

Power is fed to these 4 story racks or levels that has also steps that go to each level. The rack is about 10' wide also. Each level has a 3 gang terminal box with buss and each outlet paralleled off the buss. These are right above each other on each level. The 400A Branch Bkr feeds the 4- 3 gang reefer female outlets. 12 total. Each outlet has a 30A breaker and is fed from the main with 3 hots and 1 EGC. So we have an EGC run from the panel to all outlets. The steel structure and the outlet boxes are tied into a grounding grid system. However the reefer container sit 2' away and don't touch in any way. The reefer container has a male plug on SOW cord that connects to the outlet mounted on the structure. The container relies on the EGC in the cord to wired correctly. Therefore if the ungrounded conductor and the EGC in the cord are swapped the container can become energized. I hope this helps.[/QUOTE]



suemarkp

I may have missed it, but is there a panel at the refer rack? You say it has 12 receptacles, and is fed from a 400A upstream breaker. I hope there are not twelve 40A circuits all hanging off a 400A breaker. There should be a panel somewhere feeding these 12 circuits. Where that panel is may need a ground electrode system per 250.32 if it is on a detached structure from what is feeding it or it is fed from a transformer. You could argue the rack needs a ground electrode system per 250.32, but grounding (earthing) isn't your problem. Although the container is a structure, a ground electrode system is not required when only one branch circuit feeds a structure (its an exception to 250.32).

Bonding is the issue, not grounding. The idea of providing a separate bonding path to the container outside of the cord is a good solution. If this could be automatic in that setting the container onto a piercing point biting into metal that is bonded to the rack which is then bonded to the source equipment ground could work. But I don't know how those containers are designed to know if that could be ensured. Running a separate grounding jumper that a worker attaches to the container before plugging it in seems like a more reliable solution.

I'm surprised that just plugging in a container that is sitting on a metal rack did not trip the breaker when it had an ungrounded conductor connected to its metal frame. Is that rack effectively bonded now, or are there plastic or rubber parts in it providing isolation or heavy paint or coatings? I would verify that the rack is well bonded to the power source, and run bonding jumpers across the rack if there are isolated parts where the containers sit.

suemarkp, I believe 250.50 serves my purpose. Were none are present, one or more shall be used. Plus I think we can go beyond the code to make things safer. Please read all post.