Mobile trailer grounding preventing shock at work place?

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roger said:
Sahib seems to be missing this is what the OP is talking about, and 99.999999% of single or double rods driven will never get to a low enough measurement to prevent the OP's scenario from being a hazard.

Roger
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More so in the US where voltage to ground is typically 120V and less attention is paid to ground electrode resistance, compared to EU and other areas with 240V to ground or more and performance standards for grounds.
 
Sahib said:
I think artificial treatment of soil that lowers earth resistance would change that. In the extreme case, assume that a metal plate buried in the soil extends from ground rod for some distance. In that case there could be no shock hazard at least for the time being.
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If plate is in close proximity to the rod - I suppose that may be called soil treatement, if solidly bonded to the rod I would call that an extension of the electrode or maybe a supplementary electrode.
 
There is a realistic installation where the ground rod could prevent a shock.

As has been noted, a normal ground rod in normal soil will not significantly change the voltage of the energized trailer, nor will it conduct enough current to trip the breaker and remove the source of the voltage.

However if the supply to the trailer has GFCI or GFPE protection (6mA or 30mA ground fault detection and breaker opening) then a ground rod will likely carry enough current to open the breaker.

To the original poster: the addition of the grounding electrodes is not enough to remove the shock hazard from the mis-wired plug. Only solid bonding between the trailers and the supply, or ground fault detection would do this.

Solid bonding could be provided as part of your 'grounding electrode system', say by jumpering all the ground rods for all of the trailers and the supply together using a buried wire. The key point is that the safety is not provided by the connection to the soil, but by the _metallic_ connection back to the source.

-Jon
 
I am not willing to dismiss the ground rod as completely useless because in very moist soil a ground rod of length 3 meters has an earth resistance of 10 ohms and 3 ohms for a length of 10 meters. The same rod has 10 ohms for 10 meters length in farming soil and clay soil.
 
Sahib said:
I am not willing to dismiss the ground rod as completely useless because in very moist soil a ground rod of length 3 meters has an earth resistance of 10 ohms and 3 ohms for a length of 10 meters. The same rod has 10 ohms for 10 meters length in farming soil and clay soil.
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Must be Magical Ground Rods.

A 10' rod in NC would be 500-1000 ohms or more.
 
Sahib said:
I got the data from the article ' Earth ground resistance-Fluke' available on the web.
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Now I understand some of your confusion.

That table is very misleading. Your 'very moist' example was for wet swamp soil. Your second example was for farming soil consisting of loam or clay. That would be topsoil and not be 10-30' deep.

For real world examples, the truth is closer to 'dry sandy soil' or 'dry gravel'. I live in an area with moist sandy soil, and tests done here showed a single ground rod with 8' in contact was 1300 ohms. One test trumps pages of predictions every single time, in the case of your pdf, it missed the mark by over 1200 ohms.
 
K8MHZ said:
Now I understand some of your confusion.

That table is very misleading. Your 'very moist' example was for wet swamp soil. Your second example was for farming soil consisting of loam or clay. That would be topsoil and not be 10-30' deep.

For real world examples, the truth is closer to 'dry sandy soil' or 'dry gravel'. I live in an area with moist sandy soil, and tests done here showed a single ground rod with 8' in contact was 1300 ohms. One test trumps pages of predictions every single time, in the case of your pdf, it missed the mark by over 1200 ohms.
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I can't believe a company like Fluke supply misleading information when they gain nothing by it.
 
Sahib said:
I can't believe a company like Fluke supply misleading information when they gain nothing by it.
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What is misleading about the information? They presented a table. At one end of the table are the extremely low ground rod resistances associated with high conductivity soil. At the other end of table are the essentially infinite resistances associated with solid rock. Common values are right in the middle.

And yes, if these trailers were in a swamp, a ground rod might have a pretty significant effect and might even trip the OCPD. I'd still rather depend on a solid metallic bonding conductor.

-Jon
 
Sahib said:
This thread is not about code compliance.
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Jon's post wasn't necessarily referring to code. A metallic bond to the source neutral would be a far greater low impedance path to the source than earth.

Roger
 
Sahib said:
This thread is not about code compliance.
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I was not talking about code.

This thread is about the safety provided by grounding electrodes at the trailers.

One _possibility_ for the grounding electrodes is to add bonding jumpers between the different trailer grounding electrodes, as a bare copper wire in the soil (mentioned in post 23). This 'unified' ground electrode system is not required by code, but would provide the sort of redundant safety that the OP's employer is hoping to get from simple ground rods. The solidly bonded grounding electrode system would function as a redundant bonding path to the EGC that is supposed to be in the feeders to the trailers.

As you noted, in certain (atypically beneficial soil conditions) the simple ground rods could provide this redundant path. But a copper wire will provide this path as long as it is intact.

-Jon
 
roger said:
A metallic bond to the source neutral would be a far greater low impedance path to the source than earth.

Roger
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But if you measure earth resistance of the ground rod with GEC conected to it, you would have a value less than ohm even with EGC disconnected. It is because of a very large number of ground rods already connected to neutral of supply upstream getting in parallel connection with the ground rod at the trailer end, thereby lowering earth resistance of the ground rod at the trailer to less than 1 ohm!
 
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Sahib said:
But if you measure earth resistance of the ground rod with GEC conected to it, you would have a value less than ohm even with EGC disconnected. It is because of a very large number of ground rods already connected to neutral of supply upstream getting in parallel connection with the ground rod at the trailer end, thereby lowering earth resistance of the ground rod at the trailer to less than 1 ohm!
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You can measure anything you want at the very many ground rods, a ground rod still does nothing to reduce shock hazards.
 
Sahib said:
But if you measure earth resistance of the ground rod with GEC conected to it, you would have a value less than ohm even with EGC disconnected. It is because of a very large number of ground rods already connected to neutral of supply upstream getting in parallel connection with the ground rod at the trailer end, thereby lowering earth resistance of the ground rod at the trailer to less than 1 ohm!
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Go back and review that logic please.
If you leave the GEC connected to the service neutral you would measure a very low resistance to earth if you use the three point fall of potential method over a few thousand foot baseline (all those other electrodes expand the sphere of influence.) Or find some other way to make a single ended measurement.
But that does not mean the the addition of a single ground rod is sufficient to make use of that low impedance.
And in fact it is the connection to the service neutral that provides the low impedance return path, not just the multiple ground points. The MGN provide a low earth impedance on the POCO side, not the customer side.
In this particular case, with the neutral and hot reversed, there is no path to earth on the trailer end except through the single ground electrode. And that by itself is almost always not good enough to operate OCPD.

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GoldDigger said:
Go back and review that logic please.
If you leave the GEC connected to the service neutral you would measure a very low resistance to earth if you use the three point fall of potential method over a few thousand foot baseline (all those other electrodes expand the sphere of influence.) Or find some other way to make a single ended measurement.
But that does not mean the the addition of a single ground rod is sufficient to make use of that low impedance.
And in fact it is the connection to the service neutral that provides the low impedance return path, not just the multiple ground points. The MGN provide a low earth impedance on the POCO side, not the customer side.
In this particular case, with the neutral and hot reversed, there is no path to earth on the trailer end except
through the single ground electrode. And that by itself is almost always not good enough to operate OCPD.

Sent from my XT1585 using Tapatalk
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1)Why do you stress sphere of influence as the other ground electrodes of neutral are normally spaced apart?
2) Really both neutral and multigrounds connected to neutral provide return path. 3)The MGN is an effective way of reducing earth resistance of a ground rod at customer side. I do not know why you do not accept it.4)The OP ground rod may be connected to EGC which in turn may be connected with other grounds and so the overall earth resistance of the
rod may be low.
 
Sahib said:
1)Why do you stress sphere of influence as the other ground electrodes of neutral are normally spaced apart?
2) Really both neutral and multigrounds connected to neutral provide return path. 3)The MGN is an effective way of reducing earth resistance of a ground rod at customer side. I do not know why you do not accept it.4)The OP ground rod may be connected to EGC which in turn may be connected with other grounds and so the overall earth resistance of the
rod may be low.
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Ground rods do nothing to reduce shock hazards.
 
Sahib said:
.... 3)The MGN is an effective way of reducing earth resistance of a ground rod at customer side. I do not know why you do not accept it.
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My main reason for not accepting it is that it is not true.
In the absence of a wire interconnection, which is the only measurement that applies to the OP's situation, the MGN does not have any measurable effect on the ground rod to earth resistance. It can reduce the overall impedance between the GEC and the POCO transformer secondary, but it does that only by its effect on the secondary to earth resistance and cannot reduce the overall resistance in the circuit below the resistance between the ground rod and remote earth.

It is an effective way of reducing the overall impedance of the earth circuit on the POCO side, and that is one reason that it is used.
 
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