Mobile trailer grounding preventing shock at work place?

[Robert Merritt]

We had an incident at my work place where a contractor wired a plug incorrectly. The equipment grounding conductor was crossed with a phase conductor, inadvertently energizing the mobile work trailer. The contractor placed his hand on the trailer disconnect and was held there until the power was removed by manually opening the supply breaker. In response to this event it was determined that had a ground rod been installed at the trailer bonding it to ground, that the contractor would not have received a shock. After watching Mikes video on grounding and bonding, I believe this is not the cause due to the fact that without the low impedance path back to the power source on the equipment grounding conductor the trailer would have remained energized even with the ground rod installed.

Could you please respond with an answer as to if installing the ground rod would have been any protection preventing the contractor from getting shocked? Also, now all trailers that come on site are required to be bonded to ground, is this giving a false sense of security against receiving an electric shock?

 

[roger]

In response to this event it was determined that had a ground rod been installed at the trailer bonding it to ground, that the contractor would not have received a shock. After watching Mikes video on grounding and bonding, I believe this is not the cause due to the fact that without the low impedance path back to the power source on the equipment grounding conductor the trailer would have remained energized even with the ground rod installed.

You are correct, a connection to earth would not have provided protection against the shock and in some instances could actually increase the danger.

Could you please respond with an answer as to if installing the ground rod would have been any protection preventing the contractor from getting shocked? Also, now all trailers that come on site are required to be bonded to ground, is this giving a false sense of security against receiving an electric shock?

Earthing the trailers will not provide personnel protection however, it will help with lightning events.

Roger

 

[zbang]

Also, now all trailers that come on site are required to be bonded to ground, is this giving a false sense of security against receiving an electric shock?

The trailer chassis/frame be bonded to the EGC from the supply feeder (NOT the neutral), which ought to have prevented, at least mitigated, the shock hazard. It sounds like there wasn't an effective EGC from the supply.

Also, the trailers probably needed ground rods anyway.

Take a look at articles 550 and 552.

 

[tom baker]

If the trailer ground rod had a resistance of 25 ohms at 120 volts there would be only 4.5 amps via the ground rod not enough to open the over current device....

 

[kwired]

The trailer chassis/frame be bonded to the EGC from the supply feeder (NOT the neutral), which ought to have prevented, at least mitigated, the shock hazard. It sounds like there wasn't an effective EGC from the supply.

And this all was likely present but was not function correctly because of:

The equipment grounding conductor was crossed with a phase conductor, inadvertently energizing the mobile work trailer.

 

[kwired]

If the trailer ground rod had a resistance of 25 ohms at 120 volts there would be only 4.5 amps via the ground rod not enough to open the over current device....

Which the rod also lowers the voltage from frame of trailer to "ground" but only to about 112.5 volts.

 

[don_resqcapt19]

Which the rod also lowers the voltage from frame of trailer to "ground" but only to about 112.5 volts.

How does it do that. The circuit is a current divider, not a voltage divider.

 

[kwired]

How does it do that. The circuit is a current divider, not a voltage divider.

Never mind, though you may be exposed to a lesser voltage if standing nearly right on the electrode then if you take a step or two away from it.

 

[zbang]

And this all was likely present...

I don't know how you can assume that from the original post.

 

[GoldDigger]

We had an incident at my work place where a contractor wired a plug incorrectly. The equipment grounding conductor was crossed with a phase conductor, inadvertently energizing the mobile work trailer. ....

I see two possible interpretations here:

1. The miswired plug was an equipment plug for something which had an incidental connection to the trailer frame and so energized it via the hot EGC wire. This should have immediately tripped a breaker unless the trailer frame itself was not properly bonded to the real incoming EGC.
2. The miswired plug was the disconnect for the whole feeder to the trailer. This would energize all "grounded" metal in and around the trailer and even with a ground rod connected not enough current would be drawn to open a breaker. And the trailer would be hot with respect to local earth (touch/step potential). The only reliable counter to this (other than proper commissioning testing) would be a GFCI breaker.

 

[Sahib]

Could you please respond with an answer as to if installing the ground rod would have been any protection preventing the contractor from getting shocked? Also, now all trailers that come on site are required to be bonded to ground, is this giving a false sense of security against receiving an electric shock?

It may or may not provide protection depending on the ground resistance. If the ground resistance is low enough to trip the breaker, the ground rod can provide protection in absence of EGC. If the ground resistance is not low enough to trip the breaker, the ground rod cannot provide protection. So simply providing a ground rod without checking ground resistance and size of the breaker may not work. But depending on only ground rod without EGC for protection is a code violation, anyway.

 

[roger]

I may be wrong but I think the OP meant the trailer itself is being plugged into a supply receptacle, I don't think he means it was a piece of equipment that was wired wrong.

If that is the case the EGC and one leg was reversed so the trailer and it's frame was energized and a connection to earth won't change this or somehow make it safe.

Roger

 

[Sahib]

the EGC and one leg was reversed so the trailer and it's frame was energized and a connection to earth won't change this or somehow make it safe.

Roger

A connection to earth in this case could trip the supply to the trailer if the earth resistance is low enough and make it safe because the neutral of the power supply to the trailer might also be grounded upstream.

 

[ptonsparky]

A connection to earth in this case could trip the supply to the trailer if (IF and higly unlikely) the earth resistance is low enough and make it safe because the neutral of the power supply to the trailer might also be grounded upstream.

.

Had the neutral been floating, there would have been little to no shock hazard.

A simple continuity check Is all it takes to prevent such mishaps.

 

[kwired]

I don't know how you can assume that from the original post.

Well that is why I said "likely", giving whoever did that some slack in that they did did do things correctly and was competent in performing the task- but still accidentally swapped leads making up a supply cord or receptacle, I've even done that sort of thing when in a hurry or distracted.

This is also why you should make it a standard practice to take voltage readings after connecting something like this even though it is something you may feel you can about do blindfolded, mistakes do happen.

 

[zbang]

This is also why you should make it a standard practice to take voltage readings after connecting something like this even though it is something you may feel you can about do blindfolded, mistakes do happen.

Complete agreement, there.

 

[don_resqcapt19]

Never mind, though you may be exposed to a lesser voltage if standing nearly right on the electrode then if you take a step or two away from it.

That is correct. If you are standing on, or very close to the grounding electrode, the earth around the electrode will be at a higher voltage and the shock hazard will be reduced. It quickly goes up as you move away from the electrode. About 68% at 1', 75% at 3' and 85% at 5' from the grounding electrode.

 

[Sahib]

It quickly goes up as you move away from the electrode. About 68% at 1', 75% at 3' and 85% at 5' from the grounding electrode.

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.

 

[ActionDave]

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.

Nobody is talking about what kind of artificial treatments to use on soil or burying metal plates around job trailers. A simple ground rod does nothing to lower shock hazard.

 

[roger]

Nobody is talking about what kind of artificial treatments to use on soil or burying metal plates around job trailers. A simple ground rod does nothing to lower shock hazard.

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