Mobile trailer grounding preventing shock at work place?

[Sahib]

My main reason for not accepting it is that it is not true.
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.

If you present your objection with a diagram or calculation, it will be very helpful.


The other ground electrodes connected to the neutral are in parallel with the ground rod at the customer side through GEC. Correct?

If yes, then if you measure the earth resistance of the rod without disconnecting the GEC, then the earth resistance of rod would be the equivalent resistance of all the parallel resistances of ground electrodes connected to the neutral. Correct?

 

[GoldDigger]

If you present your objection with a diagram or calculation, it will be very helpful.


The other ground electrodes connected to the neutral are in parallel with the ground rod at the customer side through GEC. Correct?

If yes, then if you measure the earth resistance of the rod without disconnecting the GEC, then the earth resistance of rod would be the equivalent resistance of all the parallel resistances of ground electrodes connected to the neutral. Correct?

The other electrodes are in parallel with the ground rod via the EGC. But that does not change the resistance of the ground rod itself!

The earth resistance at the bond point (from GEC and neutral to earth) will be low because of the parallelism. But remove the ground rod itself and you do not change things much. So just adding a ground rod to a properly bonded system changes little.

Once again, in the OP case the supply neutral is connected to the L1 or L2 bus in the building disconnect. It is therefore not bonded to anything and so the ground rod is acting entirely on its own and will not usually carry enough fault current to trip the upstream EGC. Your descriptions are partially accurate (I disagree with your interpretation of the "ground rod resistance", but simply do not apply to the OP's situation. Go back and reread the first post please.

 

[Sahib]

The other electrodes are in parallel with the ground rod via the EGC. But that does not change the resistance of the ground rod itself!

I am only talking about the equivalent resistance of the ground rod after its parallel connection to other ground electrodes through the neutral.

The earth resistance at the bond point (from GEC and neutral to earth) will be low because of the parallelism. But remove the ground rod itself and you do not change things much. So just adding a ground rod to a properly bonded system changes little.

NESC Rule 096 C in the section on the National Electrical Safety Code states "with a multigrounded neutral distribution system it isnecessary to have an electrical connection to earth at least 4times per mile to keep the voltage on the multigroundedneutral from exceeding approximately 25 volts making it safefor the linemen should they come into contact with the neutraland the earth". So even though all ground electrodes are in parallel, reducing their number below the minimum is unsafe per above rule. In the same way, dispensing with ground rod may not be safe in OP case also.

 

[winnie]

1)Why do you stress sphere of influence as the other ground electrodes of neutral are normally spaced apart?

In normal soil the sphere of influence of a single ground rod is small enough that significant human 'step potentials' can exist.

Take a normal ground rod and connect it to a 120V 'hot', with the neutral of that supply connected to a low impedance ground. A human standing with one foot on that ground rod and one foot on the soil with normal foot spacing would experience a shock.

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.

Re-read the original post being discussed. In this situation, due to a wiring error, the chassis of the trailer was connected to one of the supply 'hots' (ungrounded conductors) and the neutral was connected in place of the hot. So some of the wiring of the trailer itself was connected to utility grounded neutral system and at very low impedance to earth, but the chassis of the trailer was being held at 120V relative to that low impedance grounding electrode. The _question_ was would a _single_ ground rod connected to the chassis of the trailer make any difference to the shock hazard presented by the trailer.

Secondly a connection to a multi-earth neutral will clearly reduce the voltage between chassis and _distant earth_, but may not make any difference to the voltage relative to _local earth_. Due to the 'sphere of influence' of these electrodes, local soil potential may be elevated relative to distant earth. For purpose of discussing shock hazard, the potential relative to distant earth is virtually irrelevant; what matters if the voltage between two points that a person might touch.
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[kwired]

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.

The MGN is that solid link between electrodes. Now put an ungrounded conductor on an isolated electrode, maybe a couple electrodes, like what happened in the OP - that solid link is not there and you only have resistance of that single rod to contend with

 

[Sahib]

Jon: I already replied to OP question in post#11.