Current through body as a result of touching a neutral

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A little off topic but another subject that comes up time to time around here and came up again this morning, is the subject of ground mats. On several of our outdoor cabinets we have metal grated ground mats that are sitting on the ground and are bonded to the cabinets as well as bonded to a ground rod driven into the earth.

The argument comes up here all the time that these ground mats are unsafe because they make a better connection to ground during a fault. I tried to explain that the purpose of these ground mats were to provide an equal potential plane to eliminate touch potential in the event that there is something faulted inside the cabinet.

Folks here dont like this and keep asking the question what if there is no fault in the cabinet and they accidently touch a hot wire then isn't the grated mat decreasing the resistance to groud allowing more current to flow? I did not really have a good answer for this. They questioned weather a rubber mat would be better?

Does anybody have any experience with these metal grated mats or know what the correct answer is for these mats increasing the risk of harm if accidently touching a hot wire? Aren't these mats just a smaller version of ground grids or other grids used to eliminate touch potential?
 
090804-0723 EST

philly:

The insulated mat should be the best solution. It will only provide capacitive coupling and the amount of current flow at 60 Hz to a pair of feet will be negligible.

The capacitive reactance of 500 pfd at 60 Hz is about 10,000,000 ohms. It would take 10,000 V to produce 1 MA if I did my calculations correctly. My guess is that the capacitive coupling thru shoes will be well below 500 pfd. The capacitance of a foot's worth of area on a rubber or plastic 1/4" thick is quite small.

See
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/pplate.html
Using A = 0.02, d = 0.01, and k = 3 produces a result of 53 pfd.

The insulated pad also eliminates a problem from one foot on the earth and the other on the pad as you step from a hot earth to the insulated pad.

.
 
It looks like an insulated mat is the way to go.

However without a rubber mat and only having a metal mat are u increasing the risk of injury when accidently touching a live circuit by being solidly grounded via the mat?
 
090804-1034 EST

philly:

Probably.

Another advantage of a soft insulating mat is that if for some other reason you get knocked on the floor there may be a less severe head injury.

Also consider a "Chop Stick" for probing in a panel and one hand in your pocket. Chop stick is a dowel rod. In many Ford plants you see many places with a sign Chop Stick, a container, and a wood dowel rod. This is largely intended to keep limbs of electricians and various mechanical trades from being in a hazardous location. A very good electrician that I had worked with at times was working on a positive transfer machine, put his hand into the machine to operate a limit switch, the machine transferred, and he lost his hand. Had he used the Cop Stick he would still have his hand.

.
 
steve66 said:
That's pretty much the case as long as the neutral wire has a good connection back to complete the circuit.

It's also important to remember as soon at that connection is gone, the neutral wire becomes a "hot" wire.

I've seen a journeymen splicing neutral wires on live circuits. He didn't realize (and neither did I at the time) that as soon as he cut the neutral wire (to install a wire nut) that one of those ends suddenly had 120 Volts on it.

Steve
Click to expand...

Haha....that brings back a bad memory! :roll:
 
daleuger said:
Haha....that brings back a bad memory! :roll:
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I can't imagine that outdoor insulating mats will stay clean and dry enough to do the job that you want them to do. Grounding mats are required in substation type installations to provide an equi-potential area. A fault on a high voltage substation can result in a ground potential rise with step potentials in the thousands of volts. The grounding mat brings this voltage down to a safe level.
 
I would think an insulated mat would be the last thing you want in philly's situation. If the guys are worried about contacting an ungrounded phase, their PPE should protect them from that, insulated rubber gloves, and dielectric boots, plus any required arc flash ppe. As others have said, a grounding mat tied to the grounding system is typical in a substation to eliminate any potential difference between the grounded case and the worker.
 
philly said:
I'm interested to hear your reason why?
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My statement was more directed at a substation...not sure that is what you have there.

From the USDA Design Guide for Rural Substations,

An effective substation grounding system typically consists of driven ground rods, buried interconnecting
grounding cables or grid, equipment ground mats, connecting cables from the buried grounding grid to
metallic parts of structures and equipment, connections to grounded system neutrals, and the ground
surface insulating covering material. Currents flowing into the grounding grid from lightning arrester
operations, impulse or switching surge flashover of insulators, and line-to-ground fault currents from the
bus or connected transmission lines all cause potential differences between grounded points in the
substation and remote earth. Without a properly designed grounding system, large potential differences
can exist between different points within the substation itself. Under normal circumstances, it is the
current flow through the grounding grid from line-to-ground faults that constitutes the main threat to
personnel.
An effective grounding system has the following objectives:
1. Ensure such a degree of human safety that a person working or walking in the vicinity of
grounded facilities is not exposed to the danger of a critical electric shock. The touch and step
voltages produced in a fault condition have to be at safe values. A safe value is one that will
not produce enough current within a body to cause ventricular fibrillation.
2. Provide means to carry and dissipate electric currents into earth under normal and fault
conditions without exceeding any operating and equipment limits or adversely affecting
continuity of service.
3. Provide grounding for lightning impulses and the surges occurring from the switching of
substation equipment, which reduces damage to equipment and cable.
4. Provide a low resistance for the protective relays to see and clear ground faults, which
improves protective equipment performance, particularly at minimum fault.
Click to expand...

To me, the above dangers are something you cannot see or expect. An accidental contact with an energized conductor or surface is dangerous as well, but I can take steps to mitigate that hazard with the proper PPE. So my ideal situation would be to mitigate the ground fault currents/step potential hazard with a ground mat tied to the grounding grid, and then mitigate the touch potential hazard with the proper gloves and/or dielectric boots.
 
wirenut1980 said:
My statement was more directed at a substation...not sure that is what you have there.

From the USDA Design Guide for Rural Substations,



To me, the above dangers are something you cannot see or expect. An accidental contact with an energized conductor or surface is dangerous as well, but I can take steps to mitigate that hazard with the proper PPE. So my ideal situation would be to mitigate the ground fault currents/step potential hazard with a ground mat tied to the grounding grid, and then mitigate the touch potential hazard with the proper gloves and/or dielectric boots.
Click to expand...

It sounds like what you are saying is that we have two different scenarios here. One is a scenario where someone (electrician or other personel) walks up to a cabinet or structure and the structure is energized due to an internal fault, in which the person in unknowinlgy then exposed to this hazardous voltage.

The second situation is one in which a trained electrican is working on a panel and accidently comes in contact with an energized circuit.

From what I've heard a grounding mat or grounding grid for that matter will help reduce injury in the first scenario but may make things worse for the second scenario. Since you cannot technically design these ground mats or grids to satisfy both scenarios appropriately, it is best to design for scenario one. The reason being is that this scenario applies to personal unknowingly coming in contact with energized equipment in which they have no chance to take appropriate protective action. Whereas in the second scenario personal can take the necessary protective action by wearing equipment or maybe even brining a portable rubber mat. Its best to design the ground grid or ground mat for the case where people are unexpectadely exposed.

After all in an industrial setting, where there is grating or other metalic flooring, there are ground mats everywhere by virtue of these structures being bonded together. I argue to the guys here that these other floors and structures in the plant are basically one big grounding mat and are no different then the small ones we have on the outdoor cabinets. Again the reason for having everything bonded and essientally creating large ground mats within structures is to prevent a person from unknowingly being exposed to a step or touch potential during a fault. This is the design intent of an industrial grounding system. For a case where someone accidently again comes in contact with any energized circuits in these areas, then they should be wearing their PPE to prevent this.

Am I grasping the points that you were trying to get across?
 
philly said:
Am I grasping the points that you were trying to get across?
Click to expand...

That is what I am saying. You got it. And I don't know that I buy the argument that a grounding mat will make things worse in scenario two. In my opinion, for an accidental contact with an energized surface, the majority of the impedance of the current path is going to be through the person and their footwear (10-20 k-ohms), even if their footwear is regular boots with rubber soles. The difference in impedance of the current path between a person standing on the ground and a person standing on a grounding mat will be insignificant relative to the impedance of the person and their shoes. I admit I do not know what that difference would be. Anyone have any ideas?:)
 
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