Protective Multiple earthing 3

[Elec17]

I have been asked to justify my recommendation that protective multiple earthing (PME) should not be used in a quarry environment. My initial justification is that differnet items in a quarry could be in close proximity (2 conveyors for example) but with different earths causing a potential voltage difference between 2 pieces of equipment in the event of an earth fault. Any other thoughts on the issue of PME in quarrys?

 

[waross]

A grounding electrode or grid near a machine will help to maintain an equipotential zone around that machine and reduce or eliminate touch and step potentials.
I think that multiple grounds are a good idea. It extends your ground grid.
But, if you ground a neutral conductor at each machine, if you have line to neutral loads you may have neutral current sharing between the neutral and the ground.
There may be code interpretation issues with multiple ground to neutral connections. (I would try to apply the rule that allows multiple grounds for separate buildings sub fed from a main building.)
With the neutral and round conductor bonded at each machine, you will have lower impedance and faster tripping of over current breakers in the event of a ground fault, BUT if you have ground fault monitoring, multiple neutral ground connections may cause serious issues with your grond fault protection.
Go with grounding connected to the machine frame at each location and connected to the ground conductor but not connected to the neutral and you should be safe.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Which regulations are you working to?


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If we learn from our mistakes I'm getting a great education!

 

[7anoter4]

I am not sure you intend to ground the neutral wire in multiple points but the protection wire[if it is 5 wire distribution].The motor conveyors, I think, are supplied by 3 conductors cable without neutral [at all] or may be a protection wire included[as a 4th conductor].
If the quarry area will be covered with a grounding grid the touch and step potential will be tolerable [if the grid is well designed and executed] in a phase to ground fault case. In the contrary the absence of a suitable grounding the touch and step could be more dangerous.
If you could isolate the equipment and put an insulated carpet around it you would avoid the touch dangerous potential. The multiple grounding is indicated if it is so constructed to reduce the danger. See IEEE 80 or DIN VDE 0141
If you can assure an electrical continuity between metallic parts of all conveyors then the protection conductor multiple grounding will help any way. See NEC art 250-81.
There is also [I could not recommend it as I didn't read it yet]: BS 6907-2:1988
"Electrical installations for open-cast mines and quarries. General recommendations for protection against direct contact and electric shock".

 

[ScottyUK]

"I am not sure you intend to ground the neutral wire in multiple points but the protection wire[if it is 5 wire distribution]."

That's the whole point: the neutral and earth conductors are combined and there is no 'fifth wire'. It is a TNC system in IEC terminology. PME is a distributed grounding system for the neutral electrode, as compared with a single point ground at the source.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

As Scotty says, PME is usually applied to overhead 4 wire distribution systems where the neutral is connected to a ground electrode several times a mile. This is not recommended for your application.
Consider a ground fault caused by contact between a hot conductor and your conveyor frame. Due to the impedance of the supply and grounding conductors there will be a voltage drop in the hot or supply conductor and a voltage rise in the grounding conductor. This voltage rise will show up as a potential difference between the conveyor frame and ground. This is a touch voltage and may be lethal. Connecting the conveyor frame to a local ground grid will create an equi-potential zone around the conveyor and greatly lessen the magnitude of touch voltages. This is a simplification, note that the operative word in the previous sentence is "lessen" not "eliminate".
A ground rod will lessen hazard in the area of the ground rod. A buried cable grid surrounding the conveyor will provide much better protection.
As an example, not a rigorous solution, a ground rod at one end of the conveyor frame may not protect a person touching the other end of the conveyor frame, but a buried cable surrounding the conveyor frame so that a person touching the conveyor frame must be standing over the buried cable or between the buried cable and the conveyor frame will be much better protection than driven rod electrodes. The buried cable will be connected to the conveyor frame at intervals that may be from 10 feet to 50 feet.
This is not a design but an example to show the difference between the protection afforded by a rod electrode and a ring cable electrode.
If you must design to a standard the analysis will include factors such as the depth of burial, the conductivity of the soil and the fault current magnitude.
A theory of a rod electrode is to provide a low impedance return path so that the machine frame stays at ground potential. However, with serious ground faults, there may be a voltage gradient in the ground. This voltage gradient gives rise to step and touch potentials which may be lethal.
The theory of a grid surrounding the machine is, in addition to providing a low impedance path to ground, to create an equipotential zone surrounding the machine (based on the theory of the Faraday cage). Should a fault be so severe as to cause a voltage rise on the machine frame, the voltage rise will extend out to the area of the buried cable and even though the frame of the machine rise above ground potential, the buried cable will cause the earth encircled by the cable electrode to rise to the same potential as the machine frame. Again this is a simplification subject to a rigorous analysis.

Bill
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"Why not the best?"
Jimmy Carter

 

[Elec17]

Thanks all.Much obliged. Have exactly what I need from your answers. I am also tryin to get hold of BS 6907 as recommended to be fully armed.