Earthing Arrangement Issues

[jc4477]

Hi,
I am assisting in the design of an earth grid that spans quite some distance ~400m x ~800m.
An issue that I have encountered is the reliance on equipotential bonds to provide earthing on long conveyor runs up to 300-400m as opposed to physically placing a principal earth in the ground and creating a grid for the whole run of the conveyor.

I have gone through what I can of Australian Standards and found a document ENA-EG1 for substation earthing to be very useful.

Currently the plan is to bond all cable trays and structures back to the switch room main earth and have a rod or 2 placed into the ground at each support structure but not link each support structure rod together underground ie 20 supports for a long conveyor and each support to have its own earthing rod tied to it but not bonding each rod together (each rod having calculated resistance to earth of ~100ohm).

I have tried to work out an equivalent circuit and realistically I don’t think much if any current will pass through the local earthing rods at each structure.

I can see a lot of issue with this arrangement and fear the earthing system will behave in some sort of battery or neutral situation that may not trigger protective devices quick. (if I had my way I would be bonding all rods underground to create some sort of grid back to the switchroom main earth)

1. Even with multiple rods at each structure I anticipate each support structure to have total resistance to earth via the local rod arrangement at ~20ohm, with a potential 415V single phase to Earth Short, what will happen?

2. Will the equipotential bond via the cable trays and structure, will this provide the path to earth triggering the protection devices?

3. Will the support structure rods do anything? If the support structure arrangement does nothing how high can the step and touch potential get?

Sorry for the long one I am confused.
JC

 

[jc4477]

Uploaded some diagrams, just assume there are more support structures

 

[rcw retired EE]

It has been several years since I did a job in Australia, but I beleive there are some basic grounding/earthing principles that apply.

The objective for low voltage industrial systems is to not have any current flow through the earth. During a ground fault, almost all current should return to the source transformer through the Equipment Grounding Conductor (EGC). In North America, the EGC is the green or green/yellow wire run with every power circuit, bonding the metal frames and equipment to the earth system back at the supply transformer. Metallic conduit, cable tray, cable armour and equipemnt enclosures can also serve as an EGC when properly installed.

Those current return paths have much less impedance than any path through the earth, mostly due to their proximity of the power conductors.

Placing a ground rod at each structure may help with lightining protection and help control local step and touch potentials. But I would be suprised if the ground rod did a better job than the concrete and rebar foundation.

 

[acog]

Hi JC,

I don't know much about the physical construction of conveyors, but consider the following:

USE OF EQUIPOTENTIAL BONDING:
I assume the conveyor is steel. What is the problem with using equipotential bonds to earth your structure? e.g. use copper braids or strand Cu with lugs where there is no good bolted connection between sections. The perhaps just bring back two (N-1) bonds from the closest conveyor section to the main earth bar at the switchroom.

EARTHING VIA GENERAL MASS OF THE GROUND:
If the supports are steel with no insulating sections and you are using concrete foundations with steel reinforcement, then the structure is already earthed. Simulate a the foundation in a popular earthing software package GDEGs, Ground MAT, etc. to get your earth resistance at each structure. these are all in parallel. Adding the earth electrodes may be a waste of time.

If it is not already constructed, maybe welding a galvanised tag to the foundation reinforcement which penetrates the concrete and has a hole for a further connection to the steelwork/conveyor would guarantee a more solid connection if the steelwork is painted instead of galvanised.

PROTECTION:
If you are worried about the impedance of the steelwork, you can work out the resistance of the conveyor if you know the cross sectional area and length. Then just put this resistance in your short circuit and see if the current will be large enough to trip out your breaker if there is a line to ground fault at the furthest end of the cable.

Cheers