Electrical Earth / Bond - Safety / Equipment Protection û Enquiries

[PersonalProfile]

Hello,
I have a number of general enquiries regarding electrical bonding / earthing from a safety and equipment protection aspect (I would like to keep things simple before delving into interference etc).

I have searched the internet in general, reviewed a number of text sources (Google books, text books, local standards, codes of practices / guidelines etc), though things are still not clear to me. My background is mechanical. I am investigating for general interest and improving safety of those around me if I can.

My initial enquiry regarding earthing (via earth stake to earthen ground) of mobile generating plant (i.e. for portable hand tools and or welding via inbuilt power source - not attached to a permanent system – in AU this would generally be a MEN system), though I didn’t obtain an answer to my satisfaction. In fact I found a lot of contradictory information. Some sources indicate earthing of the mobile generating plant is essential (with a link to the neutral winding point recommended though some do not), then some say it is dangerous for ostensibly the same application (i.e. in both situations not connected to a permanent what appears to be or explicitly stated to be a MEN system). I am confused as to what is required and more importantly why.

My other enquiry is regarding underground mine applications. In AU a MEN system is explicitly precluded in a number of our states safety act / regs (i.e. with words to the effect that an earth stake shall not be installed underground and a continuous earth to the source shall be reticulated). I understand some of the issues, though again some of them appear to be contradictory. Of course my next question, which has been explained to me though I do not completely understand is, why is MEN not permitted (by law – at least in some states of AU) underground though recommended almost everywhere else?
In both cases the real issue may be due to my lack of understanding.

I would like to review in sufficient detail to understand the situation and make my own conclusions.
I would never implement any changes to any system based on my limited understanding.

I am interested in learning about earthing systems in general.
If someone could point me in the direction on an online resource or suitable text (i.e. that I should go and buy) it would be appreciated.

Thanks and regards,
Lyle

 

[mmt019]

IEEE Standard 142-1991 (Green Book).

 

[PersonalProfile]

Thanks for that.
It was a good read, though I believe I am looking for something aimed at level before this (i.e. why did we arrive at this configuration).
I assume this is something all all electrical engineers (and electricians?) are taught or learn / come across in their career.

Maybe my left field background is the source of my (or at lease some) of my woes.

Thanks and regards,
Lyle

 

[waross]

Consider a world without earthing.
Due to capacitance and insulation resistance there will always be a high impedance path to ground. In a large system the impedance may be low enough that lethal currents are possible in a parallel ground path.
In the example of a motor winding failed to the motor enclosure there will be an elevated voltage to ground on the motor external parts. In all but the smallest isolated systems there will be enough current available from leakage to ground from the unfaulted phases to injure or kill anyone who completes the path to ground with part of his body.
Multiple Earthed Neutrals to the rescue. If all exposed conducting surfaces are earthed, any earth fault will cause enough fault current to operate the overload protection and de-energize the circuit.
However, the protective action of an MEN system often results in an arc and the release of energy. In a potentially explosive atmosphere that may replace shock hazard with an explosion hazard.
MEN is not the only scheme that protects against shock hazards. Other schemes such as earth fault monitoring and isolation (for very small systems) will provide shock protection without introducing an explosion hazard.
The alternate schemes often entail extra costs, extra equipment, extra monitoring and staff training/awareness.


Bill
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"Why not the best?"
Jimmy Carter