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P.M.E. and Potato Peelers.

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About 2 years ago I was asked to undertake electrical work at a local chip shop/fast food outlet. I was concerned about large metal drummed wet potato peeling machines located on a wet floor in an old outbuilding and the P.M.E. earthing system. I could not devise a safe system in my mind where the large potato peeling machines positioned on a wet stone floor could be made safe with the P.M.E. earthing.


Anyway, unfortunately recently the place burned down and is now boarded up. The cause may not be electrical, but due to oil catching fire.


I have not done any work in the building at all.


Because the machines could not be effectively TT earthed I did not proceed with any work.

https://www.edp24.co.uk/news/norfolk-norfolk-fire-at-mundesley-business-7985784


Z.
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    I just winder if there is too much paranoia surrounding the loss of a neutral in a PME system in a built-up area. Maybe different out in the sticks, but with local substations being relatively plentiful in urban areas what is really the risk?
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    whjohnson:

    I just winder if there is too much paranoia surrounding the loss of a neutral in a PME system in a built-up area. Maybe different out in the sticks, but with local substations being relatively plentiful in urban areas what is really the risk?


    You could well be right.  Lower risk than we perceive perhaps. I was going through a concerned phase at the time. The 14th edition used to require that single phase sockets on different phases be kept at least 2m apart. Was there really a risk if single phase appliances were supplied by sockets on different phases? What were the real life reported accidents?


    Z.


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    real life data on lost neutral incidents was gathered for the IET by the  Health and Safety Labs report here..     rare, but certainly not 'never'
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    Zoomup:

    The 14th edition used to require that single phase sockets on different phases be kept at least 2m apart.


    I knew of a technician who tried to power a high-current piece of equipment by attaching two supply leads in parallel with BS 1363 plugs and proceeded to plug it into two sockets on separate phases.


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    wallywombat:
    Zoomup:

    The 14th edition used to require that single phase sockets on different phases be kept at least 2m apart.


    I knew of a technician who tried to power a high-current piece of equipment by attaching two supply leads in parallel with BS 1363 plugs and proceeded to plug it into two sockets on separate phases.




    I feel sure that he achieved his high current! ?


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    Zoomup:
    whjohnson:

    I just winder if there is too much paranoia surrounding the loss of a neutral in a PME system in a built-up area. Maybe different out in the sticks, but with local substations being relatively plentiful in urban areas what is really the risk?


    You could well be right.  Lower risk than we perceive perhaps. I was going through a concerned phase at the time. The 14th edition used to require that single phase sockets on different phases be kept at least 2m apart. Was there really a risk if single phase appliances were supplied by sockets on different phases? What were the real life reported accidents?


    Z.







    Back in the days of unearthed lighting circuits I suspect that the risks were significant.

    An unearthed brass light switch or pendant lamp holder was fairly low low risk in an office or factory with wooden floors.

    However two such switches or lamp holders and connected to different phases (or poles of a DC supply) and within reach would be dangerous.

    Hence the old regulation of one pole or phase per room, with an exception for large rooms where this was impractical, in which case different phases or poles of a DC supply were permitted if 6 feet, later 2 meters apart.


    The general use of plastic lamp holders and light switches  and the earthing of lighting circuits rendered the regulation pointless, and it was removed, correctly in my view.


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    wallywombat:
    Zoomup:

    The 14th edition used to require that single phase sockets on different phases be kept at least 2m apart.


    I knew of a technician who tried to power a high-current piece of equipment by attaching two supply leads in parallel with BS 1363 plugs and proceeded to plug it into two sockets on separate phases.




    Now that is stupid, whichever way you look at it.


    Z.


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    mapj1:

    real life data on lost neutral incidents was gathered for the IET by the  Health and Safety Labs report here..     rare, but certainly not 'never'


    The chance of injury or death is just over one in a million. Knowing my luck that will be me.


    Quote from report: "Equipotential bonding will also not provide protection to a person in good direct contact with earth (for example standing on a conductive concrete floor) whilst also in contact with the enclosure of the equipment under fault conditions."


    That was my potato peeling machines' concern as they sat on low tables on a wet brick floor that ran with water.


    Z.


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    Z that is very unlikely. You accept much higher risks all the time. When you drive you accept a risk in the same terms of about 1 in 2000. Not driving or using the roads at all reduces this risk to a higher level to the electrical risk, that of being hit by a vehicle whilst walking. The only way to get zero risk from that particular accident is to stay in bed, where the risk is about 1500 in 60 million every day! There is a mad idea circulating in recent times that risk can be reduced to zero, (or perhaps zero Covid, but that's another story). This is impossible. Just living has many risks, some of which may be somewhat reduced with countermeasures, but they cannot ever be made zero. The only zero "risk to life" existence is to be dead! You need to get a grip on the magnitudes of various risks because otherwise, they will ruin your life completely and you will never enjoy anything. Using Electricity is extremely safe for everyone, but the risk cannot be made zero, although some highly deluded people seem to think it can. You will not even be safe from electricity if you live in a tent with no supply within 20 miles, the lightning will get you!
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    Answering the OP, you can do a good job of equipotential bonding of a wet concrete slab, you just need to fix a large area of metal to the slab, or access the reinforcing or whatever, and connect this to the bonding system. This will give similar effect to the "foundation Earthing" being pushed at one point by the IEC. It will be sufficient to prevent much danger of death, although shocks will still be possible. You also need to remember that operatives in food processing areas tend to wear rubber boots (often white) and these probably provide a good degree of insulation. It is nothing like as dangerous as you seem to think, otherwise the entire food industry would have deaths all the time.

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