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Welding bay with metal sheet floor do i earth or not ?

Discussion in 'Electrics UK' started by deadshort, 17 Aug 2017.

  1. deadshort

    deadshort

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    I am in the process of fitting out a welding bay in a fairly large workshop. The floor is sheet metal screwed to the timber floor. My question is do i need to earth the metal floor and metal work bench ?

    Thanks for any advice,

    DS
     
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  3. JohnW2

    JohnW2

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    I haven't come across people asking about a metal floor before, but we've had several discussions (which tend to become protracted, with no clear answer!) here about "do I or don't I?" (earth) metal (or metal clad) work benches, particularly in the context of electric welding. Have you, for example, see this one ?

    As far as I can see, it seems that it usually comes down to a 'swings and roundabouts' situation with, as I said, rarely a consensus! I imagine that similar consideration apply to a metal floor as to a metal work bench.

    Kind Regards, John
     
  4. deadshort

    deadshort

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    My inclination is not to earth the floor sheets or bench as this may create a 'hazard' others my think differently, or have experienced of situations in which they wished they had earthed !

    We shall see ….

    DS
     
  5. ericmark

    ericmark

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    You can earth but if you do it needs at least 35 mm² cable and it should not be run with other cables unless at least 50 mm² as it can take the welding current so you don't want it to burn out if that should happen, with the old oil cooled welding pods earthing the bench and if metal the floor was good, as the welder had no internal insulation and if dropped the buzz bars could bend and cause 230 volt on the output, however with air cooled units this is no longer a problem.

    I would use 50 mm² welding cable and this also helped the welder as he did not need to clamp the earth, however the problem with this is if there are bearings on the item being welded then you can end up with welding current passing through the bearings and damaging them, so much depends on what is being welded? It is not only the installation but also what is being welded. In the old days with metal cased drills with earth to casing it was not unknown for the welding current to end up travelling through the cable of the drill and burning out the cable with metal benches, earthing the bench with 50 mm² removed this problem. Today we still have the odd mag mount drill which is class I so there is still a case for earthing.

    However using 6 or 10 mm² earth wire is asking for problems, specially if run with other cables, as it will melt and can damage items which it crosses. Sorry not a yes or no answer, but hope it helps.
     
  6. bernardgreen

    bernardgreen

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    A German friend who did a lot of welding in his home workshop ( hobby and proffesional ) had a metal bench insulated from the floor but with a "bolt on" link for times when it needed to be earthed. If I recall correctly the floor had a similar optional earth arrangement.
     
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  8. FrodoOne

    FrodoOne

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    The metal cases of objects which may be connected to a Low Voltage (e. g. 230 V AC) source of electrical energy and all "fixed" metal objects should be connected to the "Protective Earth" of any installation.

    The current carrying capacity of the "Protective Earth" connection needs only to be sufficient to operate the circuit protection equipment involved (Fuses, Circuit Breakers etc.) and the minimum size of this PE conductor is set by the "Rules" in the country concerned.

    With electric welding, the return current carrying conductor/connection/clamp is often referred to as an "Earth" connection.
    It must be sufficient to carry the large current involved in the welding process.
    However, there is no (welding) need for it to have any connection with an actual "Earth" nor with the "Protective Earthing" conductor.

    (Similarly, in a Motor Vehicle the common "Pole" of the battery connected to the chassis of the vehicle is often referred to as an "Earth" connection when it is merely a "chassis" connection.)

    In a welding situation, such as proposed by deadshort, all metal floor plates and metal work-benches or surfaces should be "bonded" together by cables capable of carrying any likely welding return currents (should that be necessary). The connection from these "bonded" items to the "Protective Earth" should be made at only one point, via a conductor of the current carrying capacity required for circuit protection by the appropriate regulations.
    This "single" point connection of the PE to the items adequately bonded for welding purposes will ensure that the PE provides the required (Low Voltage) circuit protection yet never needs to carry any welding "return current".

    It should be noted that the "bolt on" link was not for an "Earth" connection but was for the "return current" connection of the welding process.
     
  9. bernardgreen

    bernardgreen

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    It was noted while I was in the workshop that the "bolt on" link, when fitted, connected the table to a substantial ground electrode. The return circuit for the welding had a separate "bolt on" link to the welding equipment.
     
  10. FrodoOne

    FrodoOne

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    Please, PLEASE see
    from 55:48 onward.
     
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  11. bernardgreen

    bernardgreen

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    What happens to the person welding standing on the ground potential floor ( green ) when the transformer insulation breaks down ( purple ) and the bench comes Live at 230 volts above ground.
    welder fail.jpg


    If the table is Earthed then the Earth leakage protection will ( should ) operate to remove the 230 supply to the welder before the person gets a prolonged shock.
     
  12. FrodoOne

    FrodoOne

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    Assuming that an RCD or RCBO is installed !!!

    However, if all metal components are bonded together and connected to the PE (and, hence to the return to the source) the Circuit Breaker/Fuse will operate.

    If the "Ground" is a metal plate bonded to the metal work bench and BOTH are connected to the PE, as I discussed, the person concerned will be even safer - since all parts of his body will be at the SAME potential and he won't receive any shock (even if his potential is raised somewhat towards 230 V AC until the circuit protection operates).
     
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