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 ?
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".
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.
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.