i connected probe of digital mutimeter to the earth socket of ac mains and held the other probe. it shows resistance of 1.8Mohm .but when i touched that end (one i'd held) to the floor. it displays beyond 200Mohm. why is it so ? shouldnt impedance be lesser?doesnt any rms current flow if the live wire touches the floor(directly)?
edit : floor is concrete

 

What is your floor material?

Human voltage providing a resistance with the lacking voltage from the floor resulting in high impedence circuit?

 

What is your floor material?

concrete

 

Tried the same experiment by putting the probe into a small area of water on the floor
mutimeter showed 2.3 Mohm
also tried with a steel plate between floor and probe, shows roughly 12 Mohm

 

my conclusion -
it must have to do with contact surface area with the floor
liquids provide larger contact surface area. occupy all unevenness
same does with sweat on human skin
a flat steel plate on the floor doesnt provide that large a surface area of contact

 

my conclusion -
it must have to do with contact surface area with the floor
liquids provide larger contact surface area. occupy all unevenness
same does with sweat on human skin
a flat steel plate on the floor doesnt provide that large a surface area of contact

I would say this is correct.

 

true. You also forgot impurities from sweat and concrete like salt.

 

why does the contact surface area matter?
what is the lumped circuit abstraction for probe- skin- floor system vs probe- floor (direct contact) system

 

why does the contact surface area matter?
what is the lumped circuit abstraction for probe- skin- floor system vs probe- floor (direct contact) system

Have a read of this: http://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=1324&context=open_access_etds

 

I think it matters because it makes the resistance of the contact smaller. In the lumped circuit abstraction you have to put a resistance in between the probe and the floor (or in between the feet and the floor) that represents the resistance of the contact. The larger the contact area the smaller this resistance is.

 

t. The larger the contact area the smaller this resistance is.

Probabally correct, also the actual potential of the earth terminal and the point of earth that you are measuring to may not be equal, IOW a very small current could be present.
keeping in mind the very high impedance of a DVM this will affect any reading, if present, for e.g you only have to grasp the lead of high impedance 'scope probe to see an AC wave form picked up by one's person.
Max.

 

My skin acts as an antenna picking up all sorts of signals, from mains to local radio to motors and many more things. This injects voltages into a high impedances setup like this and gives meaningless results

 

Hi,
The earth line of the mains as your drawing is not accurately known the end of it is it properly tied to where.
2nd the surface of cemented flooring (concrete) is dried which cannot penetrate the DVM current flow
accurately that gives an idea of wrong readings.