Measuring Votage

[Carnivore]

When measuring votage with a volt meter does the reading equal the total voltage present or does it equal the difference in voltage between the two points?

I havent found a clear answer to this question and im not sure my measurment is correct. if the later of the above two lines is the truth, how can i test the supposed GROUND point im using to see if its a true GROUND or if there is some voltage present?

 

[Sceadwian]

First off, there is no such thing as true ground. Ground is a common refrence point only. Voltages have to be measured between two points, which two points those are is up to you. You can check the voltage between the base and emiter of a transistor, Between the positive and negative leads of a power supply, or between the positive lead and a rod stuck 6 feet into the earth. It's all relative. Given any two physical objects in the real world their voltage potential is NEVER going to be exactly zero. Thermal noise alone will always acount for some differential.

A multimeter will however read 0 volts if the voltage source can't source much current. Most meters have an impedance of at least 1 meg ohms.

 

[RODALCO]

With mains systems there are two common systems regarding the earthing arrangements.

M.E.N. system, Used in the UK and it's colonies like New Zealand, Australia etc.
In this system the neutral and earth cables are linked at the supply LV (240/415Volts ) transformer secondary, and earthed to the earth bank, they are linked along the cable en-route in distribtution pillars and to the local earth bank by the pillar, and at the end user at the mains switch board and to the earth pin(s).
When close near the N/E link there should be no voltage present between the E and N, or from E at switch board to the earth pin(bank).
The earth wire in the MEN case always carries a current, although it is less then the neutral current because the ground resistance usually will limit this current flow and the bulk of return current is carried by the neutral.

In case of a high resistance or broken neutral caused by a cable fault, a voltage may be present at the neutral and earth, to an independant earth away from the earth pin from the end user.
A higher (270 V) or lower (130 V) than normal voltage may be present , depending upon the balancing of the 3 phases supplying the end user.

In continental Europe a separate earth is used and is not linked to the neutral.
A low voltage may be present between the earth pin at the switchboard and the neutral from the supply, depending upon the loading of the system.

Of cause local POCO's may have some non standard systems in use as well.

 

[Carnivore]

Thanks guys,

I think i got the idea now. RODALCO's explination still has my head spinning but none the less i believe your comments have made clear what i needed to know.