100VAC between concrete floor and outlet ground

Question

I have been tinkering as an enthusiastic hobbyist in my garage 'lab' for about a year. It's an old rented house in Sydney, Australia. Puzzled by several odd small shocks and confusion as to why my oscilloscope insists that my current project's circuit ground is somewhere around +2.4VDC, I thought to put a voltmeter across the garage floor slab and the wall outlet's earth/ground terminal.

According to my DVM there is 50Hz AC present between the floor surface and 'earth' measuring over 100VAC.

Mains AC here is 240V, 50Hz.

Similarly between the floor and the project on my wooden bench which is not connected to any appliance the DVM shows 40VAC.

Voltages measured between the floor and other metal items within reach (old Tektronix scopes) are in the region of 15 to 30VAC.

The floor is reinforced concrete direct on the ground and gets damp when it rains (as was the case when I made these measurements).

I plan to tabulate voltage measurements to see if these change (with weather, say), but this suggests to me that the house's earth wiring might be wholly fictitious!

Sanity check, please - I'm enthusiastic but not experienced and I don't mess with mains power. I'd like to understand what's happening though!

Answer 1

WARNING: Death sounds like a possibility here. At an informed guess it sounds as though either

• Your system ground conductor MAY be faulty - most likely between distribution board and power outlets. I suggest that you make a major priority of testing to ensure that correct grounding exists on all power outlets.

• It's possible that the main building ground is disconnected between main distribution board and ground. Your ground and neutral SHOULD be connected at the main distribution board. Are they?

I see that you have demonstrated that my 2nd suggestion appears to be correct.
Apart from this being "not to code" and dangerous, it also suggests that other shortcuts may have been taken and/or that the installation may have been done by an unqualified / less than adequately competent person. The main distribution board (mdb) Neutral and Earth busbars should be linked (assuming you have such :-) ).
You should have earth conductors to each outlet from the mdb. If these are lacking 'there will be problems'

Given that there is obviously non-standard wiring present it is uncertain what else has been done incorrectly. Some of the following may or may not apply depending on what has been done. Regardless, urgent action is required.

The danger is two fold.

• If a fault occurs such that an item of equipment has a phase to conductive body fault then the equipment body will be live and a shock hazard. If earth wiring is provided but not connected to neutral at the switchboard then ALL equipment in the house will then have earther metal at phase voltage. eg a broken toaster element could lead to a dangerously live heater elsewhere in the house. Even without a hard fault there may be enough leakage overall to cause at least nuisance shocks and possibly a lethal situation.

• Depending on the main distribution board configuration, a phase to ground appliance fault may not blow a fuse. In such a case a blown fuse may be replaced by a fire. Not generally a good idea.

If only the main distribution board ground is missing it can be added with not much effort or expense. As it is a rented house the landlord needs to be advised asap (probably pausing only to rapidly scope out which of the above problems apply).
You can probably encourage action by pointing out that if a fire occurs due to a wiring fault their insurance may not cover it (and such deficiencies are traceable after the event with some ease). And/or if you or another person dies due to this situation they also may have some liability.

A threat of eviction MAY be a response.
A suggestion that you need to report it to a regulatory body may help in such a case.
This is a reasonable action to take as if someone else dies as a consequence after you leave then you share at least moral and possibly legal responsibilty.
No need for that initially if they act promptly to remedy the problem.

Answer 2

I don't have enough rep to leave a comment, so let me say this.

Your Fluke may be reading a phantom voltage, but this is unlikely because you actually felt a shock. What may have happened is that the grounding rod became disconnected from the bonding screw in your mains electrical panel and now the system is ungrounded. This can happen for several reasons that I've seen. A plumber installing plastic lines and the earth ground getting disconnected. Sometimes a lawn-mower can hit the grounding rod and disconnect the copper wire.

Basically, the general concept of grounding is that everything in your house should be at ground potential. At my workplace, the building has heavy-duty copper-cable embedded in the concrete around the steel columns connecting the columns to the concrete and earth. Without doing this, it is possible for voltages to exist between different locations on the floor. Copper allows a low impedance path for the current to flow so everything is at equal potential.

Try connecting a small load between the ground wire of your receptacle and the cement. Like an inductor-style voltage tester or an incandescent flashlight bulb. I've done this in my own house and got an LED to light up where there should not be any voltage at all. Figure out a way to do this SAFELY. The reason for small voltages to exist is because of naturally occurring voltage differences in the soil, but something like you are experiencing is likely due to a failed grounding system in your house.