We have just moved into a new house. I connected up the stereo, DVD, CD, tuner etc and noticed that touching the surface of metal parts of the equipment caused a tingling sensation.
So (after unplugging the equipment) I measured the voltage between the GPO earth and ‘real’ earth (the hills hoist). It was close to zero volts, so I suppose the earth is good.
Then I plugged the equipment back in and measured the voltage between hills hoist earth and the chassis of the equipment - it was 150 V AC, but only 0.4 microamps. That is with the hi-fi stuff turned off, but with the switch at the GPO on.
What the hell is going on? I am concerned about the safety of people and the equipment.
If it’s a fairly new house (<15 years) the earth should be connected to a waterpipe near the fusebox. The wire may have come off.
If the house is older, the earth could be via an earth stake driven into the ground. Again, the wire may have come off, or the stake may be driven into very dry or sandy soil.
There could also be a break in the earth wire between the GPO and the fusebox.
The actual fault in the equipment is probably a bad suppressor capacitor wired between active and earth.
Thanks Desmo. Thats what I thought at first, but how come the potential between the GPO earth and Hills Hoist* earth is zero?
I just checked - the earth is connected to a tap out the front of the house. It looks a bit dodgy though. Is there a good way to check if it is good?
*People from other lands may not know, so I should point out, that a Hills Hoist is a clothes line.
There’s no potential there, because the fault between active and ground is in the sound equipment, not in the GPO or the other house wiring.
Try measuring the resistance between the GPO earth and some exposed copper water pipe. That should give you an indication. The resistance should be only a couple of ohms if the connection is good.
Looking at this again, there’s no fault in the sound equipment. 0.4 uA implies an impedance between active and the chassis in the tens of megohm range, which is quite high.
Looks like the sole problem is that the GPO isn’t earthed.
Something else you could easily test is to use a GPO in another part of the house. (Just run an extension cord if you have one long enough, rather than moving the sound gear). If the problem disappears, that tells you that the main earth is good, and that the problem is confined to the wiring somewhere in the house, if not at the GPO itself.
But you really should consider bringing in a licensed electrician.
I did a series of experiments last night. Connected one end of a long extension lead to a good ‘ground’ (copper tap in yard) and the other to my multimeter.
Extension lead only was 2 ohms
Other earth points (not on GPO’s - such as other pipes) was 4 ohms
GPOs earth to ground was fluctuating about 10 ohms, but the digital meter alterated rapidly between minus 2 and 20 ohms. The continuity tone buzzed instead of a constant tone. Should have dug out the analogue meter.
Pulled all the hi-fi power cables out of the GPO’s. Then one at a time measured the voltage at GPO earth when only that item was plugged in and turned on at the GPO.
TV 100V
video 84V
DVD 175v
amp 11v
tuner 17v
240v lamp 11v
cd 17v
nothing plugged in 9v
On this side of the pond, we use devices called Meggers or ground rod testers. An ordinary VOM can’t evaluate the quality of a grounding electrode at the voltage potential present.
Some power companies over here will test ground rods without charge. This won’t, however tell if there is a poor or broken conductor between the distribution panel and the electrode.
In this case, the chassis ground is at, or assumed to be at, Earth ground potential. That’s what the term “ground” is usually taken to mean. Even with no physical connection to Earth, there is always a point in the circuit designated to be 0 V, whether or not said point is actually at 0 V with respect to Earth.
The words “earth” and “ground” mean exactly the same thing in this context. They both are intended to mean the earth or ground beneath your feet.
In a piece of equipment (doesn’t have to be battery powered) that is electrically isolated from planet earth, say a battery powered radio, or your car, or the space shuttle, the terms earth and ground are used to refer to point of zero potential from which other voltages are measured. It’s sometimes called a “floating” earth or ground, and it acts as a common reference point regardless of whether it’s at the same electrical potential as some random spot on planet earth.
After doing some more experiments, I realised that all the sound system appliances ** have NO earth plug**. Der. [Insert wally smiley.[
So I unplugged everything from the powerpoints, then with only one piece of equipment plugged in at a time, found that the voltage on the chassis of the unplugged item was still there. Even if the RCA leads were removed as well.
Sounds like the voltage is induced from nearby pieces of equipment.
So I am buggered if I know what is going on, but it can be that dangerous, so I have set up the sound system again.
Anyone know why there are voltages in excess of 100V AC 0.4 microamp induced in other items of sound equipment??
One advantage of all this is I discovered the DVD player stuffs up the reception of the tuner. By the way that was an item that induced 175v in the chassis of other equipment.
You are attempting to do a voltage drop test here. Current must be flowing for this test to valid. If there is no current flowing zero volts will always show regardless of the condition of the ground.
Your circuit is live. First rule of ohm meters, no voltage present to take a measurement. Anytime you use an ohm meter and you get a funny reading, switch the leads, if the reading changes or goes negative there is voltage present.
