Ground loops? Leaky current to ground? On RF equipment in the Lab

[neazoi]

Every time I touch the shields of the connectors in two different equipments in the lab, I get a nice shock. To get the shock, I have to touch two different connectors in different equipment. Any ideas why is that happening?

UPDATE:
I connected a neon bulb between the grounds in equipment and I found two machines that when touched to the grounds of the others, cause the neon bulb to light up!
Both of these machines are 60s - 70s equipment.
I give this as a hint to find the problem.

 

[c_mitra]

Every time I touch the shields of the connectors in two different equipments in the lab, I get a nice shock. To get the shock, I have to touch two different connectors in different equipment. Any ideas why is that happening?

The two equipment you mention have isolated their respective grounds. In other words, the shields are not connected to the chassis or the chassis is not connected to the circuit ground. It is also possible that the two equipment you refer are not connected to the ground of the wall socket.

Test with a good old neon lamp. Stand on the ground and touch the boxes (metal part) individually and see if the neon blinks (even once). Then check with the shield. Next test between the two shields.

With the instruments turned off, see whether the shields are grounded. They are often grounded at one end to avoid ground loops. But for communication cables, they should be grounded at both ends.

 

[neazoi]

It happens even if I touch the machines chassis with a neon bulb and even if the machines are off. It also happens even if I touch a neon bulb to the GND of the wall socket.
Single phase mains power here.
But it is really weird, when all grounds in the machines are connected together (through their coaxial RF connectors), the GND in the mains socket does not lit the neon. The machines are old, the metal enclosures are connected to the grounds.
What does this mean?

 

[c_mitra]

Your house wiring is bad; the local ground is disconnected. The third pin (ground connection) on the wall socket is NOT providing you the required protection. I do not believe that your instruments are at fault but somehow the local earth connection got lost. It happens sometimes. Need to check the electrical connections.

 

[neazoi]

Your house wiring is bad; the local ground is disconnected. The third pin (ground connection) on the wall socket is NOT providing you the required protection. I do not believe that your instruments are at fault but somehow the local earth connection got lost. It happens sometimes. Need to check the electrical connections.

Why when I connect all the coaxials in the machines together the ground pin in the mains is not under potential?
I suspect aground loop and not bad wiring, but a ground loop of more than 90v is that possible???

 

[betwixt]

Possibly, one or both units has a mains input filter BEFORE the power switch. The capacitors in the filter will still be conduct to some degree even at mains frequency so they could lift the chassis to some potential above ground. If possible, I would make a direct chassis to ground wire connection for safety. In the past some equipment designers, when regulations didn't exist, ignored safety in favor of cheapness.

Brian.

 

[c_mitra]

It is easy to check; turn off the power (remove the power cord from both the instruments) and see whether the ground pin of the power plug shows continuity with the instrument chassis. If yes, check whether the shield is also showing continuity with the chassis. If the chassis is not showing connection with the power cable ground, see whether it is connected with the shield. Now disconnect the cables that connect the two machines and see whether the two share the same ground. Also see whether the potential between the neutral and ground is too high. You can discover a lot with good old multimeter...

 

[vfone]

One of my ground-plane vertical antenna (even I use radials for each band) the antenna is connected electrically to the metal roof of the house (through the clamping system of the antenna). The metal roof is connected to an Earth ground rod. So, even if I use a two-pins mains plug (without ground) when I connect multiple devices to to this antenna, all of them will be connected indirectly to the Earth ground. Maybe this is your case also.

 

[biff44]

yes it is a problem, and possibly a serious safety issue.
the insulation on some transformers is probably deteriorating, and causing the AC line voltage to leak onto the chassis.
or some other path, like a failed capacitor from the line voltage to ground, has failed as a low impedance.
back in the day, some audio amplifiers had a capacitor from one of the input AC lines to the chassis to reduce hum, and if the hum was bad, you threw the switch to the other side, and it connected the capacitor to the other line.

clearly both of these chassis do not have an earth ground connection. OR your earth ground connection is not good. or you live in a country that does not use earth ground connections! here in the USA all 115 v AC lines have a third prong for earth grounding.