110v between the metal surface of grounded and ungrounded equipment?

sixthday

Super Member
Sorry, I am naive to electric engineering but this makes me nervous.

I have two Reel to Reel both plugged into the same surge protector. One is 2 prong ungrounded and another is 3 prong grounded. I've been used it like this for almost a year and never had a problem. However, over the weekend I happened to find that there is a 115v AC difference between the two units' metal case, even without powering on ! I was so surprise that there is electric flow on the case (am I right?) and, but, I was never been hurt by it.

Is this normal? Or something indeed wrong? I tried different surge protector and different outlet. The same thing AC voltage still measured. But I can still touch the case without feeling anything ( only touch one at a time, never touched both together)

Sorry if this is a stupid question, but I am just nervous now.
 
It's likely that on the unit with the two prong plug, one prong is connected to the chassis through a few megohm resistor. Try reversing that plug in the outlet (which may move that prong from hot to neutral on the AC line) and see if the voltage between that chassis and the other one drops.
 
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I am also nervous :) I don't have a great understanding of ground / potential / us electrical grid. But its a chance to learn. I read this: http://www.hardwaresecrets.com/grounding/

A wild guess: If you switch the plug around on one unit, maybe the potential will dissapear? Ok. Disregard that.

Someone who knows what they are talking about will chime in soon :)
 
It's likely that on the unit with the two prong plug, one prong is connected to the chassis through a few megohm resistor. Try reversing that plug in the outlet (which would may move that prong from hot to neutral on the AC line) and see if the voltage between that chassis and the other one drops.

I was still editing when you posted.
I should have played a lotto game :)
 
In the early days of computers (1979) I bought a used printer that had a non-polarized plug. The printer had one end of the plug connected to ground, both chassis ground and signal ground. Being a printer, it needed to be connected to my computer (a Commodore PET) with a printer cable, which carried the signal ground between the two devices. My computer had a grounded power plug. With the printer connected to my computer, I plugged in the printer. I was surprised with the release of the magic smoke. It took several days for the burning smell to dissipate from my apartment. :(

As suggested above, there is likely a high-resistance resistor connecting one power lead to chassis ground. If the power plug is non-polarized (both prongs the same dimensions), changing the power plug around at the power strip should drop your measured voltage to 0. Before changing your power plug, I would pull the plug and measure the resistance from each prong on the plug to chassis ground. If you measure a high resistance (greater than 1 megohm), then plug in your power cord the other way around if you want to have your chassis connected to the power line neutral. If you still have voltage after switching your power plug, you should measure the voltage between neutral and ground at the power strip. The voltage should be zero unless you have a wiring problem in your house.

But if you measure a very low resistance (less than a few k ohms) between the power plug prongs and chassis ground, you should inspect your power wiring inside the unit. It may be that the power wiring has become frayed and making contact where it should not. If your interconnect cable shield is not connected to chassis ground, you would not see the magic smoke like I did with my printer. But touching chassis ground of both RtRs could be hazardous to your health.

tl;dr: Seeing voltage is not unusual, but, for safety, you should verify there is not a problem with the wiring.
 
Most stereo equipment of the early days did not have a polarized plug, so it was the luck of the draw if you put the plug in the right way. If you switch out the plug (recommended, as by others), the wide blade of the polarized plug is the neutral and the narrow one is the "line" or "hot". The chassis should end up on the neutral side.

If the unit is fused, the hot is always fused (not the neutral). (neutral should never be fused (or switched)).
 
Thank you guys so much. I reversed the plug and the voltage is now close to 0. The resistance between the plugs and the chassis are test O.L. Now I feel much better. Thank you all again !
 
...you should measure the voltage between neutral and ground at the power strip. The voltage should be zero unless you have a wiring problem in your house.

...

Not necessarily zero but pretty low anyway. Unloaded there should be theoretically zero volts. Loaded, however, you would be measuring the voltage drop in the neutral/grounded wire so it would be something greater than zero (but, again, still should be quite low...maybe a couple volts max).
 
I had a 1950's AM tube radio. I connected up the ground to a water pipe for better reception. The radio exploded the instant I turned it on. It looked like I was welding in the room. Anyone who thinks 120v isn't enough to do much damage should have seen this.

115vac is just about the most dangerous voltage level there is. Higher voltages typically blow you away after contact, thereby saving you (at least, not killing you). But 115vac causes your muscles to clamp down, keeping the current going until you are cooked. Getting a screwdriver across 115 creates a welding machine. I've seen more than one screwdriver destroyed because an electrician got sloppy working a live circuit.
 
I had a 1950's AM tube radio. I connected up the ground to a water pipe for better reception. The radio exploded the instant I turned it on. It looked like I was welding in the room. Anyone who thinks 120v isn't enough to do much damage should have seen this.
That's one of the downsides of arbitrarily grounding what is not built that way.
 
The trouble with the line to chassis caps is that they can short. When they are good you'll measure voltage with a typical high impedance digital voltmeter, but it won't tell you the available current. Thats where it really matters. If that cap gets leaky or shorts, you've got basically whatever your breaker is rated at for current. 20 amps at 120v is enough to lock your heart right up. Its supposed to be limited to some small number of milliamps. I forget the number exactly but the way you test it is to use a resistor to ground and measure voltage across it. I'm sure the particulars have been linked or typed out on here someplace but the idea is that if you get more than a certain voltage across the resistor, the current is too high and it probably means the cap is bad or just the wrong value.


As for the 50's AM radio, many of them had a direct connection from line cord to chassis, and were known as a "hot chassis" set. They will go bang if you ground them, and they can electrocute you if you run them outside of the cabinets or do something like installing metal knobs when they did not have them originally.
 
The trouble with the line to chassis caps is that they can short. When they are good you'll measure voltage with a typical high impedance digital voltmeter, but it won't tell you the available current. Thats where it really matters. If that cap gets leaky or shorts, you've got basically whatever your breaker is rated at for current. 20 amps at 120v is enough to lock your heart right up. Its supposed to be limited to some small number of milliamps. I forget the number exactly but the way you test it is to use a resistor to ground and measure voltage across it. I'm sure the particulars have been linked or typed out on here someplace but the idea is that if you get more than a certain voltage across the resistor, the current is too high and it probably means the cap is bad or just the wrong value.


As for the 50's AM radio, many of them had a direct connection from line cord to chassis, and were known as a "hot chassis" set. They will go bang if you ground them, and they can electrocute you if you run them outside of the cabinets or do something like installing metal knobs when they did not have them originally.

Line to chassis capacitors must be marked Y(x), caps that go across the line must be marked X(x).
https://www.illinoiscapacitor.com/pdf/Papers/EMI_RFI_suppression_capacitors.pdf
 
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