Thanks guys. I thought that was how isolation transformers worked but now I have a better understanding of them. They won’t make things safe to work on while live (duh) but if you do happen to come into contact with something hot while you are grounded it won’t shock you. Just don’t touch both the live and neutral at the same time. My guess is that the compressor would eventually fail if the windings are truly compromised but after some more testing I’m not sure of anything anymore.
Oddly enough after I got done testing the D2 last night I decided to test some other things around the house. Now I must say that I technically wasn’t testing for voltage to “ground” in any of these measurements. I was testing for voltage to neutral since that is what I had handy. My house is old and there aren’t any grounded outlets in it. I figured though, grounds and neutrals are bonded in most electrical panels anyways so it should be the same right? Well, I got some more interesting results.
I tested my blender to neutral, nothing. Toaster oven to neutral, nothing. Toaster to neutral (an old 1941 model) I got around 4 volts. My 1935 flat top to neutral, around 70 volts. My 1936 Frigidaire to neutral, full 120 volts. My Maytag freezer that Travis was nice enough to give to me, full voltage to neutral.
This really left me scratching my head. The only other incident that I’ve had was one time when I was leaning over to feel the condenser on my 36 Frigidaire and I got a slight tingle. I realized that it must have been the metal trim around my counter top in the kitchen that completed the path because I had leaned over and was touching it. I then decided to test that metal trim. Neutral to the trim, nothing. No surprise there. Hot to trim, full voltage which confirms that it is grounded somehow.
So I guess my question now is, are most of the old appliances in my home trying to kill me or am I missing something here? To be fair, the wiring in the 36 Frigidaire and the freezer are original. Toaster too. But I have visually inspected everything I plug in and have not found any problems. Strangely enough the wiring in the Frigidaire is in very good shape...as far as I can tell. It is still flexible and shows no signs of cracking. Makes me wonder if it really is the wiring or the windings of the compressor or something else I’m missing entirely. The flat top has no excuse because it’s wiring was so bad that I replaced it all without hesitation!
I work around electricity every day for my job and yet things like this still make me question my sanity! What do the experts think? I will also try to flip the plug when I get home and see what difference that makes. I agree that it should make the unit dead when off but that it would become live when it turns on. That makes sense.
This is where it gets deep! And no your sanity is not at stake, believe me.
You're dealing measuring voltage potential, when the current has no where to go. If there is a static charge on a door knob, and you touch it, there's a spark then the voltage is gone. This voltage potential was sitting there, between your hand and the door knob but it had no where to go because there was no current path to carry it away.
With a digital multimeter, the meter has an extremely high resistance. It is designed to measure voltage without upsetting an electronic circuit while you test it. This meter will see the voltage between the appliance and the neutral circuit, but it won't tell you whether it;s "A" just a charge which slowly bled into the appliance and has no where to go; or if it "B" is the result of s solid connection to a live circuit, due to a fault in the machine.
When dealing with alternating current, there is capacitive coupling between the machine's wiring, motor windings, and the housing of the motor.
A capacitor is made by placing many layers of electrical conductor very close to each other, and separating them with insulating material. It's made in special ways to maximize its ability to store a charge, but it is basically two electrical conductors near each other, separated by insulation.
In an appliance, you have wiring laying against the frame work of the machine. You have a motor winding with hundreds of wraps of wire separated from the motor frame by insulation. These things are not designed to be capacitors, but "stray capacitance" is developed, none the less. This is hard to predict, and it is unavoidable.
There are several ways to determine what is happening. The most effective is with an insulation resistance tester. The original brand is Megger, so most people call this meter a Megger no matter who built it. The principle of it is very simple. It uses DC voltage to test the insulation. There is no effect of capacitive coupling, when DC is applied. Therefore, capacitive leakage won't be a factor.
The Megger does two things. It applies a DC voltage, which should ideally be higher than the peak voltage the machine will operate at. It then provides a path for any leakage current to flow. This second part, the leakage path, is critical to a valid test. When the Megger is activated, it applies voltage to the machine, and then measures the leakage current resulting from this voltage. Through use of Ohm's Law (built into the instrument) it will tell you how many ohms of insulation resistance exist between the machine's circuitry and the housing of the machine.
The issue is interpreting the readings. New motors have extremely high insulation levels, in the hundreds or megohms or more. Old motors can be lower than 1 megohm and still be safe
Another way to test older appliances where you know the insulation will not be in like-new condition, is to do an AC leakage current test. The basis of this test is to place the machine in running conditions, as well as stopped, and possible abnormal conditions. You will then actually measure the current which leaks from the machine's power supply circuit to the chassis of the machine.
To set up for this test, you will need to power the appliance from a standard supply, but if the appliance has a grounded cord, the ground will need to be disconnected. A two-wire extension cord which will allow the ground to be "hung off the side" of the plug is ideal.
The appliance must be sitting on a dry, nonconductive surface such as a wood floor. If it's on concrete, you will need to slide some insulation pads under the feet.
The test requires a resistor in a jumper lead. This resistor is for safety, in case you have a solid fault in the machine. A convenient resistor would be a 15W light bub. Assemble a light sicket, with a 15W bulb and an alligator clip on the end of each wire from the socket.
Connect one light bulb clip lead to the appliance's metal frame. The other end of the light bulb wire, will go to either of your multimeter's test probes. Select the AC Current (milliamps) range. Place the multimeter's other test probe into the electrical socket, starting with the Neutral side.
The circuit should be like this....
Appliance connected to 2-wire extension cord, without grounding.
Appliance sitting on a nonconductive surface, again to ensure no unintentional grounding.
For Monitor Top refrigerators, the cooling unit (the top) and the cabinet must have a jumper wire connecting them together for a common ground. The cabinet top gasket can act as insulation.
Light bulb socket with clip leads is clipped to appliance, and to one of the multimeter's test probes.
Test #1: Take the other meter test probe, and insert it into the Neutral side of the electrical supply which is powering the appliance. Observe for a milliamp measurement on the meter. Note the meter readings under the following conditions:
A - Appliance is powered ON and operating.
B - Appliance is cycled off, turned off, or otherwise stopped.
C - For a fridge, door open / closed for light operation. Any electrical functions test off and on.
Test #2 (for two-wire cord appliances): Remove the meter test probe from the Neutral power supply. Place into the Live (hot) connection of the electrical supply which is powering the appliance. Observe for a milliamp measurement on the meter. Note the meter readings under the following conditions:
A - Appliance is powered ON and operating.
B - Appliance is cycled off, turned off, or otherwise stopped.
C - For a fridge, door open / closed for light operation. Any electrical functions test off and on.
There is a lot of discussion as to what is too much leakage current. In general, 0.5 milliamp is enough to give a noticeable (but not dangerous) shock. Installation of a grounded cord would safely bleed off this leakage current without any harm to the appliance.
If you see 1 to 5 mA, this is in the highest acceptable range for an appliance with a grounded cord. Leakage in the 4 to 6 milliamp range will trip GFCI outlets. There are exceptions however, as heating elements often have capacitive leakage in or above above this level. That's one reason you don't use GFCI's with refrigerators, due to defrost heaters tripping them.
So while I can't give a hard and fast answer, in general:
Two-wire appliance - Over 0.5mA will give a shock. Any more than that is unsafe.
Grounded appliance - Up to 4 or 5 mA acceptable; with high capacitance designs possibly higher.
Fridges tend to have more leakage because of the size of them, the oil filled motor, and on newer ones, the heating elements in the door mullions and defrost system.
It goes without saying that if the test light bulb illuminates, there is an actual short to ground. Repair this fault and retest to get an accurate insulation leakage test.
Hope this helps!
Sincerely,
David