When I changed from a 2 prong outlet to a 3 prong outlet, and removed the old outlet, I found it had not to but 3 wires on each side of the outlet and could not identify or find a separate ground wire. The was a very old 1050 or so wiring. I put the wires on to the new 3 prong outlet screws back exactly as they were on the 2 prong outlet with one exception. I used a pigtail from the ground (green screw) on the new outlet and attached the other ent to the metal box using one or the clamp screws holding the wire in place where it entered the box. I tested from gound and it was OK but am very concerned as this now has a space heater pluged into this outlet and worred I might have made a mistake just attaching the wire to the outlet as mentioned about. Could the extral two wires have been a ground wire. All the wire were covered with heavy insulation and haven't a clue why there was no obvious ground wire. Please explain if you can.

Anything connected to the terminal screws of a 2-prong outlet is NOT a ground wire. At least if the existing wiring is correct. If you connected the grounding screw of the new 3-prong outlet to the steel box, you didn't really do anything constructive unless the box itself is already grounded.

There isn't much to concern yourself with unless the insulation on the existing wires is bad and the hot wire comes in contact with the steel box. Then the grounded chassis of your device plugged into that box will become hot. However, that could happen even if you didn't connect the green screw to the box. The outlet mounting screws do the same job until they corrode a bit. Then they provide some resistance in the circuit.

Right now I am more concerned that you reinstalled the new outlet correctly than that you added that grounding jumper.

Anything connected to the terminal screws of a 2-prong outlet is NOT a ground wire. At least if the existing wiring is correct. If you connected the grounding screw of the new 3-prong outlet to the steel box, you didn't really do anything constructive unless the box itself is already grounded.

There isn't much to concern yourself with unless the insulation on the existing wires is bad and the hot wire comes in contact with the steel box. Then the grounded chassis of your device plugged into that box will become hot. However, that could happen even if you didn't connect the green screw to the box. The outlet mounting screws do the same job until they corrode a bit. Then they provide some resistance in the circuit.

Right now I am more concerned that you reinstalled the new outlet correctly than that you added that grounding jumper.

Thanks for the response. After connecting the wires to the new outlet, again, exactly as they were connected to the original 2 prong outlet, I wound electrical tape, several times around the entire sides of the outlet to prevent the screws or wire from touching the box. My theory was that it was wired this way with 2 wires on one hot terminal and 1 wire on the othe hot side for decades. I suspect, actually hope, the ground wire was wound around a part of the box that I could not see. Last question.....when I engaged the breaker, nothing arced and the plug did not get warm after running the applicance for a hour or so. I am hoping these are good signs. Ultimately, I think I will ask an professional to look at this anyway. I've never seen three wires into the hot side and 3 into the other before. This is what threw me.

If the breaker didn't trip and the plug in the outlet didn't get hot you are most likely fine. Sleep well.

As to the two wires under one screw, that was done in times past instead of using a 'wire nut' and pigtail to connect everything together. We are not allowed to do that anymore. If there are more than two wires (one under each screw) we have to wire nut them together with an extra conductor that will then attach to the device.

Mr Miller,
Thank you for taking the time to respond to my questions. As you indicated, this must be very old wiring and that being the case, I have decided to give a local electrician a call and have him go through the entire house and replace all the old 2 prong outlets as well as eyeball my attempt to do this. To give you an idea of how old this wiring might be, the insulation looks like a fabric, not plastic. Again, thanks .

1050 is indeed Very Old Wiring, You could try a Receptacle tester to verify Voltage and polarity, anything Not working?

I didn't read the entire post, but to add: Because you do not see an equipment grounding conductor, does not mean you do not have one at the outlet. In the old days, they use to attach the EGC out of site. Normally attached to the back of the box.

To Stratmando and Washington... I am almost positive the ground wire is there and out of sight. I used a voltage tester before and after changing the outlet. After the new 3 prong outlet was installed and before it was pushed back into the box, I put the positive probe into the hot side of the outlet and the negative probe was put on the edge of the box and it lit.
I believe that proved the box was grounded.
My biggest concern as noted about in my orig post was when I removed the plate and pulled the orig outlet out, it has 3 wires, not 2 on each side. As Mr Miller suggests, this was probably permissible at one time but now two of the hot wires would be twisted together and tied to a lead to the terminal using a wire nut.
There was no problems with this outlet other than I could not plug in a 3 prong plug from a new space heater and did not want to use an adaptor. I still plan to have a pro come and check that and replace any other 2 wire outlets. This is my daughters first house built near a lake. Building codes way back when allowed this type of wiring set up but it scary to look at. To my knowledge, nothing is amiss and I have my daughter keeping an eye on the temp of the plug and socket and not running the yeater when not home. Can't say how much I appreciate everyone's comments here. This site is new to me. I hope I can be of help to someone here someday.

Inagine, great idea to have your outlets updated by a professional. It will no doubt give you piece of mind. While he's there, he can certainly check all connections. If your switches are as old as the outlets, you may want them replaced as well. And thanks for your kind comments regarding the site. It's nice to hear something positive.

Great idea on the switches MB. I will do that.