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I have a room with 2 ungrounded (2-prong) outlets and 1 switched grounded (3-prong) outlet. This has not been a serious inconvenience so far, as most of the equipment I wish to connect has ungrounded plugs and I can leave the switched outlet on. However, it would be nice to have the protection of a surge protector on more of my equipment, which requires a ground connection.

(I am renting, so let us assume for the purposes of this question that having the outlets replaced is not an option.)

For this purpose, I can imagine using a “2-prong to 3-prong” ground adapter to connect a separate wire run to the ground of the switched outlet (using another appropriate adapter, assuming such exists, to connect to the ground terminal with low risk of hot/neutral contact, or perhaps running to the outlet screw as a ground adapter would).

  • I would ensure that no 3-prong equipment is plugged into the surge protector connected this way (e.g. label and cover the ground pin sockets). This would hopefully eliminate the risk of loss of the safety ground (except for the surge protector itself).
  • I would verify that the 3-prong outlet does in fact have a ground. (Adequate testing would be a different question.)

My question(s):

  • Is this safe?
  • Is this useful? (That is, would the unusual pattern of wiring somehow interfere with the function of the surge protector?)
  • Is this permitted?

I'm not especially committed to this solution; it's just a thought that keeps coming to mind, and so I want to know how bad an idea it is.

I am in California, USA.

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3  
Unless you know for a fact otherwise, I am skeptical that the 3-prong is properly grounded, which throws the whole scheme into question. –  bcworkz Mar 13 '13 at 23:56
    
@bcworkz Very good point! I'll add that to the plan. –  Kevin Reid Mar 14 '13 at 2:12
    
It may be a lot easier to simply ground the other two outlets - open up the outlet box and see if there is a ground wire present. I know you said you're renting, but if you feel that concerned about it, you can undo it when you're done. –  Aaron Mar 14 '13 at 2:22
2  
Possible Duplicate: Is it OK to borrow a ground wire from a different circuit? –  Tester101 Mar 14 '13 at 12:13

1 Answer 1

up vote 2 down vote accepted

It is NOT safe to obtain the grounding from another outlet.

The grounding wire must follow along side the hot and neutral wires of the same circuit for the entire path back to the panel. The reason for this is to minimize the inductance in the event of a short circuit to any grounded metal, either in work boxes, or in the powered equipment itself (such as the multi-outlet box). Surge protection will be much less effective in cases where the surge is a differential between hot/neutral and ground (which actually is a common type of surge) due to this same inductance.

Inductance is minimized when the wires with the current going in both directions are close together. Their magnetic fields overlap and mostly cancel each other out (there will be a field inside the cable between the wires). When the wires are far apart, their magnetic fields will extend a greater distance and there will be more inductance. Also, metal located inside the "loop" between the wires can be affected by the magnetics (sometimes in a hazardous way).

To correctly upgrade a 2-wire circuit to 2-wire-plus-ground circuit, the cable must be replaced with one that has integrated ground. In some rare cases, the proper cable may have been run, already, but an ungrounded outlet was used.

Simply running a single wire along side the old cable is also NOT safe. The wires of a circuit, when conducting current, will try to physically move apart from each other due to the orientation of the magnetic field. Physically binding the new single wire to the cable with cable ties not further than 6 inches apart for the entire length could avoid that issue (but is still not code compliant). If you are doing that much work, just replace the cable with the proper type. FYI, this was one of the hazards of older knob and tube wiring. Single wires inside conduit are known to move around, but the conduit sufficiently confines the movement so all you get is some noise.

In many cases you can obtain much (but not all) of the safety of grounding with the use of a GFCI outlet. The grounding wire still cannot be used in this case. It just gives a safer 2-wire load. It is not sufficiently safe for appliances that have frequent human contact, like a computer.

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I have to disagree, the larger risk is that you are using the ground from a circuit on a different GCFI circuit. –  Kortuk Feb 23 at 16:46

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