Is it unsafe if some three prong outlets don't properly test as grounded?

Question

It's me again -- the pest ;-)

On the general inspection report for the house we're looking at buying, there's a note about the house's electrical:

Some three prong outlets did not test properly grounded. This may be an indication that the wiring in the walls is the older ungrounded type. We recommend considering upgrading to grounded type wiring for safety enhancement.

How big of a safety concern is this? Is there any other explanation for faulty ground tests? What corrective action would be necessary? I'm concerned that fixing this would require the entire electrical lines to be replaced... If so, any thoughts on how expensive that would be? I'm guessing quite a bit.

What's strange is that the report does mention that a ground is present on the main panal:

Grounding system is present. The grounding electrode was buried and not fully visible at the time of our inspection. Wood screws are used to secure the dead front cover to the panel. This is improper because they have sharp ends and can damage the protective coating in the wires inside the panel. We recommend installing proper screws.

The inspection report also recommended adding GFCI outlets. Would that help at all?

Answer 1

If an outlet has a ground terminal, it should be grounded. No ifs or buts. (And IMO, giving the illusion of ground protection is even worse than not having any protection.). I don't think you can even get two-prong outlets any more, so I think you'll have to fix the wiring. It sounds like you're still negotiating, so this is a good bargaining point. And if you don't get it fixed now, when it comes time to sell, any potential buyers would be within their rights to make you fix it then.

An easy way to check that your wiring has a ground conductor is to open up the service panel, look for the breaker that controls those outlets, then trace the black-shielded wire to where the cable feeds into the panel. (If it's present, ground is unshielded.)

If you have grounded wiring at the service panel, the next thing to check would be to look inside the outlets in question to see if the wiring there is grounded (it may just be a loose connection, and since it's common to daisy-chain outlets in a room, one faulty wire could affect several outlets).

If you have don't have ground at the outlets, but did at the service panel, you probably have a junction box joining the older ungrounded wiring and newer grounded wiring. Time to go exploring in the attic or crawl-space / basement.

GFCIs detect a difference between current going out on the live terminal and current coming in on the neutral terminal. If you're the easiest path to ground in an electrical fault, a GFCI will detect that and break the circuit; without a GFCI, you'd have to hope that you draw enough current -- and for long enough -- to trip a regular circuit breaker. On the whole, though, I'd rather that current was running to ground through some copper that's intended for just that purpose.

Answer 2

If the house has two-prong outlets or three-prong outlets that are not actually grounded, it may mean that there is older knob-and-tube wiring. It's common for much of the house to be have been rewired while leaving a few circuits knob-and-tube. Knob-and-tube wiring may prevent you from getting insurance and can be a fire hazard, especially if you were to blow insulation into walls without knowing that they have knob-and-tube, so it's important to find out if you have it.

It's also possible that there is modern, three-connector wiring, but that the ground was never properly connected. Finally, part of the circuit may be grounded but an extension ungrounded. That is is fine in most rooms, but if there is no actual ground, the outlet should be replaced with either (1) a two-prong outlet, (2) a GFCI-type receptacle marked with the words “No equipment ground,” or (3) a three-prong outlet protected by an upstream GFCI and marked with the words “GFCI protected” and “No equipment ground”. See section 210-7(d)(3) of the National Electrical Code.

I would start by removing the cover of one of the ungrounded outlets to look for knob-and-tube there. Next, I would look for knob-and-tube in the basement and around the service panel. Your home inspector would probably have mentioned any visible knob-and-tube wring, but it can't hurt to take another look. It's possible that parts of the circuit is still knob-and-tube even though the visible portion (connecting to the service panel) has been replaced. Try to trace the circuit from the service panel to the outlets in question, and open any outlets and junction boxes along the way to understand what kind of wiring is used and how it's connected.

If you need to rewrire a few outlets of the entire circuit, the difficulty and cost depends on how hard it is to access the affected areas. Fishing cable through old walls is a daunting task, and it's often necessary to make holes in the drywall and patch it again. A trick for first-floor outlets is to leave most of the circuit in the basement ceiling and drill up to each first-floor outlet.