Do I trust the ground tester?

I’ve yet to use my glowforge because we’ve been dealing with grounding issues. We have an old house (built in 1948) and started off by replacing the 2-prong outlet in our GF room with a 3-prong outlet, as we’ve done for other outlets throughout the house. As a novice, I thought this update meant we were now grounded. When my husband replaced the outlet, he informed me that there was no 3rd green grounding wire, so sadly this did not mean the outlet was grounded. He did point out that our fuse box does have a ground wire coming from it, so there must be some aspect of ground to work with. Our next thought was to probably hire an electrician to hook up grounding wire to this outlet…

In the meantime, though I bought the $6 ground tester everyone suggested here: and lo and behold the outlet in question tested CORRECT! After digging online a bit more, I read that:

“metal boxes attached to armored, or BX, cable—a type of wiring commonly found in old houses—generally are grounded; the cable’s flexible metal jacket serves the same purpose as a dedicated ground wire.”

Does this sound like why my outlet (w/ no 3rd green grounding wire) is testing as CORRECT?
And if so, does that mean I should be good to go?

That is very possible that it is grounded because of the armored cable. Is the wire in your house covered in a metal sleeve?
The tester is probably correct. But you really should run a bare wire from a screw in the box to the green screw on the receptacle.

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Yes and probably. I grew up in a house like that. The box was grounded to a pipe and then the cable between the box and outlets was BX (metal twisty sheathing around the wires) that attached to metal outlet boxes (today’s boxes tend to be plastic). The BX attaches to the outlet box with a clamping connector that’s also metal. So there’s a ground connection going all the way back to the grounding pipe. You probably won’t see the BX at the outlet end but should at the fuse/breaker box. You should be able to see the inside half of the metal clamp inside the outlet box that secures the BX cable if you want to be absolutely positively certain :slight_smile:

But, as @scott_trahey noted, it’s good practice to run a few inch piece of wire between the green grounding screw on the outlet and to the grounding screw in the box (if there isn’t one, just use a self-tapping metal screw to secure the wire to the box).

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I was looking for a good image and came across this article. The first image shows the green wire you will want to add. It’s basically and insurance that the ground will always be there. With out it there is a potential that you could loose ground when wiggling the plug or getting some box corrosion.


Good find. Brings back memories.

When I was a kid we didn’t have breakers in the service box - it was all fuses. Either those round screw in types or the less common (at least for us) solid cylinders. Much preferred the round ones (time delay) because I could tell if I fried one before my dad came home & I could replace it with a spare (or a dime :smile: ). My house ate spare fuses.

I was a pretty innocent looking kid - no idea why you have to keep buying spares dad…


The main problem of grounding via the conduit rather than a separate ground wire is that it often isn’t very reliable. It relies on all the conduit connections having been wrenched down and stay that way since they were put in 70 years ago. Often buildings settle or shake around a bit in earthquake country loosening up those conduit rings.
A good way to improve shock hazards on a questionable box is to install a GFCI outlet instead of the regular one. While this does not magically ground an ungrounded outlet it will trip and isolate the outlet from power if it detects a fault current (like one going through your body).
If you feel at all unsure about any of this, then you should hire an electrician to look at your wiring.


What is the conduit made from? If it is steel for example it wouldn’t have a low enough impedance to prevent it becoming live briefly in the event of a live to ground short.

Huh. Learn something every day.

Might be worth being aware of ‘ground loops’, (see wiki for reference) if you suddenly start having mains hum appearing on nearby audio equipment.
Might even impact the wifi from GF to cloud, but it’s getting outside my comfort zone now.

If you are in the US, the ground wire is just a second path back to the neutral (white wire) termiinal in you box. Another way to say that is both the neutral and ground are grounded in the same place in the box.

The green wire gives an extra path to neutral in the event a wire breaks or connection comes loose, but it is just a layer of protection from having a hot hooked up with no where to go. The ground prong is also usually longer so it is the last thing to break contact when youh pull the plug. It certainly wouldn’t be to code, but you could connect the ground lug and white wire with a jumper. Then the ground pin on the plug would actually be doing something.

That is a hack for sure, so do as you see fit.


Yes that is the case in most places but the important point is no current flows in it in normal use so it is at the same potential as your water pipes, etc, but the neutral has lots of current so may be a few hundred millivolts different at each socket.

It’s a bad idea to connect the outlet box to the neutral, especially when the box is not grounded.
If you were to lose your neutral run, your outlet box would become life through anything that is plugged into it.
Don’t do it. A GFCI is really the only safe (and code approved) way to improve the safety of ungrounded outlets.

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Thanks! When we replaced the 2-prong with 3-prong, we did install GFCI outlet, so it sounds like we’ll be in reasonable shape :slight_smile:

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