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our gas furnace has a junction box on the side of it for the 110 feed for the blower, the feed has no ground wire , if i put a meter on the hot wire and metal box i get 110/120 , without a ground wire on the feed side does this mean the furnace sitting on a concrete basement floor is creating its own ground and is this ok?

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2 Answers

I'm fairly certain it's against code not to have an electrical ground. However, your gas line should be grounded, and that is likely how the furnace is grounded. The reason it's dangerous not to have an explicit electrical ground is pretty simple: if someone is working on the gas, and has it disconnected from the furnace, and there is a short in the electrical wiring of the furnace, as soon as they touch the furnace and gas line, they become a conductor to ground, and will be electrocuted. Any type of electrocution where the current passes through your chest is incredibly dangerous: as little as 100mA can cause fibrillation (requiring a defibrillator), and >1A typically causes permanent damage.

As Shimon points out, it's also possible that it's inadvertently grounded via duct work, though ducts aren't explicitly grounded so that is unreliable at best. Again, same sorts of dangerous here, and if it is inadvertently grounded (eg: somewhere a piece of metal is touching both the duct and something else grounded), it's likely to become ungrounded at some point.

I highly doubt that it's grounded on the cement floor to the point you'd get 110V, as the resistance is likely in the megaohms, and so even if it was being grounded that way, you'd likely only see a few volts if you measured.

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Is the feed cable a metal-sheathed (aka BX) cable? If so, the metal sheathing is likely providing a ground even if there are only two wires (hot and neutral) inside the cable.

It's also possible that the metal ductwork attached to your furnace is grounded at some point. This might be done in a reliable, intentional way, or it may just be that it comes into contact with some metal pipes somewhere.

I think it's unlikely that the concrete floor is providing such a reliable ground, and it's definitely not the way you want to do it -- the whole point of having a ground conductor is to provide a lower-resistance path to ground than the one that goes through your fingers to the floor you're standing on.

In any case, there ought to be a ground conductor to the metal housing of your furnace. The best way to achieve this would be through any cables feeding electricity to your furnace, but you may also be able to add a dedicated ground wire. I'd recommend checking your local building code to ensure any change you make is compliant -- things get complicated when you're dealing with HVAC and basements. Good luck!

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