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mystery guest 07-23-2008 07:12 PM

Ground question.
I have a equipment that runs on a UPS. It is plugged into an outlet that must not have a ground connected. The light (wiring ground fault) is on. So, to help protect the equipment from power surges, I pounded a copper ground rod 6' into the ground, ran a length of #8 wire (green) from it to the ground lug on the receptacle. It still shows a wiring fault. I know the UPS is ok because it liked the wiring in another room. Why doesn't it like that ground?
I guess the proper way would be to run a dedicated circuit all the way to the breaker box, but that is not practical at this time. (Old house, block construction, shrubs in the path I'd take.)

When I look at the breaker box, the neutral and the ground are tied to the same bar. There is a ground rod near there.

Is the UPS looking for a connection between the neutral and ground? I'd like to feel I was better protected from lightning strikes.

Stubbie 07-23-2008 07:47 PM

The UPS surge protection cannot protect from surges without a low impedance equipment ground back to the panel and then to the transformer over the service neutral. It registers this by showing a completed circuit via its electronics. By connecting to a ground rod only.... your surge protecting circuitry is not showing a completed circuit due to high resistance from an earth only connection. It thinks the wire to the ground rod is open. You must have low impedance back to the source (transformer) not earth.

mystery guest 07-24-2008 08:33 AM

I believe you are correct. I'll have to run a separate circuit. So, answer this, since the ground and neutral are tied together in the breaker box, why can't I just jumper the ground and neutral in an outlet?

I'm going to attempt to answer my own question: Because of resistance in the various wires, a potential could develop between one circuit and another.

HandyPete 07-24-2008 09:08 AM

Thats a no-no to install a jumper in the box! You can however run a bare wire from the ground terminal back to the panel or to the closest copper water pipe. Don't do anything to the white neutral wire.

Stubbie 07-24-2008 12:01 PM


why can't I just jumper the ground and neutral in an outlet?
The lingo we use for doing that is called 'bootleg ground'. As for potentials this exists between all three wires in a 120/240 volt system in residential wiring.

As for tying the neutral to the ground at the receptacle you need to remember that the neutral is the return leg to the center tap of the serving transformer and is therefore a current carrying wire. The equipment ground is a low impedance path intentionally installed to bond all metal in a branch circuit or feeder and is never to carry current except during a fault event. This is to allow any fault currents from ground faults such as a hot wire coming in contact with a metal box to return to the source not earth. The source is the transformer powering the residence. Without this low impedance path back to the source you could never get enough current to flow thru the breaker and trip it on fault. So the 'primary' purpose of the equipment ground is human safety not property protection.

Now to explain why we bond the neutral and ground at the service equipment (main disconnect panel). The utility only runs 3 wires to your meter and then to the main disconnect or breaker panel with a main disconnect installed it. The service neutral is connected to the neutral bar/terminal in the enclosure that houses the main disconnect. The service neutral at the other end (source) is connected to the center tap of the transformer. It is the only low impedance path back to the source for neutral current and fault current line side of the main disconnect panel. Therefore we bond them at the service equipment (main disconnect enclosure) so that both can utilize the service neutral to return to the source.

If we were to bond neutral and ground load side of the main disconnect like at a receptacle we would be giving the neutral current another low impedance path back to the would flow on your equipment ground wires. No current from the operating system is ever wanted on the equipment ground. Remember current will take all paths given to it to return to the source. Given paths of equal impedance/resistance the current will split evenly on those paths. If in the case of a bootleg and the presence of an equipment ground in your wiring and you would have a neutral open in the receptacle box all neutral current would transfer to the equipment ground to get back to the source. Not what you want.

Your grounding system in a residence or more appropriately your grounding electrode system (GES) such as metal water pipes and ground rods are there for property protection not human safety. Thing you need to remember is current will not flow to ground (earth) in appreciable amps at low voltage like house voltages because earth (dirt) it is not a good conductor. It offers too much resistance. However in the situations where you have huge voltage events, like lighting or a utility high voltage wire falling on your overhead service to your house, current will flow too earth because it is being pushed by huge voltage. So ground rods are there for this type of event but are only supplemental electrodes. Your water pipe if it is metal plays a much more important role than a ground rod assuming the waters pipe is in contact with the ground and is a continuous metal system with the utilities.

Now these ground rods IMO play a very small role in surge protection devices at equipment. Reason is they simply cut off electrical to the equipment and divert voltage surges to the equipment ground which will take the any current back to the main panel most of which will flow back to the source. So given a good surge protector and voltages it can handle effectively (also remember current plays a big role here) it simply protects the equipment plugged into it by utilizing a low impedance path to transfer the unwanted surge. If it is simply connected to earth via a ground rod the resistance is so high it likely would not be able to function properly and simply be damaged to the point of failure.

mystery guest 07-24-2008 12:47 PM

Thanks, good answer. I didn't consider the transformer on the post.
Eventually, I'll install another outlet in this room. I'll have to run it outside in conduit (pvc) and go through the wall. (Nothing is easy).

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