Earthing a generator

Small portable generators have several possible solutions. Sometimes there is a lug on the frame Sometimes the is a terminal near the neutral/ground connection usually near the overcurrent devices or the outlets.

I recommend that you accuire a manual and see what it says.

Any screw head or nut on the frame of the generator will work fine. If it is bolted up to the engine, then add any screw or nut head on the engine.

You are quite right, a generator needs to have a local earth ground for safe operation.

Hmmm. My news feed doesn't have the original article so I don't have the full context but... generators don't always need a local ground. As in here, which explains it wrt 'separately derived systems'.

Bob

Hello, and many years ago when I was with a US Army signal battalion (501st of the 101st Airborne/Air-assauit Division) it was standard procedure to always ground (via a driven ground rod and mesh copper) our

3kW and 5kW units. If you asked any of the EM or NCOs why the answer was always "safety" but they could not explain it beyond that. The metal enclosures on truck-mounted comm equipment shelters were also earthed (probably more for lightning protection).

What a lot of the discussion in this area seems to overlook is that by earthing a generator you can introduce a potentially fatal shock hazard that would otherwise not exist (except under fault conditions). If the neutral of the generator is bonded to earth ground via a relatively low resistance connection a potential difference (capable of causing a potentially fatal current flow) will exist between a "hot" wire at the generator output and the earth. Of course the generator may have a metal frame with the neutral connected internally to the frame. If the metal frame is not insulated from earth ground you can also a have path to ground for the neutral (but most likely not as good as with a driven ground rod).

Unlike commercial power distribution systems where voltage stabilization and tripping overcurrent prodcution devices (fuses or circuit breakers) is desirable under fault conditions (e.g. a hot wire touching a conducting object that is in contact with earth), this is generally not an issue with portable generator sets intended primarily for local consumption such as by power tools, etc. and that are not connected to some extensive power distribution system/grid.

US Navy ships in particular do have extensive A.C. power distribution systems in which no current-carrying wires are intentionally connected to the steel hull. This is done to promote operation of electrical equipment under fault ("battle short") conditions (e.g. a current-carrying wire touches the hull). The ability to have a fault blow an overcurrent protection device is not desirable in this environment. You still find the standard "U" ground outlets with the ground prong in electrical contact with the hull, however. If you were to measure the voltage from the + or - prong of a shipboard 120 VAC outlet to the ground prong you would see about 60 VAC due to capacitive coupling to ground/hull.

Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

4555 Overlook Avenue, SW Washington, DC 20375-5337

There are two separate issues here.

One is whether to GROUND the generator frame.

The other is whether to intentionally GROUND one of the current carrying conductors.

A good case can be made to NOT deliberately ground a current carrying conductor.

BUT the generator frame should be grounded. If there is an internal connection between a current carrying conductior and the generator frame when grounding the generator frame will ground the "neutral." Better that than risk the possibility of a fault somewhere in the system turning your generator into a bobby trap.

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Hello, and if the neutral is already bonded to the frame you would be setting up the condition that I commented on in my previous posting, viz., creating a hot-to-earth ground potential where one previously had not existed (or at least not capable of delivering lethal current). The internal neutral/frame bond will still allow the generator's fuse/circuit breaker to blow if the generator's hot wire(s) contacts the generator case/frame. But it wouldn't necessarily electrocute you if you were to grab a hot wire while standing on earth ground. The probability of electrocution increases in this scenario if the frame is metallically grounded to the earth.

It's probably easiest to picture the generator as a voltage source. Ground one side of that source to earth and the other end becomes hot with respect to earth ground. Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

4555 Overlook Avenue, SW Washington, DC 20375-5337

Couldn't have said it better myself ;-)

Former EMC(SS)

Maybe so. But the HOT piece of metal will not be the generator.

If you ground the generator, there is a chance that a ground fault will trip the circuit breaker. If you don't, that same fault makes the generator a bobby trap.

The

I say it would be about the same.

If for no other reason than the rotor is a Wimhurst static electricity generator that can build up several thousand volts of potential on the generator frame. That is certainly enough to for a spark to ignite a gasoline fuel tank under some conditions.

I would have NEVER considered that!

I just figured you have a good sized metal box with all kinds of "lectricity running about. I want that box GROUNDED.

Hello, and to give another scenario to the one I recently provided:

The generator set has a metal frame that is bonded to the generator's neutral wire. The generator is on a rolling cart with rubber wheels and is presumed to be isolated from earth ground. The generator is distributing power to appliances located within a metal equipment shelter located a short distance away. The shelter is connected to earth ground via a ground rod. An appliance in the shelter develops a fault resulting in the connection of a hot wire to earth ground. It is further assumed that no bonding exists between the neutral wire and the shelter's ground connection. Since the generator is ungrounded its frame is now hot with respect to earth ground. In this scenario the frame should be bonded to earth ground. Whether or not sufficient fault current will flow to trip the generator's breaker/fuse depends on the resistance of the ground rod interfaces and the soil between the generator and equipment shelter.

Suppose, on the other hand, that the grounded frame/neutral generator is only connected to a metal-shell power tool that is supposed to be fed by a

3-wire cord but the ground prong on the tool's cord has been cut off. An internal hot-to-shell fault occurs in the tool. Since the neutral on the generator has been connected to earth ground the tool's shell is now hot with respect to earth. Had the generator been floating off ground this situation would be more benign. Of course the primary mistake was operation off of two wires when three are required for safety.

There are two safety considerations here: Protection of personnel at the source (generator) and at the sink/load (actually three considerations if you include the distribution system). From the perspective of the source, if I had a metal-frame single-phase portable generator that was to suppiy power via two wires (designated hot and neutral) to something(s) hidden behind a curtain I would ground the generator frame to earth ground. I suspect this is why we did it in the Army signal battalion since equipment and tactical power distrubution wires were often being reconfigured.

I don't think grounding is necessary if you are providing connection via a

3-wire (hot/black, neutral/white, ground/green) cord between the generator and an appliance such as a power tool. A two-wire connection is also OK if the appliance has a doubly-insulated shell.

The need for grounding of a portable generator therefore is dependent upon how the generator is internally wired and the external infrastructure to which the generator is connected. The NEC covers the most likely scenarios. Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

4555 Overlook Avenue, SW Washington, DC 20375-5337

Excellent response, John.

I agree completely, except for the part that says "a generator needs to have a local earth ground for safe operation." I would eliminate the word "NEEDS" and say it's highly recommended. I do believe it is safer, but absolutely not required. It also depends on your uses. If you are running a large generator for outdoor construction, I'd definately ground it, using any bolt connected to the generator frame. (meaning the actual generator, or anything connected metal to metal to it.). But if you are running a

500watt generator to run the razor and tv set in your camper, I'd not as important (but preferred), Mark

But if you are running a

In that case, it would be just as important to ensure that the generator frame is connected to the frame of the camper.

The only time is would be completely unnecessary is where the generator is just running a single load that a power drill.

If you are running strings of lights it's a very good idea to ground as faults can easily occur.