Earthing a generator

[Livewire]

I've just bought a cheapo Honda clone generator off eBay. It's a great
machine.. . . up to a point!

But I wonder whether it should be earthed/grounded and if so, how? The
supplier is playing silly beggers and won't give a straight answer.

I think I need to find a proper earth relay on the machine. But how do I
recognise it?

 

[SQLit]

I've just bought a cheapo Honda clone generator off eBay. It's a great
machine.. . . up to a point!

But I wonder whether it should be earthed/grounded and if so, how? The
supplier is playing silly beggers and won't give a straight answer.

I think I need to find a proper earth relay on the machine. But how do I
recognise it?

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.

 

[Bob Rahe]

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'.

http://www.imsasafety.org/journal/marapr/ma5.htm

Bob

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[john mccarron]

I am not surprised that the manufacturer will not give you a straight
answer. To earth a small generator ( ground) or not ...................it
depends on the situation and the application. Many systems operate on
'floating' earths especially overhead systems in 3rd world countries and
also military applications.

 

[J. B. Wood]

I am not surprised that the manufacturer will not give you a straight
answer. To earth a small generator ( ground) or not ...................it
depends on the situation and the application. Many systems operate on
'floating' earths especially overhead systems in 3rd world countries and
also military applications.

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: [email protected]
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

 

[John Gilmer]

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).

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.

 

[J. B. Wood]

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.

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: [email protected]
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

 

[daestrom]

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.

Couldn't have said it better myself ;-)

Former EMC(SS)

 

[John Gilmer]

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).

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

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.

I say it would be about the same.

 

[John Gilmer]

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.

 

[J. B. Wood]

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.

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: [email protected]
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

 

[SQLit]

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: [email protected]
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

Excellent response, John.

 

[John Gilmer]

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),

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.