I work (when there IS work) as a gas/oil drilling consultant. There are
generally several work trailers on the drilling location powered from
the generator house - a large semi-trailer itself.
Suffice it to say the generators provide fluctuating power levels at
best which is why most trailers utilize a couple of layers of UPS
protection to level things out. My question is about grounding, which
can sometimes be an issue.
Some folks have taken to hammering in their own ground stakes outside
their trailers. These trailers are usually 50-150 feet from the
Some have taken the added measure of NOT using the ground line from the
generator and use only the ground stake and they swear this has saved
their equipment on numerous occasions.
How dangerous is it to have these separate grounds that far apart? If
they insist on their own ground stakes, would it be better to keep the
generator ground and bond the two grounds via the ground bus on the
I probably won't be able to convince anyone to alter their own beliefs -
I'm just trying to assess the level of risk I'm at when entering/exiting
these trailers in a snow/mud mix ankle deep (ie: should I wear my thick
rubber gloves when grabbing the metal door handles).
Most of these trailers use a split phase as they need 220 for the
Thanks in advance.
Although I am a licensed PE, this topic has not been my field of
practice. My comments should not be taken as definitive. They can very
well be wrong.
Power should be applied through the hot conductors and their returns or
neutral. If there is significant ground current, something is wrong. A
local ground not connected to a ground wire from the source should
protect against shock when a grounded person contacts the grounded metal
of equipment with insulation failure to the shell. That could be tough
on equipment, but probably less paper work compared to having a few dead
bodies lying around. I can picture that a long grounding conductor could
be less safe than a local ground.
That said, it seems that ground fault protection (GFCI) is in order.
There may be nuisance trips, but that only shows that something is wrong.
Ï "Bill" Ýãñáøå óôï ìÞíõìá
A floating neutral like what you describe can be very dangerous-all metal
frames of equipment would be electrified. Best to make sure about neutral
return integrity- especially if N is bonded to ground.
That would be _roughly_ equivalent to a TN-C-S earthing system
in Europe (also called Protective Multiple Earthing).
That's a TT earthing system in Europe (or an IT earthing
system if the neutral also isn't grounded at the generator).
It's regarded as safer than using a distant ground connection, but
(in UK at least), you would have to have an RCD/GFI as ground rods
often can't be relied upon to have a low enough impedance to handle
a fault current (short to earth) and pass enough current to blow the
circuit fault current protection within the required disconnect time.
I would say local ground rod is a really good idea.
TN-C-S requires a high integrity PEN (combined Protective
Earth and Neutral) conductor, so there's pretty much no
chance of it breaking with the live still connected.
PEN conductor also has to be sized to carry other stray
ground currents in the area which are nothing to do with
the trailer supply.
TT is probably safer but the mandatory RCD/GFI could be
an issue in some circumstances.
Sorry, I don't know the US regs in this area, so what I've said
above may not be permitted or customary in that location.
Its like deja vou all over again.
"Grounding" has 2 major functions.
1 To keep the voltage with respect to earth at a reasonable value.
Connecting the system neutral to earth provides this earth reference. A
ground rod is among the worst earth electrodes. The resistance to earth
for a quite good rod is 10 ohms. [There might be a lot better electrode
at the drill rig - like a metal well casing.]
2. To provide a high current path for a short-to-ground (ground fault)
to rapidly open protective breakers or fuses. This *requires* a metal
path back to the power source. The National Electrical Code (NEC) does
not allow the earth to be used as part of this path because an earth
path is far too high a resistance to be reliable.
I don't see any problem with adding earthing electrodes. Probably
required by the NEC.
Assume there is a ground fault at the trailer. The intended path is
through the source ground wire back to the generator trailer, through a
required ground-to-neutral bond, and to the generator neutral. That
gives a high current path to trip a breaker.
If you remove the source ground wire connection, the path is through the
local ground rod, through the earth, through the generator trailer
earthing, through the generator ground wire-to-neutral bond and back to
the generator. Look at only one piece of this path - the local ground
rod. If you have 120V connected to the ground rod (via the ground
fault), you have 120V with maybe 10 ohms (likely a lot higher) resulting
in a fault current of 12A (likely a lot less). That is unlikely to trip
any breaker. So the ground rod (and trailer "ground") are at 120V with
respect to "absolute earth".
As a rule of thumb, the 80% of the voltage drop away from a ground rod
is in the first 3 feet from the rod. Standing on the earth over 3 feet
from the rod and touching the trailer you have at least 96 volts - not
exactly safe. (Part of this could be at the generator earthing - so you
could electrocute someone there.)
Before the 2008 NEC it was probably allowed to bond the neutral to the
panel ground at the trailer. This would provide the metalic fault path
that is required (via the source neutral). (There remain the possible
hazards in Tzortzakakis's post.)
Multiple earthing electrodes that are bonded by a source ground wire are
IMHO a disconnected source ground, if there is not a N-G bond at the
trailer panel, is very hazardous.
In Andrew's (nice) post this is a TT system and requires GFI/GFCI
protection (RCD is the same thing in the UK). GFCI protection overcomes
the lack of an effective ground fault path. I don't believe this is
allowed in the NEC.
A few manufacturers used to want an "isolated ground" receptacle for
their data equipment, with the receptacle ground connected only its own
ground rod. (This was not only a major NEC violation, it was dangerous
Geez - what a bunch of wimps.
| This is an interesting question!
| The problem would be if the neutral became disconnected between the
| generator and the trailer (Not an unusual occurrence with high amperage
| connections!). Electricity would then want to flow through the ground (if
| the ground was bonded to neutral at the trailer main electric panel - which
| would be a necessary thing for safety of grounded appliances in the
| But this is the way the electric company provides electricity - no ground
| wire to a home/business - just the hots and a neutral and the neutral
| grounded at the transformer. Then the neutral and ground bonded at the main
| electric panel for the home/business and grounded there.
So I see no difference between generator to trailer wiring as described,
and utility to home wiring, aside from there being that upstream MV risk
for those sticking their hands inside the pad transformer, and whatever
difference might exist in terms of the chance of a bad neutral connection.
| I suppose you could say a generator is different because it is on the
| ground! But the electric company's "pad mount transformers" are on the
| ground as well, and no ground wire is run from this to the home/business!
I would prefer to keep the generator grounded just so there isn't a slight
voltage rise due to the voltage drop over the neutral between the generator
and the bonding point. But that could be just the generator frame itself
and not actually create a bond.
| Perhaps it is because unskilled persons would not tend to touch a "pad mount
| transformer" metal case? Yet they would have their hands all over a
| generator if there was an electrical problem???
A generator is not as likely to have 7200 volts.
| Or perhaps because generator wiring to a trailer would tend to be temporary
| with lines strung on the ground. Whereas the wiring from the electric
| company would be in the air or underground in well protected conduit????
I guess that is where the risk of bad neutral comes from. If you really
want to protect against that, put an isolation transformer at the trailer
fed with just 2W 240V. You can get your 120/240 from that with a ground
bond on the secondary center tap.
Nah - just got to keep the water in the bucket in a liquid state for
washing the samples of the cuttings from the bottom of the hole.
Otherwise all those clay particles from the drilling fluid makes it hard
to see the pretty colors under the microscope...
Without that need we'd all be like, "phht - below zero? Bring it on!"
...at least as far as you know...
Thanks to everyone for all the good info.
The information that we are missing is how is the neutral conductor
from the generator house terminated at the individual trailers. If
the neutral buss bar of the individual trailers is bonded to the
Building Disconnecting Means enclosure and thus to the local Grounding
Electrode Conductor then there should be no problem with having a
local grounding electrode for the trailer in question. In fact some
form of local grounding electrode is required at each building
served. If, however, the enclosure and the Equipment Grounding
Conductors are only connected to the local electrode and are not
bonded to the neutral then a real hazard does exist. In the event of
a fault to the frame of the trailer via the frame of some piece of
equipment the entire trailer would be at 120 volts relative to the
ground right beneath it. Within a few feet of the local electrode
that would not be true but I doubt that the ground rod is right near
the entrance. Are the steps up into the trailer metal or wood? If
they are metal then just don't grab the hand rail until you have both
feet on the steps. If the steps are metal and not physically attached
to the trailer then the danger point would be between the step
platform and the trailer door. In that case make sure you let go of
the hand rail before you touch the door handle. If the neutral is
floating at the trailer and the EGC back to the generator is not
intact that is a major OSHA violation for your employer. It is one of
the violations that can cost them thousands of dollars in fines alone
and void their workers compensation act protection opening them up to
The information that we are missing is how is the neutral conductor
from the generator house terminated at the individual trailers. For
quite some time manufactured buildings have been required to be
supplied by a feeder that has separate neutral and Equipment Grounding
Conductors. My use of the term Feeder is quite deliberate because
except for a recently added exception the National Electric Code has
required that the Service / Building Disconnecting Means be located
off of the manufactured building. If the neutral buss bar of the
individual trailers is bonded to the Building Disconnecting Means
enclosure and thus to the local Grounding Electrode Conductor then
there should be no problem with having a local grounding electrode for
the trailer in question. In fact some form of local grounding
electrode is required at each building served. If, however, the
enclosure and the Equipment Grounding Conductors are only connected to
the local electrode and are not bonded to the neutral; which seams to
be what you were saying in your original post then a real hazard does
exist. In the event of a fault to the frame of the trailer via the
frame of some piece of equipment the entire trailer would be at 120
volts relative to the ground right beneath it. Within a few feet of
the local electrode that would not be true but I doubt that the ground
rod is right near the entrance. Are the steps up into the trailer
metal or wood? If they are metal then just don't grab the hand rail
until you have both feet on the steps. If the steps are metal and not
physically attached to the trailer then the danger point would be
between the step platform and the trailer door. In that case make
sure you let go of the hand rail before you touch the door handle.
If the neutral is floating at the trailer and the EGC back to the
generator is not intact that is a major OSHA violation for your
employer. It is one of the violations that can cost them thousands of
dollars in fines alone and void their workers compensation act
protection opening them up to unlimited liability.