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Re: Can This Be Done? #96868 01/02/06 12:04 AM
Joined: Oct 2004
Posts: 135
B
BobH Offline
Member
The warning label was probably written by a lawyer, not an electrician, based on misunderstood information. Thank God electricians fully understand bonding and grounding, huh! [Linked Image]

2017 / 2014 NEC & Related Books and Study Guides
Re: Can This Be Done? #96869 01/02/06 01:28 AM
Joined: Apr 2005
Posts: 116
X
XtheEdgeX Offline
Member
Here's a definition of impedance.
http://www.encyclopedia.com/html/i1/impedanc.asp

Re: Can This Be Done? #96870 01/02/06 05:00 AM
Joined: Jul 2002
Posts: 717
M
macmikeman Offline
Member
In order to properly measure the impedance of Earth", one must first measure from a ground rod driven to an 8' depth into the Earth and one driven to an 8' depth driven into the Moon, and one driven to an 8' depth driven into Mars. Now we have a good 3 point system from which to measure the impedence of the Earth. Its the impedance of space which will get you every time. It is over 25 ohms.

Re: Can This Be Done? #96871 01/02/06 11:28 AM
Joined: May 2001
Posts: 717
G
George Corron Offline
Member
Guys,
Good answers, and lots of good thoughts.

The sticker is there to indicate that you should insure there is less than 25 ohms (though an arbitrary figure, CAT and others, use it because every electrician understands it) between the generator and what you are standing on.

The last thing anyone (and certainly CAT) needs is a difference of potential between a trailer sitting there in the open where anyone can touch it, and the trailer which is mounted on rubber tires.

A fault or inductive build up is one thing, the gen has plenty of safeties to deal with that, but if someone is touching it during that period, we want to make certain they don't get hurt.

They also don't want the electrician, or tech, to get zapped trying to crawl up on the trailer when trying to service.

There's a good half dozen proper ways to hook up a ground to a gen depending on your service type, etc. They're just trying to make sure you know it needs to be grounded at the frame to nearby ground, if you don't, it's on you, cause they put the sticker there.

Re: Can This Be Done? #96872 01/02/06 11:41 AM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline OP
Moderator
Quote
A fault or inductive build up is one thing, the gen has plenty of safeties to deal with that, but if someone is touching it during that period, we want to make certain they don't get hurt.


George that can not be prevented no mate how low the resistance to ground is.

It will be imposable to 'connect' the generator frame to earth in a way that would eliminate the danger during a fault.

This is exactly why I posted this thread, there are no real answers.

In some ways ways it would be better if the generator and the building it was suppling where isolated from the earth but that is not possible either.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Can This Be Done? #96873 01/02/06 11:59 AM
Joined: Jul 2004
Posts: 9,392
G
gfretwell Offline
Member
I was just over at my wife's construction trailer, powered by one of those 36kva generators. They have a 4 wire feeder into the trailer. the gen set is bonded and they have 2 ground rods. The trailer frame is also somewhat grounded by the 16 tie down anchors screwed into the dirt.

For the purposes of insuring there is no difference in potential between the trailer and the generator I think the EGC in the 1/0 feeder is doing a great job. Who cares if it is not "ground" as compared to the sales trailer a quarter mile away?

Now if they strung a LAN cable between them we would have a problem. They are letting Sprint DSL deal with that.


Greg Fretwell
Re: Can This Be Done? #96874 01/02/06 12:05 PM
Joined: Oct 2004
Posts: 135
B
BobH Offline
Member
Iwire, personally I think this label leads to misunderstanding and possibly a false sense of safety for those who do not understand grounding and bonding. They would be better off just stating that the generator should be hooked up by a 'qualified electrician' and leave it at that.

Re: Can This Be Done? #96875 01/05/06 04:04 AM
Joined: Jan 2005
Posts: 39
S
skipr Offline
Member
The human body has roughly 25 ohm's of resistance and heart beat pulses at 60hz. So the path to ground needs to be less, so current choose's the least resistant path to ground in the event of contacting live parts. try checking resistance on yourself, you will see a many different reading
s as you place probes at different locations on yourself. OH and DSA (department of state architect's) does a earth ground test every time I install rods on high school campus electrical systems.If 25 or less is not achieved, I must correct, either by soil treament or driving another rod no more than 6 feet from first.

Re: Can This Be Done? #96876 01/05/06 05:39 AM
Joined: May 2003
Posts: 2,876
E
e57 Offline
Member
"heart beat pulses at 60hz" I would not rely on that... On average humans, 72 beats a minute, higher if you smoke and drink coffee non-stop like myself. [Linked Image] Far cry from 60 cycles, a second.

Also be wary of placing meter probes on the body, many have voltage that could be not cool going across the heart. Especially those used to check ground resistance.

Anyway, I think this sticker is intended for situations where the the gen set is not part of a building ground, and not perminatly bonded to the neutral, making it a seperately drived system. It would then require its own rod(s) at 25 ohms or less. Or, if it were a stand alone power source.

Tied to the buildings ground system (Bonded neutrals and all), and with a EGC sized for it, you would not need a seperate rod at all. Making said sticker a superphilus decoration.


Mark Heller
"Well - I oughta....." -Jackie Gleason
Re: Can This Be Done? #96877 01/05/06 08:14 AM
Joined: Nov 2000
Posts: 2,148
R
resqcapt19 Offline
Member
skipr,
Quote
The human body has roughly 25 ohm's of resistance

Most studies on this show that the average person has a resistance of 500 to 1000 ohms, not 25.
Quote
so current choose's the least resistant path to ground in the event of contacting live parts.

It just doesn't work that way. Current takes all paths and as long a the fault exists, the grounding electrode provides no increase in safety for people. Ground rods and grounding electrodes do not get rid of the voltage, it is there until the fault clears and the grounding electrode system does not play much of a part in the fault clearing process. The big players in that process are the main bonding jumper and the equipment grounding conductors.
Quote
I must correct, either by soil treament or driving another rod no more than 6 feet from first.

The code requires that the second rod be at least 6' from the first, and it is most effective when it is spaced twice the length of the rod from the first rod.
Don


Don(resqcapt19)
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