Why are electrical systems grounded?

F

Thread Starter

Fluoronator

Our service truck has a generator onboard to supply power for power tools, lighting etc. The truck also has a cord that can be plugged into a local outlet to power up the truck's accessories from another utility source. While being trained on the generator's operation, we were instructed to always bond the truck to ground (it has a cable with a ground rod we can jab into the ground) before starting the generator to prevent electric shock.

This seems backwards to me. It makes sense to ground the truck if taking-on external power because the oncoming power probably has a neutral that's tied to ground. But wouldn't grounding the truck for only generator use be creating a shock hazard?

For example, if I'm standing by the truck, barefooted in water, and I stick my hair pin the the hot hole of the truck's receptacle (truck ungrounded) am I completing a circuit? I think not. Now ground the truck and try the same thing and I'll get shocked, right?
It seems that an ungrounded truck is safer for generator use, while grounding the truck would be safer if taking on power from a grounded system.

During this training I had to speak up and ask, but I got dumb looks and comments about everybody knowing that grounding makes things safer. So... I did what I usually do in these situations, I made up a lie to back up my reasoning.

The lie goes something like this (I probably heard bits and pieces for real from somewhere).

Back in the day, when power was first being distributed, the power system was ungrounded. This was safer for people and pets, because simply touching a hot wire would not shock you. In order to be shocked you had to come between the hot and neutral wires and complete a circuit between them. Actually they weren't called hot and neutral because they were essentially the same. But there was a problem with the ungrounded system. Lighting would strike the power lines and huge balls of molten burning copper would erupt, burning houses to the ground for miles.

In order to fix the lighting problem, the system was grounded. But... This created a new problem. Now if you touch a hot wire while grounded, you complete the circuit and receive a shock. To help with this problem, we started grounding everything that could possibly become "Hot". That way, when the ringer washer motor shorted to it's casing, an over current condition would be created and the fuse would blow. Of course, before our systems were grounded the short wouldn't have been dangerous.

So... Our truck doesn't need lightning protecting when running the generator. Why create hazards by grounding the truck???

I thought my lie sounded pretty good so now I'm wondering how close it is to the real story?
 
C

Curt Wuollet

You have a point as long as everything stays isolated. Suppose it's a rainy day and you have an extension cord out. And suppose the hot wire has a bare spot. That is the wire that is not hooked to the frame of the present day generator and very likely the vehicle. Now the truck has a potential of 120 volts above ground because the other wire is. You grab the door handle to get out of the rain and have an epiphany, the understanding overwhelms you and maybe the person that finds you. That's why you drive the ground rod, the gfci pops and you fix or toss the extension cord. Rain soaked (or even damp) power tools used to teach many the fear of God. And if you ever wore an electric guitar and grabbed a mike (or worse yet, brushed it with your lips) in the days when the chassis were not grounded, you would appreciate today's obsession with grounding. We like objects we touch to be equipotential, and ground is the safest potential and the easiest to find and be sure of.

Regards
cww
 
> I thought my lie sounded pretty good so now I'm wondering how close it is to the real story?

Here it is.

All electrons are searching for "Earth Ground" it is there "Home".
When we give them a clear path back to "Earth" they are happy and do our work for us on the way back.

If we do not give them a clear path to home (Earth Ground) they get upset and hurt people or damage things.

Very simple. :)
 
Fluoronator...

1) Are the receptacles mounted in/on the truck powered by only the on-board generator?

2) Are the receptacles mounted in/on the truck powered by EITHER the on-board generator OR another-source remote to the truck?

3) Are you in a country complying with NEC or CSA?

Regards,
Phil Corso
 
C

Curt Wuollet

Yes, but it would notice current going out that is not coming back and trip nonetheless.

My point is that these things are not without reason, even if not obvious at the moment.

It is hopefully, rare that the precautions come into play. But having seen some of the interesting wiring systems kludged into being by non-believers, I check grounds before I even connect equipment.

Regards
cww
 
B

Bob Peterson

There is a basic misconception that electricity wants to return to Earth. That is just not so. It wants to return to its source. In fact, it has to so to complete a circuit. Earth might be part of the path but is generally not the destination for electricity.
 
F
I never realized that electrons were so emotional! I was discussing the question with one of our EEs who told me about a barge loading station he once worked on. He said the electrical systems on the loading docks had a "Floating Ground". This was for safety reasons as mentioned in earlier posts. However, they also had instrumentation to detect faults and trip the system that would otherwise throw a breaker in a normal earth grounded system. He also mentioned hi impedance static draining devices.
 
> You have a point as long as everything stays isolated. Suppose it's a rainy day
> and you have an extension cord out. And suppose the hot wire has a bare spot.
> That is the wire that is not hooked to the frame of the present day generator
> and very likely the vehicle. Now the truck has a potential of 120 volts above
> ground because the other wire is.
----- rest of message snipped by moderator -----

Rain water is a poor conductor of electricity, as is distilled water. Until the water is contaminated by salt and other soluble minerals that is, even then it has a high resistance value.

Yes, it can carry enough current to do damage to equipment and people but it is not nearly as dangerous as the tools we work with.
 
Top