ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1] 2  All   Go Down

Author Topic: Mobile trailer truck grounding  (Read 1094 times)

Franz Francis

  • Newbie
  • *
  • Offline Offline
  • Posts: 29
Mobile trailer truck grounding
« on: April 08, 2014, 10:00:44 am »

Sometimes we deploy our PA on a mobile trailer truck for carnival street parades and this question is always raised, how does one ground the 110Kva generator used to power over 80,000 watts of amplifier power for these mobile deployments? The neutral is bonded but earth ground does not exist, Anyone wish to share their experience?

Franz
Logged

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Online Online
  • Posts: 2442
  • St. Paul, MN
Re: Mobile trailer truck grounding
« Reply #1 on: April 08, 2014, 11:45:56 am »

Sometimes we deploy our PA on a mobile trailer truck for carnival street parades and this question is always raised, how does one ground the 110Kva generator used to power over 80,000 watts of amplifier power for these mobile deployments? The neutral is bonded but earth ground does not exist, Anyone wish to share their experience?

Franz
Bond to the truck frame.  This prevents a potential difference between the generator/grounded things and the truck that you're standing on.  There is the possibility of a potential difference between the truck and the ground, however that's isolated by the tires, and floating. Even if there is a potential difference between something conductive on the ground and something conductive on the truck, unless there were two faults, there's still no circuit.
Logged

Chris Hindle

  • SR Forums
  • Sr. Member
  • *
  • Offline Offline
  • Posts: 351
  • Montreal, Quebec, Canada, Earth, Sol System,......
Re: Mobile trailer truck grounding
« Reply #2 on: April 08, 2014, 11:54:21 am »

Bond to the truck frame.  This prevents a potential difference between the generator/grounded things and the truck that you're standing on.  There is the possibility of a potential difference between the truck and the ground, however that's isolated by the tires, and floating. Even if there is a potential difference between something conductive on the ground and something conductive on the truck, unless there were two faults, there's still no circuit.
+1 to the truck frame.
(Get some SERIOUS hearing protection for the poor SOB that's gonna be driving that rig.......)
(No, closing the windows (with no A/C) isn't going to help at all  ;D)
Logged
Ya, Whatever. Just throw a '57 on it, and get off my stage.

Jonathan Johnson

  • Church and H.O.W. Forums
  • Hero Member
  • *
  • Offline Offline
  • Posts: 1072
  • Southwest Washington (state, not DC)
Re: Mobile trailer truck grounding
« Reply #3 on: April 08, 2014, 03:08:28 pm »

Bond to the truck frame.  This prevents a potential difference between the generator/grounded things and the truck that you're standing on.  There is the possibility of a potential difference between the truck and the ground, however that's isolated by the tires, and floating. Even if there is a potential difference between something conductive on the ground and something conductive on the truck, unless there were two faults, there's still no circuit.

Would two people standing on the ground touching the truck frame qualify as two faults?

Sometimes we deploy our PA on a mobile trailer truck for carnival street parades and this question is always raised, how does one ground the 110Kva generator used to power over 80,000 watts of amplifier power for these mobile deployments?

+1 to the fact that the question is raised! It's good to know there are concerned people out there who think about these things.
Logged
Stop confusing the issue with facts and logic!

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Online Online
  • Posts: 2442
  • St. Paul, MN
Re: Mobile trailer truck grounding
« Reply #4 on: April 08, 2014, 03:26:11 pm »

Would two people standing on the ground touching the truck frame qualify as two faults?
It's possible for two people to be the two fault paths, yes, but not simply if two people are touching between the truck frame and ground - they would be more like two parallel conductors of the same fault - i.e. two neutral wires.

For their to be a life-safety issue, a human(s) must complete the circuit - i.e. one hand touching a hot wire, the other touching either a ground that is ground -> neutral bonded or a neutral wire. 

In the US electrical system, because the neutral must be bonded to ground, we basically make the earth a part of the electrical system.  This has pros and cons.  One of the downsides is that a person standing on conductive ground - i.e. a wet concrete slab or soil with bare feet is already a part of the circuit, and touching a hot wire will cause a shock.

In the mobile scenario, there's no neutral -> ground bond, so one person can be standing barefoot on wet ground and touch the hot wire on the trailer with no risk, as there's no completed circuit path from the ground to the neutral of the trailer.  Hypothetically person A could be earth grounded and touching the hot wire and person B could be earth-grounded and touching the neutral wire, which would indeed complete the circuit, albeit with a fairly high impedance.

If you can ensure that your electrical environment will always be floating - as with a small single-circuit generator like a Honda EU2000, you can make an argument that not bonding is better, but this becomes problematic when the size of your system and the potential for current sources from multiple places grows.
Logged

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Online Online
  • Posts: 1043
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Mobile trailer truck grounding
« Reply #5 on: April 08, 2014, 03:58:02 pm »

If you can ensure that your electrical environment will always be floating - as with a small single-circuit generator like a Honda EU2000, you can make an argument that not bonding is better, but this becomes problematic when the size of your system and the potential for current sources from multiple places grows.

I'm not comfortable with a distributed power system with a floated ground. Yes, bonding the generator neutral to the truck frame is a good idea, and there's likely to be a safety-ground to neutral bond at the generator. So now imagine the following scenario where a power cable is trapped between a metal stairway and the stage which then "shorts" the hot to the grounded stage. Instead of tipping any circuit breakers, the entire generator frame and the body of the truck is now elevated above earth potential, and at the full circuit breaker amperage. Anyone stepping to the truck and grabbing the door handle would be in jeopardy of a severe shock and possible electrocution, especially if the ground was wet. The same thing would occur if a stage amp with a short to chassis comes in contact with the grounded metal stage or sits on the ground in a puddle. Yes, a GFCI in the proper path should trip and open the primary current fault, but I don't like to count on such things. So I still think any metal stage should be bonded to the generator N-G bonding point, and a real ground rod should be driven and single-point bonded to this same G-N bonding point. This earth grounding is also important for construction sites with multiple power drops for the similar situation where somebody drops a power tool in a puddle which would then energize all other grounded tools throughout the construction site. And remember, on any performance stage or construction site, it's not just the electrocution potential that's dangerous. Simply getting a non-lethal shock can startle a worker or musician enough to take a bad fall. And that can lead to serious lawsuits.

See this article on EC&M about an electrician who took a fall from a ladder after being shocked from a roof-mounted sign. He shattered two leg bones after the fall and eventually won a big dollar lawsuit, even though he was on an unscheduled visit to check for a wiring problem in that same sign. http://ecmweb.com/shock-amp-electrocution/case-hurried-electrician or see below.

A business man who had recently bought a building for use as a car repair shop knew from the start that he had a number of electrical problems to solve. So after taking possession of the space, one of the first things he did was call a master electrician in to address the lighting issues. A very busy man in high demand, this electrician eventually agreed to fit the garage owner into his tight schedule.

The scene

Arriving one morning at 4 a.m. unannounced, the electrician asked the one employee on duty at the time to turn on the parking lights. The front of the one-story building, which featured a flat roof, faced the road and had two roll-up doors with a large fluorescent sign placed on the front facade. The parking lot contained four lighting poles with feeds from the roof, where a sub-panel controlled the four sets of lights for the four poles.

Even a short fall from a ladder can result in a life-changing injury.

The accident

After receiving verbal permission by phone from the owner, the employee turned on the lights for the electrician, who proceeded to climb his ladder to test the connections on the lights at the weatherhead on the roof. Once he finished testing, the electrician began to climb down the ladder. As he descended, something unexpected happened. Touching his hand to the metal frame of the fluorescent sign, he received a shock — despite the fact that his ladder was made of fiberglass. This unexpected jolt caused him to lose balance and fall approximately 6 ft to the ground.

Although the effect of the shock was negligible — there were no visible signs of burning or any inconsistencies in the behavior of his heart or nerves upon medical evaluation — the way in which the victim fell caused severe fractures in both of his legs. Approximately 15 min. after the accident, EMTs arrived at the scene and administered drugs to help the electrician cope with the extreme pain. After medical evaluation, it was later determined that the fractures would never heal properly, requiring the 30-year-old victim to walk with a cane for the rest of his life.
The lawsuit and allegations

After the accident, the electrician (plaintiff) sued the owner (defendant) for damages, testifying that he was hired to check the non-working parking lights — not to look at the fluorescent sign. He maintained that he should have been made aware of this unforeseen danger at the work site before starting the job.

His attorney further argued that the defendant knew the sign was malfunctioning and had prior knowledge that it possessed a short circuit, causing the outer metal frame of the sign to become electrified. Although the owner did suspect there was trouble with the sign, he claimed it had a buzzing sound when turned on, which he asserted should have been reason enough for the electrician to be careful. The defendant’s legal team argued that as a non-electrician, the building owner took the proper course of action in hiring a master electrician to fix his wiring problems; therefore, he was not liable for the accident.

The plaintiff’s attorney stated that buzzing in fluorescent lights is a common phenomenon and cannot be considered a reasonable warning. He further claimed that it would be unreasonable for an electrician to assume that all electrical wiring on the premises was malfunctioning, basically requiring him to check each and every device unless specifically asked to do so.

Not surprisingly, the two parties disagreed on who said what and at what point in time. The electrician claimed he had not been informed about the sign by the shop owner, who purported that he did, in fact, give the electrician a verbal warning about all of the electrical wiring on the premises, including the sign.

The plaintiff’s attorney alleged that the employer had sufficient knowledge about the malfunctioning fluorescent sign but had not told the electrician about it. This failure to communicate such an important piece of information about the premises led to the accident.

Investigation and analysis

The plaintiff’s expert stated that exposed non-current-carrying parts of electrical equipment must be effectively grounded. The sign should have followed this requirement. By failing to ground the casing of the sign, the defendant had failed to comply with NEC regulations. The expert also claimed that by failing to implement the lockout/tagout of the sign or fully disconnecting the sign, the owner failed to properly guard the sign, thus failing to provide reasonable and adequate protection. The grounding to the sign was found to be connected but rusted. The prosecution argued that had the employer turned off and locked out the power source to the electrical supply to the sign, it would not have been energized — ultimately avoiding the dangerous situation.
« Last Edit: April 08, 2014, 08:15:27 pm by Mike Sokol »
Logged

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Online Online
  • Posts: 2442
  • St. Paul, MN
Re: Mobile trailer truck grounding
« Reply #6 on: April 08, 2014, 05:14:03 pm »

I'm not comfortable with a distributed power system with a floated ground.
No argument there.  Your view and mine do reflect the regional bias of North America; other countries see the balance of risk the other way.  I'm not sure what country Franz is from - his rules may be different.

Viva la GFCI.
Logged

Franz Francis

  • Newbie
  • *
  • Offline Offline
  • Posts: 29
Re: Mobile trailer truck grounding
« Reply #7 on: April 09, 2014, 09:18:24 am »

I am located in the Caribbean islands,

The electrical codes are slightly different from those of North America but at the end of the day it’s all about safety and not getting electrocuted.

Brief overview of the electrical setup. The 110Kva generator’s A.C alternator is configured for 415 volts three phase with a neutral. This is the same voltage configuration generated by the utility company.

The engine speed is factory calibrated for the A.C alternator to generate 50 Hz (1500 R.P.M).

Since most of the electronic load (Amplifier etc) power supplies are wired for 240 volts everything is wired to the Generator’s A.C output with taps from any of the phase legs to neutral.
We try to get as close as possible in getting the load balanced on all three phases.

The very few equipment most times backline devices (U.S built keyboards, guitar amps etc) power supplies are 120 volts; we step down 240 volts to 120 volts through a step down transformer. Because these devices don’t draw much current the size of this transformer can be anywhere between 2kVA and 5 kVA.

We have never had any complains of voltage hazards (People complaining of getting shock when touching the truck) sometimes these carnival parades conditions are under rainy tropical weather with everyone socked wet and a few of them holding on to the truck while enjoying themselves.

I measured between the bonded Neutral and a pool of water a potential difference of 16 volts some time back and the reason why I posted the original question.

Almost forget it is very loud, the sub bass can actually cause things to get lose in the truck. The might of the P.A is always at the back therefore there is even more system in the back :-)

Franz
Logged

BobWitte

  • Full Member
  • ***
  • Offline Offline
  • Posts: 114
  • Soundman, Consultant, Love all things audio
    • Cascading Waters Audio
Re: Mobile trailer truck grounding
« Reply #8 on: April 09, 2014, 12:51:47 pm »

Sorry if I don't understand something here:

I'm not comfortable with a distributed power system with a floated ground. Yes, bonding the generator neutral to the truck frame is a good idea, and there's likely to be a safety-ground to neutral bond at the generator. So now imagine the following scenario where a power cable is trapped between a metal stairway and the stage which then "shorts" the hot to the grounded stage. Instead of tipping any circuit breakers, the entire generator frame and the body of the truck is now elevated above earth potential, and at the full circuit breaker amperage.

IF the generator neutral was bonded to the truck frame and the safety-ground to neutral was made, how could the body of the truck be elevated above earth potential (although without a ground rod/connection it would be floating)? Wouldn't this cause the circuit breaker to clear the fault of a "hot" to safety-ground (neutral) short circuit? isn't there a clear short circuit path - hot to neutral? I must be missing something "again"? I could understand that if the stage was not bonded to the truck frame, then the stage could be energized and then someone touching the stage and the truck - well that would be bad. But it was stated above that the truck was bonded to the safety ground and that was bonded to the neutral.


Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Online Online
  • Posts: 1043
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Mobile trailer truck grounding
« Reply #9 on: April 09, 2014, 09:13:55 pm »

Sorry if I don't understand something here:

I'm not comfortable with a distributed power system with a floated ground. Yes, bonding the generator neutral to the truck frame is a good idea, and there's likely to be a safety-ground to neutral bond at the generator. So now imagine the following scenario where a power cable is trapped between a metal stairway and the stage which then "shorts" the hot to the grounded stage. Instead of tipping any circuit breakers, the entire generator frame and the body of the truck is now elevated above earth potential, and at the full circuit breaker amperage.

IF the generator neutral was bonded to the truck frame and the safety-ground to neutral was made, how could the body of the truck be elevated above earth potential (although without a ground rod/connection it would be floating)? Wouldn't this cause the circuit breaker to clear the fault of a "hot" to safety-ground (neutral) short circuit? isn't there a clear short circuit path - hot to neutral? I must be missing something "again"? I could understand that if the stage was not bonded to the truck frame, then the stage could be energized and then someone touching the stage and the truck - well that would be bad. But it was stated above that the truck was bonded to the safety ground and that was bonded to the neutral.

I'm away from my office for the next week teaching seminars in Little Rock, but I have a basic generator distro diagram I made a while back which I'll modify the show the current paths. That should clarify what I'm talking about as well as solidify my logic. I'm pretty sure I'm correct, but I could just be saying it badly.
Pages: [1] 2  All   Go Up
 

Page created in 0.157 seconds with 23 queries.