Construction Cranes Radio Frequency Induction

[SafetyConsult]

Can anyone explain why cranes in construction sometimes experience induced currents from antennas radiating from distances of up to several Kilometers.

I have looked into the phenomenon several times, and have not been able to find a study or baselines to follow. I have a basic understanding of how it happens (antenna theory and reradiation), but not a solid proof. Can anyone point me on the right direction.

 

[John37309]

Can anyone explain why cranes in construction sometimes experience induced currents from antennas radiating from distances of up to several Kilometers.

I have looked into the phenomenon several times, and have not been able to find a study or baselines to follow. I have a basic understanding of how it happens (antenna theory and reradiation), but not a solid proof. Can anyone point me on the right direction.

Yea, i can help you with this one :)

First you need to explain as best you can what you mean? Why are you asking the question? Is the cane driver suggesting he is getting "static shocks" touching the metal ladder maybe? What exactly is the original complaint that's causing you to investigate this?

If its a tower crane, and its sitting on a per-poured concrete base, and the soil conditions are very dry, the metal frame could be kind of "insulated" from the body of the planet earth. If that's the case, you need to get an electrician to do some Earth bonding of the metal frame of the crane, to several Earth rods spread out over a large area. Or even better, bond the metal frame of the crane to the local supply transformer Earth point (The centre tap of the 3-phase transformer). Your electrician could test this afterwards by doing what's called an "Earth loop impedance test" to check the impedance of the Earth back to the transformer.

If the crane is "Earth bonded" to the local supply transformer, and grounded with several Earth rods using 50mm squared copper Earth wire to the Earth itself, the crane will not pick up any static electricity, or it won't pick up stray radio or microwaves.

Hmmmmm... Or are the cranes mobile cranes that sit on rubber wheels? I need more info.

John.

 

[SafetyConsult]

John Thank you for taking the time to reply. You can assume that the Tower crane (or it could be any mobile, gantry ect) is effectively grounded. The second assumption is that the crane is outside of the close proximity radius of the generating antenna. The phenomenon is usually associated with AM signals, but not exclusively. The end result is RF burns of the employees (contact), but only for certain locations of the crane parts I.E. only when the boom is perpendicular to the ground (to give you an example). finally RF grounding is a solution, but not necesary the most feasible way to abate the issue. Once again thank you for taking the time to reply.

 

[Studiot]

but only for certain locations of the crane parts I.E. only when the boom is perpendicular to the ground

?

Confused

 

[SafetyConsult]

?

Confused

Presumably is because at those positions the crane creates a tune antenna (1/4, 1/2... wave). Reradiating the frequency (or frequencies), this could be a possible explanation why this occurs. I know this much in theory, however, I have never encountered the issue myself. I just gathering information becuase I have not been able to find a standard or study that explain the phenomenon. My deductions have been on inference looking into the permit process for new antenna installation which requires an interference case study to preven re-radiation nothing to do with cranes. Near field its pretty much address by the FCC and OSHA standards and there is a solid basis for it. Far field is another issue, but as I mentioned I will welcome any input.

 

[John37309]

John Thank you for taking the time to reply. You can assume that the Tower crane (or it could be any mobile, gantry ect) is effectively grounded. The second assumption is that the crane is outside of the close proximity radius of the generating antenna. The phenomenon is usually associated with AM signals, but not exclusively. The end result is RF burns of the employees (contact), but only for certain locations of the crane parts I.E. only when the boom is perpendicular to the ground (to give you an example). finally RF grounding is a solution, but not necesary the most feasible way to abate the issue. Once again thank you for taking the time to reply.

SafetyConsult,
You need to understand something here. What you are suggesting is almost impossible! So some of the information you have just given me MUST be wrong!

So no, its a very bad idea to "assume" the crane, and all its parts, are grounded. Let me tell you why!

The only way to get current to flow in a circuit is to create a voltage difference between two points. That is physically the only way you could get a shock or a burn. If every single metal part of the crane is electrically bonded, that means joined with heavy copper wire, to every other part, and the whole crane structure is "earth bonded" with copper wire and Earth rods into the local ground, then it would be very difficult to get a shock from any part of the machine.

This is what it sounds like to me. In some electrical panel on the crane, probably a messy looking panel that guys have been pulling at the wires, one wire has shorted off whatever people are getting the shock from. If this is a port ship-to-shore gantry crane, and they are getting the shock while raising and lowering the boom, then the boom of the crane is not properly bonded to earth. Or the Boom operators cab is not bonded to earth. If its a luffing-jib tower crane, then the jib needs to be bonded to earth, and the cab.

I will repeat it again. If every metal part is Earth bonded to every other metal part, and Earth bonded back to the supply transformer, it would be almost impossible to get a shock or burn.

I am very very skeptical of your radio antenna theory. I promise you, its really unlikely unless your crane is within 100 yards of some massive TV transmitter.

On very large cranes, just because there might be massive steel bolts and pins joining the structure together, this does NOT mean its electrically bonded. It might look like its bonded, but its NOT. Across the pivot hinges on the boom of a crane, the steel pins will most likely be greased. This could have the effect of kind of "insulating" the boom from the rest of the crane. Its will definatly cause increased "impedence" in the electrical connection for short circuit currrents. Do you physically see a very heavy copper wire that bonds the electrical panels on the crane boom, to the main electrical supply?

Also, what is the "generating antenna" you suspect might be involved here? And is this a legal case you are fighting in court or just a general complaint from crane drivers that you want to solve? Describe the type of crane if you can, this might help me? The more specific you are with details, the more help i can give you.

John.

 

[SafetyConsult]

You are absolutely correct John. Correction: The crane is electrically (power source) grounded. This does not mean it is grounded for RF current because it has to deal with impedances (impedance matching). Most antennas are a little bit diferent than your normal electrical circuit (look into lighting protection for antennas)I hope that this clarifies my response.

 

[John37309]

What about this question;

Also, what is the "generating antenna" you suspect might be involved here? And is this a legal case you are fighting in court or just a general complaint from crane drivers that you want to solve? Describe the type of crane if you can, this might help me? The more specific you are with details, the more help i can give you.

John.

 

[Studiot]

I still think the clue is in the statement that it only happens when the jib is vertical.

There is only one type of tower crane where this can happen.

But you did not answer my earlier question.

 

[John37309]

The end result is RF burns of the employees (contact), but only for certain locations of the crane parts I.E. only when the boom is perpendicular to the ground (to give you an example). finally RF grounding is a solution, but not necesary the most feasible way to abate the issue. Once again thank you for taking the time to reply.

Describe the "RF burns"? Do they only happen on peoples hands? Like when they touch something? Is it a burn to the another part of the body, like the face? Are they getting a rash? Did any of them see a doctor? How do you know its not an infrared heater they have in the cab that burning them?

John.

 

[SafetyConsult]

Generating antennas means just that (at the radio frequencies of course), however, the phenomenon is mostly attributed to AM frequencies. My motives are based on a general request. I will be doing a presentation about the case study. No much of specifics as that goes because it the nature of being a consultant for a regulatory agency ( not all pertinent information available), but have to do the best I can with what I am given. The boom being perpendicular to the ground refers to one case (mobile crane), but not the others.

I have found serveral instances of the case in question. It has happened in tower cranes, gantries (rail mounted cranes ex. port based), barge mounted cranes, and mobile cranes. in a variety of places like bridge construction, port activities, general construction etc. The nature of the injuries is contact-burns (RF) to the hands. Sometimes it has been due to shock (posible transformer action on the coiled lines) not rash, in most cases it involves a doctor visit. Heaters not a plausible explanation due to most shock cases involves the riggers. I wish I had the luxury of more information.

This is a good area to go into research if that is your area, not many facts to go on. Interest from construction/building community could mean the opportunity for monitazation.Thanks again for at least trying.

 

[SafetyConsult]

?

Confused

I was not clear about that. What I meant was that it only happens at certian set ups. for example if the hook is at ground level and the boom is at 60 degrees from the horizontal. However it does not happens when the boom is at 30 degrees and the hook is at ground level. I was trying to say that the crane is not constantly energized only when it is at certain positions or set ups.

 

[John37309]

Generating antennas means just that (at the radio frequencies of course), however, the phenomenon is mostly attributed to AM frequencies. .

You seem to be very confident about the burns being "radio frequencies" and specifically AM radio. Why? Why are you so confident about that? Why are you so confident that these burns are caused by radio transmission antenna's and not from something else?

From an electrical engineering point of view, this is really unlikely. Its so unlikely from what your describing, it sounds like your just repeating "hear-say" information and speculation by crane drivers with little or no understanding of electrical engineering.

You keep talking in general terms, and not about any specific one event that you can describe in detail. So I'm just going to give you some general facts.

Mobile crane operators almost NEVER ground the crane to the body of planet Earth with an Earth wire, for the simple reason, the crane is Mobile. As a result of this, yes, its very likely that the crane could build up a "static" electric charge. This is nothing to do with radio antenna's. It happens on hot humid days just by the wind blowing past the metal crane building up static on the metal. It also happens in your car. Sometimes you can get a static shock as you get out of your car and your feet touch the ground on a hot day.

Riggers and tower crane electricians rarely ever ground the body of the crane to the earth. I know that as a fact!

And i simply don't believe you about the rail mounted gantry cranes. That just sounds like tea-time small talk among dockers with no electrical engineering education. I never heard of anyone getting any kind of shock or burn from the main body of a rail mounted gantry crane. It sounds like an old wives tale. You need to give me a specific case before i could elaborate any further on that one.

This is the science;

If the body of the crane, or any part of the crane is sitting on something that acts as an electrical insulator, then the crane could very easily build up "static charge". Electrical insulators could be;
1. Very dry concrete
2. Very dry soil
3. Asphalt or bitumen road surface
4. Rubber wheels of the vehicle
5. Putting timber boards under the out-riggers of a Mobile could insulate the crane from the earth.

So if the crane is sitting on any of those surfaces, the surface could be insulating the crane from the Earth and allowing static charge to build up on the crane. And the driver could get a shock.

If you want i can write this out as an electrical engineering report on a professional basis for publication and proper reference. You can contact me at john37309 at hotmail dot com, if you need something more professional. I worked on the electrical engineering side of cranes for 15 years.

John.

 

[SafetyConsult]

I am sorry you take this position about my responses because I wish I can contribute more specifics for you.

It is evident to me that you have not knowledge about radio frequency transmission, near, far field characteristics, much less to the question at hand "the phenomenon of re-radiation, and detuning towers". The phenomenon of contact burns due to RF induction is well know in the telecom industry, therefore, it's addressed in ANSI C95.1-2005 which is an IEEE based standard. The FCC addresses the issue for new or renovation for Cell and Microwave towers (public mobile service) in CFR 47 Ch. 1 subart D paragraph 22.371. The OSHA standard addresses it in general terms in CFR 29 paragraph 1926.550-0-15-vii. The Navy addresses in its NEET Module 17 p 3-40. There are several studies for example Gandhi, O. P. "radio-frequency hazards in VLF to MF band" quoted from IEEE, Vol 70 No. 12 P. 1462-1464., or Rogers, S.J. "RadioFrequency Burn hazards in the MF/HF Band.", USAAFSAM Aeromedical Review 3-81. And so on, so forth.

I was only looking to find someone that was willing to share their first hand knowledge with me. However, I do appreciate your efforts, time and conderation.

 

[Studiot]

I have no knowledge of dockside cranes or other apparatus.

I have, however, had considerable experience of construction and maintenance craneage and other equipment, and once sat on the (UK) Institute of Structural Engineers panel on the same and contributed to their code on the same. At that time I was responsible for the operation and maintenance of three permanent large electrically driven viaduct gantry cranes.

I am also aware of the induction hazards when working on pylons, masts and similar structures.

Having said all that I agree with john that you are too vague and further observe that you keep shifting your ground when pressed. That is not the way to prepare and present a good solid engineering report.

I have never come across RF burns from AM radiation. The only time I have seen these are from microwaves. That does not mean it can't happen, but just consider. The wavelength at AM is from several tens of metres to several hundreds of metres.
You will only get burns at antinodes eg at quarter wavelength. This can be an anormous length as noted.
If you have not heard of them you should google lecher lines.

You are also pretty vague with your description of the equipment. The whole point of any occurrence that is positional is that cranes have articulation joints. And sometimes with poorly maintained equipment a connection may not be properly transferred across the joint at a particular configuration.

The difference between static and many other effects can be distinguished by effect reproducability. Static will be random and local condition dependent. RF induction will occur every time the equiment is set to a particular attitude.

So to echo what John and Steelye Span said.

More meat. More meat.

 

[John37309]

Electrical engineering fact;
An antenna ceases to be an antenna if its grounded to the body of planet earth. I don't care if there is a million articles in a million academic journals discussing the topic. You cannot induce a radio signal in a crane, or any other metal structure that's bonded to planet Earth with 50mm squared copper wire. If you want a better bond, use 200mm copper lightning conductor!

SafetyConsult
And yes, that is the ultimate and final solution to all the scenarios you describe. It is the electrical engineering solution! No more Rf, no more burns, no more small talk in the crane driver canteen, no more old-wives-tales and myths. If the metal is 100% properly bonded, end of problem! Steel is NOT a good conductor, it conducts electrical signals, but it not a good conductor.

See if you will find a moble crane driver who's going to hammer Earth rods into the gound before he puts his jib up into the air. That sums up the problem, it doesn't happen! Thats why his jib acts as an antenna!

John.

 

[Studiot]

@John,

There's a world (pun internded) of difference between Earth bonding and RF Earthing.

 

[John37309]

@John,

There's a world (pun internded) of difference between Earth bonding and RF Earthing.

You must be reading different books than i do.

John.

 

[Studiot]

Perhaps, have you heard of an Earth plane?

 

[John37309]

Perhaps, have you heard of an Earth plane?

No, go on...enlighten me :)

Is it a sub-orbital virgin galactic spacecraft ?

John.

 

[Studiot]

I first thought your answer rather flippant, but looking at the first page of references google provides I can see where you may have got these non scientific ideas.

An Earth plane is a usually constructed from a mesh of wire cables spread out at ground level and earthed locally beneath a vertical whip aerial.

The use of the Earth plane alters the feed transmission line impedance and allows the whip to operate as a quarter wave (or perhaps smaller fraction) resonant aerial.
This mode of operation has a node at the Earth (plane) end and an antinode at the free end.

This brings me to my comment about lecher lines. These are basically pairs of parallel conductors acting as transmission lines for RF signals. For certain signals standing waves occur with nodes and antinodes. Note that many structures have parallel conductive members. No amount of Earth bonding, applied at the nodes, will make any difference whatsoever to these standing waves since they are already at zero voltage.

go well

 

[John37309]

Ok, sound fair enough! Do you think it might be contributing to the phenomena the safety guy is describing? Or could we alter the advice i gave the man about Earth bonding the metal structure of cranes?

I apologise if i was flippant, I'm not an expert in radio transmission and antenna's.

John.

 

[Studiot]

Or could we alter the advice i gave the man about Earth bonding the metal structure of cranes?

Nothing wrong with your advice mate. However remember that many (most?) cranes are self powered by onboard generators, if electrically operated. This modifies earthing advice.

We also need to distinguish between the conductive elements. (Structural) steel has much higher resistivity than copper or aluminium and different characteristics. this is why some saucepans work on induction hobs and some do not.

Next we need to ascertain the actual frequencies (or better, wavelengths involved).
Powerline frequencies have wavelengths measured in many kilometers.
The AM range the OP mentions varies from a few tens of metres to a few hundreds of metres.
Short wave and above vary from a few decimetres to a few metres.
Microwaves and above vary from sub millimetric to a few centimetres.

I have only come across induced burns from radar and microwaves in small metal personal objects, such as rings, chains and watchstraps. But these can be pretty bad. I know of one friend, working at Marconis, who lost a finger, cooked in a radar beam.

Then there is the arrangement and type of equipment. The OP starts out by saying that the effect happened with a vertical jib and AM transmissions.

Firstly there is only one type that is capable of verticality - a luffing jib - and this would normally only be vertical on assembly or disassembly as it serves no other purpose - and at this time protection may well not be in place.

But the problem with this is that at AM frequencies it is the gound wave that is used, and so called long wire AM aerials are horizontal for this reason. Sometimes a loop is used.

Finally we need to know who was doing what when alleged injuries were received. That includes what were they wearing especially any metal that might interact with the structure.

So I agree 110% - a great deal more information is required.

 

[John37309]

So I agree 110% - a great deal more information is required.

Yea, its a tricky one. I also understand where the safety guy is coming from. Some crowd asks him to carry out a safety analysis on this topic, and give him very little info to work with. So he is also just trying his best to find out what's happening. All he has to work with is second-hand, hear-say information.

I suppose i would add this advice to the safety guy;

If your employer is serious about solving this problem, its going to cost them money to really get to the bottom of the problem. Individual cases will have to be assessed, each on their own merit. Ever single situation will be different in its own way. Its a bit like solving crime, if you generalise all burglaries into one category, you will never solve any of them individually.

The quick solution that might "very generally" solve some problems will be Earth bonding. But as Studiot suggests, it might not solve ever case.

John.

 

[berkeman]

(Thread moved from General Physics to the EE forum)

I didn't see if it was already mentioned, but grounding the bottom of the metal structure of the crane wouldn't prevent the rest of the crane from acting like a receiving antenna. If the incoming RF has a wavelength that can resonate the metal structure, having a node at the bottom won't prevent getting anti-nodes along the height of the structure.

But if the suspect transmitting antenna is as far away as the OP suggests, then I agree that the source of whatever problems they are seeing is not from incoming RF.

 

[SafetyConsult]

The near field radius for AM antennas is 3km and effective range is up to 20 miles. Here is a case from a reliable source: http://www.osha.gov/dts/hib/hib_data/hib19900905.html

However, and again not much as far as specifics goes. I would not disagree with most the statements (there should be a pun in here). I have found others, but they are hearsay as so cleverly mentioned before.

The problem with the phenomenon is that SAR rates (current non-ionizing energy standard) do not really apply to the cases observed because the rates deal strictly with power density, or the amount of RF energy that is emitted from an antenna. in some instances power density were measured at 60 times below permissible limits. When crane tower is a significant portion of a wavelength in the AM broadcast band, mutual coupling occurs between the tower and the AM tower. This mutual coupling induces AM RF power in the crane which then will reradiate a portion of this AM power. Thus, the crane effectively becomes another tower in the AM station's system, changing the radiation pattern.

As Studiot mentioned the problem can occur at 1/4 to full wavelength (range ~50 ft to 620 ft.), however most common is 1/4 wavelength due to Max power output at the config. The problem is not exclusive to the tower itself (as berkeman) for instance the case above (link) mentiones that it was the line and hook the one that were reradiating.

 

[berkeman]

The near field radius for AM antennas is 3km and effective range is up to 20 miles. Here is a case from a reliable source: http://www.osha.gov/dts/hib/hib_data/hib19900905.html

However, and again not much as far as specifics goes. I would not disagree with most the statements (there should be a pun in here). I have found others, but they are hearsay as so cleverly mentioned before.

The problem with the phenomenon is that SAR rates (current non-ionizing energy standard) do not really apply to the cases observed because the rates deal strictly with power density, or the amount of RF energy that is emitted from an antenna. in some instances power density were measured at 60 times below permissible limits. When crane tower is a significant portion of a wavelength in the AM broadcast band, mutual coupling occurs between the tower and the AM tower. This mutual coupling induces AM RF power in the crane which then will reradiate a portion of this AM power. Thus, the crane effectively becomes another tower in the AM station's system, changing the radiation pattern.

As Studiot mentioned the problem can occur at 1/4 to full wavelength (range ~50 ft to 620 ft.), however most common is 1/4 wavelength due to Max power output at the config. The problem is not exclusive to the tower itself (as berkeman) for instance the case above (link) mentiones that it was the line and hook the one that were reradiating.

No, the near field is on the order of a wavelength, which is about 300 meters for a 1MHz AM radio broadcast band signal.

This quote from your link is important:

located in close proximity to several AM radio station transmitting towers.

Close proximity means pretty much next to the transmitting antennas. And yes, when you are close by and you have a resonant length piece of metal, you can get the 100mA-200mA currents that the article mentions. But you have to be in "close proximity", not kilometers away.

If you suspect the radio transmitters, you can rent a field strength meter for the appropriate frequency, and just measure the field strengths near the cranes. That will tell you if the source of the problem is the transmitter (unlikely) or perhaps some other electrical source.

 

[John37309]

The near field radius for AM antennas is 3km and effective range is up to 20 miles. Here is a case from a reliable source: http://www.osha.gov/dts/hib/hib_data/hib19900905.html

Yep, the article also suggests a few different ways to prevent the problem, namely grounding the crane's wire rope to earth. Or grounding the rigger who is handeling the lifting hook.

John.

 

[SafetyConsult]

No, the near field is on the order of a wavelength, which is about 300 meters for a 1MHz AM radio broadcast band signal.

This quote from your link is important:



Close proximity means pretty much next to the transmitting antennas. And yes, when you are close by and you have a resonant length piece of metal, you can get the 100mA-200mA currents that the article mentions. But you have to be in "close proximity", not kilometers away.

If you suspect the radio transmitters, you can rent a field strength meter for the appropriate frequency, and just measure the field strengths near the cranes. That will tell you if the source of the problem is the transmitter (unlikely) or perhaps some other electrical source.

Close proximity is a loose term which in this case any where that an induced current can occur since there is not enough science to set an standard for RF current induction besides AM broadcast as permitted by the FCC today could have a near field of up to 3km depending on the Antenna configuration.

The grounding of the line or hook does not eliminates the hazard. There is still RF current present when the ground is attached. John you have to be a fanatic of Ohms law.

 

[berkeman]

Close proximity is a loose term which in this case any where that an induced current can occur since there is not enough science to set an standard for RF current induction besides AM broadcast as permitted by the FCC today could have a near field of up to 3km depending on the Antenna configuration.

Can you please provide a technical source for the part of your statement that I've bolded?

Here is a counter-example, which matches my antenna textbooks:

http://en.wikipedia.org/wiki/Near_and_far_field

.

 

[Studiot]

there is not enough science to set an standard for RF current induction

I take exception to this claim.
Indeed if you truly think this why did you come here?

I have told you some of my bckground, although I do not know any of yours. I had understood you had come to PF seeking scientific discussion and enlightenment but you seem to avoid any request for the necessary detail that would lead to such an aim.

When I was a young pup I was told

"Good Engineering is all about attention to detail."

 

[SafetyConsult]

I must clarified once again. SAR rates is measure based on power density which is good predictor for twa exposure levels over 6 minutes. At this point there is not a form of measurement as I understand (e field, h field, power density etc) that will predict the total current induced on a tower; for that reason the burden is put on new telecom tower installations within the prescribe radius (1km to 3km) to do the studies and take precaution to prevent reradiation. In the event of a crane induction (one example of many possible) there is not set standard as I mentioned before.

Berkeman look into AM Directional antenna arrays for the answer to your question.

This is my first posting on a forum of this kind, and have learned many valuable lessons from it. One is you are a very well knowledgeable group in your respective areas, but most of all TOUGH Crowd!

 

[John37309]

SafetyConsult,
There is enough scientific information posted in this message to help you analyse the problem, and solve the problem. Its up to yourself if you choose to use the information.

Lot of people trying to help you solve this, people with different knowledge and from different scientific backgrounds.

John.

 

[SafetyConsult]

I would definitely agreed with you John. I have enough to go on. I was hoping for a silver bullet.

 

[berkeman]

I must clarified once again. SAR rates is measure based on power density which is good predictor for twa exposure levels over 6 minutes. At this point there is not a form of measurement as I understand (e field, h field, power density etc) that will predict the total current induced on a tower; for that reason the burden is put on new telecom tower installations within the prescribe radius (1km to 3km) to do the studies and take precaution to prevent reradiation. In the event of a crane induction (one example of many possible) there is not set standard as I mentioned before.

Berkeman look into AM Directional antenna arrays for the answer to your question.

I am pretty familiar with antenna arrays. For AM radio transmitters, one of the most common is a 3-tower array, used for gain of a few dBi in the direction of the tower line. Using an array would not affect the definition of the near/far field by more than the added size of the antenna array, I don't think.

And I still think that with a simple field strength measurement at the frequency of interest around the crane area, you should be able to predict what the maximum currents will be in the crane's metal structure. You would just assume an efficient monopole element antenna as the model for the crane, and look to see how the metal structure sizes compared to resonant antenna lengths.

Glad that you are getting some useful info in the thread. The PF is a great place for discussing questions.