RF Safety resources?

[Puffie40]

Hey all,

I recently landed a job at a lumber fingerjoint plant as an apprentice technician to look after the RF presses they use to cure the glue.  I'm scheduled to start this January, and I've been spending December researching and preparing.

From my research, the generators operate in the 2-30 MHz range.  My experience so far has been with UPS and power systems, not so much radio frequency equipment.

While I'm certain the company will have safety briefings on the equipment, I'd like to walk in with an idea of the do's and don'ts with radio frequency radiation.

Are there any safety tips you can share with me?

[ogden]

Know field strength and compare it to RF exposure limits. Look for RF safety standards (IEEE C95.1–2005): https://www.rfsafetysolutions.com/RF%20Radiation%20Pages/IEEE_Standards.html

Also you shall know everything about RF radiator itself and read all the operation/safety related documentation you can get from manufacturer (or it's competitors).

[edit] You may become quite knowledgeable after reading this: http://www.ilo.org/wcmsp5/groups/public/@ed_protect/@protrav/@safework/documents/publication/wcms_107796.pdf

[T3sl4co1l]

I'd expect it's well enclosed, if for no other reason, leaking RF is heat they could be putting into the board.

Also that there wouldn't be much fields around, say, resonant coils and such inside the machine -- an extended field would mean more induction in the enclosure, wasted power.  Keep a similar distance from those.

When it comes to maintenance, follow OSHA guidelines.  Lockout and tagout.  If it's tube based, it will reach out and touch you -- many kilovolts AC or DC, plus rapidly branching RF sparks if it's active.

Remember, those rules are there for a reason.  They may seem annoying or irritating, but every rule is written in blood.  Most of all, OSHA* understands that nothing is more ineffective than a safety protocol that isn't followed -- safety must be practical!

*Actually, I may be overstating this.  Safety officers, at least, I should say.  I would hope OSHA's official policy is similar.  Look it up if in doubt!

And above all, you are responsible for your safety.  Inspect the breakers and cutoffs and such.  Put your own lockouts on them.  Verify that the system is in a zero-energy state.  If it can't be worked on in a zero-energy state, take precautions to protect yourself (e.g., get out the rubber gloves -- and test them with a high voltage wand!).  Mind that rubber gloves probably won't help you much against RF.

Coming from power systems, you should already know this well (consider this an educational opportunity for others watching.. ).  Regarding RF, I don't think there should be anything that stands out beyond the usual procedures -- just keep extra distance from operating surfaces, and don't, like, stick your arm inside a resonant coil or something.  And that, more just because of the voltage present.  At these frequencies, the current will heat you up (diathermy), but that's probably not something you won't notice getting warm on the way in!

Definitely do take off conductive jewelry, rings and piercings and such, before operating such equipment.  Induced current in metal objects will heat them up much faster than flesh alone!  (I worked with induction heaters before, which use much higher currents -- a ring is deadly to your finger there!)  There's probably a lot of moving machinery in that place, too, and you don't want to get caught on it -- long sleeves, loose clothes, loose hair, rings, bracelets and such, are probably discouraged.

But most of all, don't take our word for it, find the manufacturer's operating and service manuals, and read them.  If they don't discuss safe procedures, refuse to work on it -- survival is your right!

Tim

[dmills]

RF burns are also a thing, nothing quite like the smell of charing human because some bit of metal was only earthed at one end and you are accidentally leaning on the other end, just because something is earthed at one point does not mean there is not significant voltage to earth from some other point a few feet away....

Not safety as such, but do NOT power up any RF generator without the load being correctly connected, they don't like it and you can get things flashing over (Also, disconnecting an N or 7/16th type RF cable under serious load is a cockup you only make once!).

73 Dan.

[coppercone2]

is this like the machine from the movie Kickass?

Fucking scary.

[ogden]

is this like the machine from the movie Kickass?

LOL no int's not. Also not scary at all. Those modern machines shall be properly RF-shielded, shall not leak any RF radiation. If not broken and properly operated you can stand right next to it and nothing will happen to you or machine. Check demonstration in public area during exhibition, first one shown in following video:

[disclaimer] Similarity with my nick is pure coincidence

[coppercone2]

its also small. I don't like large hot things at all.

[Puffie40]

Thanks for the input guys.  I'll read the safety book Ogden linked to. I have some experience doing summer sawmill cleanup, and the importance of lockout was hammered into us at every opportunity.

The generator is definitely going to be tube-based.  The plant does a combination of fingerjointing and edge-gluing to make value-added product, so I would imagine a fairly high-powered generator would be needed for rapid curing, especially for material pushing 16-20 feet long.

[cdev]

One of my best friends from many years ago was a lady lumberjack who was killed in an instant at work, when a huge redwood tree shifted. That was a really tragic end to a rich life.

RIP Edie.

[coppercone2]

fyi to protect people, you should look at 'funny tree fails' on youtube. Their not funny and quite dangerous and even tragic but you seriously get an idea of what kind of weird shit trees can do to hurt you. I was genuinely surprised. After watching these videos for a good hour I was seriously worried about doing any more cutting by myself without more planning.



There is a whole series of these videos. I don't mean to bring humor to this thread but I seriously went 'what the fuck' at some of those clips because to me, it looked completely safe. I would  some of those cuts that ended up causing problems in those videos but you might need to watch a whole bunch of the videos to get an idea of how evil trees can be when they fail. Like splitting down the middle during a cut and hitting you upside the head with a ton of force in what can be described as a 'water wheel' motion. They also end up containing spring force in a weird way.

Also the tree industry is extremely scary in some places, like China, where people are literary running around with chain saws mounted on wheels and all sorts of crazy shit.

Be very careful cutting trees. Coordinate with your co workers well and positively ignore any management concerns related to throughput in this industry.

[vk6zgo]

Hey all,

I recently landed a job at a lumber fingerjoint plant as an apprentice technician to look after the RF presses they use to cure the glue.  I'm scheduled to start this January, and I've been spending December researching and preparing.

From my research, the generators operate in the 2-30 MHz range.  My experience so far has been with UPS and power systems, not so much radio frequency equipment.

While I'm certain the company will have safety briefings on the equipment, I'd like to walk in with an idea of the do's and don'ts with radio frequency radiation.

Are there any safety tips you can share with me?

Transmitters of reasonably high power of all kinds offer the possibllity of injury, but usually are quite benign.

They are also large, & fairly easy to work on, with the overwhelming majority of faults being repairable with the units switched OFF at the Main breaker associated with this equipment on the building power distribution panel.

They also usually (especially in Tube designs), offer extensive metering, so you should not have to apply a DMM to an active device.

From the little I know about RF heating devices, they are fairly simple, & do not have the fairly complex "Exciter" circuitry common to Broadcast & Comms equipment.

If you inadvertently remove a panel while the device is operating, two things normally happen:-
(1) The equipment's own main breaker  ( this NOT the one on the building distribution panel, referrred to above), drops out, removing HT & filaments from the device ( sometimes it  only removes the HT.)

(2) A shorting bar is placed across the large HT reservoir capacitor, discharging it.

When this happens, it will go "Bang!!" & scare the pants off you!

If you then wish to delve into the "guts" go & turn off the associated main breaker on the building distribution panel, then use a shorting staff, which is a long piece of rod of insulating material, with a hook like a "shepherd's  crook" on one end, & a long length of sturdy cable with a large alligator clip on the end.
In use, you attach the clip to a good " ground"  (normally the metal chassis/cabinet).
The hook is now placed so it sits around the  "hot" terminal of the reservoir cap.

All this should be done, whether or not (2) seems to have happened.

Much is made of "RF leakage", but , in modern equipment, it is not that hard to obtain excellent shielding.

[dmills]

If you then wish to delve into the "guts" go & turn off the associated main breaker on the building distribution panel, then use a shorting staff, which is a long piece of rod of insulating material, with a hook like a "shepherd's  crook" on one end, & a long length of sturdy cable with a large alligator clip on the end.

Aka the "Chicken stick", in tube based gear it is the plate voltage that is the real killer, think a few kV or so with a few amps behind it.

One thing to be aware of is dielectric memory in the HT caps (especially if they are film types), the bleeders should deal with this, but bleed resistors fail and a big film cap can creep back up to 10% or so charge over a few hours after being turned off and discharged, with 8kV as designed plate voltage that can be enough to surprise.... I have a nice heavy bit of braid with two crocodile clips that goes across the HT bank right after using the chicken stick that also serves as a last line of defense. It has a big red flag on it as a sort of 'remove before flight' reminder.

Regards, Dan.

[cdev]

I had never heard of this dielectric memory before, thanks for sharing it.

This is another good reason to get a DVM with a NCV feature, basically you can wave it around inside a piece of equipment and it will beep when it gets near a high potential voltage. I have found from owning one (my UT210E has one) that a lot of circuits contain high voltage somewhere, even low voltage circuits. For example, somewhere in the keyboard I am typing on right now there is some HV, enough to trigger the alarm.

So now even when something is disconnected completely and especially when not, I use the UT-210E to do a little survey to find any voltages. Combined with its ability to measure current down to 1 ma in a non-invasive manner by means of its (insulated) clamp its a great addition to the toolbox for safety.

[Puffie40]

Know field strength and compare it to RF exposure limits. Look for RF safety standards (IEEE C95.1–2005): https://www.rfsafetysolutions.com/RF%20Radiation%20Pages/IEEE_Standards.html

Some quick googling brought up this PDF of the IEEE standard:
http://emfguide.itu.int/pdfs/c95.1-2005.pdf

[coppercone2]

I asked about this in a previous thread but I learned more about capacitor manufacturing since then, since the capacitor uses some kind of metal spray process to short out the rolled up conductor to make something you can bond a wire or pad to for the lead (like in a film cylinder), Is it possible for a capacitor to get charged up and internally break a bond(viberational fracture) temporarily and remake that bond after it gets disturbed mechanically?

 I could kinda imagine it turning into something like a land mine particularly since the bus bars would already be shorted out during maintenance (maybe) so the discharge would make plasma in the capacitor and blow it up.

A solution would be a complex impedance measurement to make sure there is no disconnected capacitors, since a resistance test after a voltage test would not tell you if the capacitor is part of the circuit if conventional meters are used.

Anyway if I ever set up a high power capacitor system I will be sure to note its complex impedance.. but if you have many parallel capacitors they need to be stable/matched or you can't tell without doing a system breakdown. And you take the reading carefully.

[dmills]

This is another good reason to get a DVM with a NCV feature, basically you can wave it around inside a piece of equipment and it will beep when it gets near a high potential voltage.

Non contact voltage detectors are sort of useful for confirming that something is live, but they should NEVER be used to confirm the power is off.

Not only do some of them fail for DC (like say the plate voltage line in an RF generator), but they are difficult to verify. For me, plug out and put where I can see it plus bleeder resistors plus door interlock plus chicken stick, plus  shorting jumper beats non contact that I cannot trust to prove something safe.

Regards, Dan.

[m3vuv]

once had an rf burn on my fingertip running a homebrew 1kw linear amplifier thru a homebrew atu,the roller coaster handle was metal,first time i touched it it felt like a hot needle stabbed into my fingertip!,the only thing visible was a tiny black mark the size of a pin head,it hurt like a bitch for about 3 weeks,once you get an rf burn you wont want a 2nd!! m3vuv 73.

[dmills]

Ain't that the truth, the things are like arc eye that way.

Grabbed an inductor in a VHF output filter once to give it a squeeze to raise the inductance, forgot that the rf deck was hot....  Very sweary few weeks.

Regards, Dan.

[coppercone2]

that sounds like a interesting tool a full dielectric pliers for squeezing larger components .

I wonder what jaw grip would be the best. Probobly like the ones knipex makes for pipes with the inserts but full nylon or something

[coppercone2]

I wonder if you can just make a mold to make a pair out of epoxy or something... I wonder what a good mold material would be to make a pair.

I have plastic tipped ones (for fine wire use) but nothing that can grab a big RF choke.

[coppercone2]

there is also a RF burn/injury thread in this subforum that has some stories

[vk6zgo]

once had an rf burn on my fingertip running a homebrew 1kw linear amplifier thru a homebrew atu,the roller coaster handle was metal,first time i touched it it felt like a hot needle stabbed into my fingertip!,the only thing visible was a tiny black mark the size of a pin head,it hurt like a bitch for about 3 weeks,once you get an rf burn you wont want a 2nd!! m3vuv 73.

When I first worked at a high powered AM Broadscast site, I was dispatched with a broom & a dustpan to clean up the "tuning huts" of the 6WF standby antenna.
This "Triple Tuned Alexanderson" had, as the name implies, three such huts.
A job like this was really "a bit of a plum" as you could take your time, away from the Boss's attention.

After sweeping, & knocking down cobwebs, I turned my attention to the tuned circuits. (Large coils made of silver plated copper pipe, & big "cheese slicer" variable capacitors.)

The ones in the feeder hut & one end one looked good, but the third one was a bit corroded.
I strolled over to the Comms Building & borrowed some "Silvo" and a couple of lengths of paper towel, with the idea of cleaning it.

On placing my hand holding a Silvo wetted sheet of paper towel on the coil, I copped a painful RF burn.
Who'd have thought that an antenna, resonant at the same frequency as an adjacent active one radiating
55kW would pick up so much?