Safety wire spec'ing

[ramdram]

Thanks for that PT. Your post raises an interesting question ref the Selecon issue. If the cert issued is worthless then what happens if you rely on this test cert to protect yourself in court say? Doubtless a field day for the lawyers.

 

Further, if as you point out that the OD of the wire is not really related to the rating per se then are we all (except those qualified of course) being misled on the ratings issue? I would say it is questionable to quote a figure which relates to a static load and not a falling load, where the force is applied to the bond in an instant as the lantern, say, snubs on the wire as its fall is arrested.

 

In fact do the makers or perhaps sellers truly understand their own comments if as you say the Selecon cert is worthless?

 

I suspect the term mass is omitted on the grounds that "everyone" knows about weight but "mass" may be interpreted as a "mass of stuff". It may be too that the "proper" terms for force such as Newton are not at all widely understood (when I were a lad we used poundals as well and how often do you hear that term?).

 

I note too that you were careful to say worst-case inflexible anchor point for the top of the bond. Another interesting point in that if a lighting bar was fixed to whatever then would/could that imply if a single lantern fixing "broke" and the secondary safety bond arrested the fall that the resultant impulse (right term?) generated to the rest of the bar fixings could weaken the grip of the fixings to the bar itself, if said bar was fixed by sleeve bolts (or similar) into masonry?

 

I find all this stuff fascinating and a bit of a wake up call because hitherto I have been blithely fixing (and insisting other folk do the same) secondary safeties to everything that is hanging, smugly secure in the knowledge that the safeties were properly designed, tested and fit for purpose. We now read that this may not be the case at all, viz Selecon.

 

Perhaps some enterprising soul might think about designing a secondary safety system which involved an energy absorber which de-accelerated the load over a second or so. Perhaps using a compressible spring or synthetic rubber block or deformable tube?

 

I can recall some of the Beeb riggers building tow ropes for their kit using steel cables through a spring from a quite large Macpherson strut to absorb the "snatch", so at least the theory works. Should emphasise too that they were strictly for off road use in an aerial field...

[johnlinford]

I note too that you were careful to say worst-case inflexible anchor point for the top of the bond. Another interesting point in that if a lighting bar was fixed to whatever then would/could that imply if a single lantern fixing "broke" and the secondary safety bond arrested the fall that the resultant impulse (right term?) generated to the rest of the bar fixings could weaken the grip of the fixings to the bar itself, if said bar was fixed by sleeve bolts (or similar) into masonry?

 

You're quite correct to point this out - after any large dynamic load (someone falling and being caught by a fall arrest system) you would likely have to perform a thorough inspection of the whole structure. It's important to consider the size of the dynamic loads from lanterns catching on safeties, and compare them to (say), the same effects when rigging them in the first place - I very much doubt with a heavy fixture that it is carefully placed on the bar imparting no additional impulse!

 

If you're rigging on a bar screwed in to the ceiling made of who-knows-what the amount you may want to worry about this would be different from on a counterweight/motor/hemp system on a stage.

 

I find all this stuff fascinating and a bit of a wake up call because hitherto I have been blithely fixing (and insisting other folk do the same) secondary safeties to everything that is hanging, smugly secure in the knowledge that the safeties were properly designed, tested and fit for purpose. We now read that this may not be the case at all, viz Selecon.

 

Perhaps some enterprising soul might think about designing a secondary safety system which involved an energy absorber which de-accelerated the load over a second or so. Perhaps using a compressible spring or synthetic rubber block or deformable tube?

 

For a <50kg fixture I think you may be over-doing it somewhat! I'd suggest care should be taken not to undersize the SWR and not to use unrated connectors and then, with the guidance of engineers/suppliers, it's extremely unlikely to go wrong.

 

To be perfectly honest, I think we're going a little OTT here. I'm much more worried about badly maintained fixtures falling apart (either of their own accord, or because someone's just flown the set in to it), which no safety will stop, than I am about the dynamic loadings of dropping lanterns on to safety bonds.

 

Often, working in the events world where a fixture may be rigged for only a few hours, I'm left wondering what the safety bond is actually making safer. A blind adherence to a rule ("All rigged things must have secondary suspension") is about as much use as not having that rule.

[Brian]

To be perfectly honest, I think we're going a little OTT here.

And some.

 

The way some people are going on it's as if we are talking about arresting loads in the 100's of kg region tethered on the end of 10m safeties. Not 20kg loads which can never drop more than 300mm before the safety catches.

 

Falls of 1 second?

 

Come on guys. Look at the real world.

[andy_s]

Nice one Brian.

but sadly not as much fun to talk about....?

[ramdram]

Alas no, it's not nice one Brian, at all.

 

Brian, the real world to me was the amount of care the Beeb put into cobbling up its masts and towers and curtain arrays and dishes etc, etc to survive in the worst of UK weather.

 

When was the last time you read of a Beeb mast falling over? Believe me we all had a vested interest about rigging when strolling around the aerial fields, especially the bods on a SW transmitter station who had to cycle all over the site on the cable duct covers to do the array changes.

 

As mentioned before we have all worked with chancers and the very last thing I would want going through my mind is a lump of lantern because the secondary failed...gag intended.

 

You could die just as easily from a 3kg lantern falling 3 mtrs onto your head as a 3kg yagi from 30 mtrs.

 

As for being OTT(?) it's the principles of the thing surely? If the nippers on here believe we are being dismissive of safety aspects then there is a danger they won't take it seriously. (I speak from experience on this one.)

 

Falls of one second? You might have missed the point on that one.

[Andrew Edwards]

I think all this is a very interesting topic. I have had numerous conversations about it over the years. Usually after someone regales me with a story about someone going on a course somewhere and coming back condemning all their gear or needing to swap out all their safety chains. ;)

 

Is I badly said last time, the fixture would need to travel for at least 1s to be anywhere near the minimum breaking strain of any of the safeties supplied with my fixtures. As for an external force, any scenario where the primary anchor point receives large enough force (which could be made up of any combination of force over time) to snap the primary fixing, will in all likelihood snap any safety you care to attach, especially if it has little to no slack.

 

The real world scenario for which I believe the safety bond to be of use is with a fixture with a standard hook clamp being momentarily clipped by moving scenery sufficient for it to be lifted off the bar. The amount of force required to do this would vary wildly as I am sure that most people do not tighten the wing bolt with a torque wrench. In many cases, I suspect it wouldn't take much.

For me this is the most likely cause and why I secondary suspend all my fixtures. Although sometimes I do see little point. So purveying is the 'need' to secondary suspend, I see safeties supplied with moving fixtures that have 2 independent trigger clamps. If both of these somehow go, I can't see the supplied SWL35kg safety making much difference. Can anyone think of scenario where one of these could come down and the safety stop it. Again based on the assumption that everything else is properly maintained.

 

Going back to Mr. Higgs comment about longer safeties. Perhaps we should be having long enough safeties attached away from the primary. This way, if a big piece of scenery were to apply a sufficient force as to knock the fixture from the rig it would swing safely out of the way before it snapped the safety.

 

The only thing I attach the safety to the yoke is a Par, all the rest have proper anchor points.

 

I don't choke the safety as shown in the acclaim picture. I loop the safety over the bar and put the eye in the carbine and then carbine to the designated anchor point on the fixture. There are a couple of scenarios where this does not give me enough slack to focus properly and in this instance I have a few short equivalent rated SWR drifts which I shackle to the fixture.

[Seano]

Blimey, this thread has moved on some.

Many thoughts but little time, so here are a few of the thoughts and I'll keep the rest to myself for now...

 

There have been several attempts to get heads around the physics of the situation above, none of them (so far) at all successful.

I too have found myself making a few mistakes and misconceptions while pondering, so it would be very foolish imo for anyone to do some fag-packet calculations and think they have a firm grasp of the subject (even only 'in theory') based on that alone.

 

The definitive answer is...

With the end of the secondary suspension remote from the luminaire securely fixed, the luminaire is allowed to hang freely by means of the secondary suspension alone. The luminaire is raised in its hanging mode a distance of 300mm vertically and allowed to fall freely. This test is made 30 times.The secondary suspension shall not fail and no part of the luminaire shall fall.

 

Where is this from Brian? I feel I ought to know, but I dont. Its not a BS-EN standard though is it? ABTT thingamabob?

 

This seems impossible to be honest, I'd have a very tough time believing that any but a minority of all but the most floaty-light fixtures out there comply with this. Certainly there's no way a chandler selling a conventional safety bond could address it, because its pretty much all about the fixture. And in a system with no energy absorption designed-in swinging, bouncing and progressive failure of something or other is pretty much all that remains. It is simply impossible to stop something instantaneously - infinite acceleration = infinite force.

 

Rapid acceleration (deceleration if you prefer) = large force. Lets say a 20kg fixture free-falls over 300mm in this test and is then brought to rest over 30mm - that's an average force equivalent to 220kg. Not much shy of a quarter of a tonne, through 30mm of what? Something stretching, bending, tearing?

 

Like the analogy with PPE fall factor, had never really occurred to me before.

Not a helpful analogy though, imo.

Though its possibly more relevant here than it is with PPE of the lanyard + energy absorber variety (where it really *is* a useless concept).

 

Fall-factor is useful when thinking about climbing protected by dynamic ropes because its a variable that can be controlled, to a degree, by placing protection, or at least considered when comparing situation A to situation B and thinking about risk.

 

(Very) theoretically, in that situation the severity of the forces experienced as a fall is arrested are solely related to the fall-factor and the actual distance fallen is not that relevant. (Because a long fall arrested by a lot of rope may involve a 'softer' landing than a shorter fall arrested by an even shorter rope.)

 

I say 'very theoretical' because of course that neglects the increased possibility with a longer fall of bouncing off a ledge or hitting the ground, as well as various other 'real world' factors that are difficult to predict with any accuracy at all.

 

In the case of a safety bond, the fall-factor is not a variable. The fall-factor is 1. So no point thinking about it over much.

 

the effect of the initial swing is to increase the time it takes for the lantern to reach its lowest vertical point, thus reducing the maximum deceleration and so force.

 

Interesting point. I suspect kinetic energy would likely be a the best way to look at that too.

 

If a bond with an eye one end and carabiner on the other were choked around a bar through its eye and then attached to a fixture with the carabiner, this would be perfectly acceptable practise.

'Less bad' practice than side-loading a connector - acceptable maybe, but I wouldn't go so far as 'perfectly' acceptable. A ferrule ideally shouldn't be side-loaded either. Running a wire through a 'soft' eye isn't ideal either.

 

I'm left wondering what the safety bond is actually making safer. A blind adherence to a rule ("All rigged things must have secondary suspension") is about as much use as not having that rule.

I think you're onto something there, at least in that its a very bad habit to automatically assume "it has a safety on it, therefore its safe".

 

I had a discussion about 'safeties' (on a truss, rather than an individual fixture) not so long ago, and was asked (paraphrasing) "Well, if you thought the primary suspension was a bit dodgy, would you safety it then?" The answer was no, I bloody well wouldn't - because if I thought the primary suspension was 'a bit dodgy' it wouldn't be hanging up there in the first place.

 

Not 20kg loads which can never drop more than 300mm before the safety catches.Falls of 1 second?Come on guys. Look at the real world.

 

Equivalent to 10kg dropping 600mm? *Far* from a second of free-fall, but nevertheless also far from trivial as you'll probably agree if you've ever dropped a stage weight on your toe. ;)

 

The main thing to take away from this discussion, for most of us, imo:

 

Minimise the potential fall distance. (300mm is too far!)

 

If you think something might fall, don't put a safety bond on it - sort it out so you that you no longer think it might fall (and then put a safety bond on it). Safeties may be less safe than you think - better not to test them.

 

 

When was the last time you read of a Beeb mast falling over?

March 19th, 1969? Oct 30th, 2004?

Last time I read about it was about 30 secs ago. ;)

[Brian]

Where is this from Brian? I feel I ought to know, but I dont. Its not a BS-EN standard though is it? ABTT thingamabob?

BN EN 60598-2-17:1989 Amendment No 3 Clause 17.6.6

[ramdram]

(Well there you are Seano, Beeb mast failures years ago...and there must be thousands of BBC masts, towers, arrays, not to mention miles of copper wire and steel cables and tons of steel work in this country alone, so not too bad a record. And I'll presume you did have to google for the info.)

 

I too had a look around the .www following the remarks and am now aware that the secondary safety issue is far, far more complicated than I (certainly) had considered.

 

If you look at the various web sites which "teach" mechanics you will see that the falling issue is addressed all over. What does complicate things, in some examples/explanations, is the assumption of an object falling into a surface. I believe this is to do with the time of de-acceleration and energy absorption.

 

This, I would suggest, muddies the waters a bit, because the equivalent in the case a falling lantern "against" a wire or chain safety would be, as mentioned earlier, how elastic the wire was and how much give there was. Any comments on my assumption welcome btw.

 

We mentioned lanterns weighing in at possibly 3kgs, yet an old Cantata profile (largest type) weighs over 16kgs or 35lbs. We have at our venue used exactly the same safety chain or wire for them as our much lighter Minims, so either over the top for a Minim or woefully underspecced for the Cantata.

 

With ref to Pars, they can be fitted with a colour changer device which has to weigh something...is there any recommendation to up the rating on the safety?

 

(To satisfy our curiosity/concern, and because we can, we now plan to "sacrifice" an old scrapper Cantata by doing a drop test exercise with the bonds we now use, as in with the bond loose. Those familiar with Cantatas will know there is a hard point to which you might clip one end of a safety bond and choke the other round the bar. Will repeat the exercise with bond through the yoke; might be that the yoke could be damaged so useful stuff to know.

 

To avoid the impulse issue bringing down the entire structure, which I might find a bit difficult to explain, we'll cobble up a bar outside.

 

Granted the experiments won't be terribly scientific, controlled or subject to peer review, but it might be useful to discover if the swaging holds up, or we get a break a third of the length along the wire, although how you determine that distance might be debatable...or a yoke parts company with the body, or if the crab' or similar can cope with a sudden increase in load. The bonds would have to scrapped after the event and it might be prudent to just buy a load more bonds anyway, whatever we find. Might even film it.)

[jonathanhill]

 

(To satisfy our curiosity/concern, and because we can, we now plan to "sacrifice" an old scrapper Cantata by doing a drop test exercise with the bonds we now use, as in with the bond loose. Those familiar with Cantatas will know there is a hard point to which you might clip one end of a safety bond and choke the other round the bar. Will repeat the exercise with bond through the yoke; might be that the yoke could be damaged so useful stuff to know.

I trust that you will use a separate bond for each test?

 

It would be good if you took some photographs and posted them into this topic.

[fireball40k]

... The bonds would have to scrapped after the event and it might be prudent to just buy a load more bonds anyway, whatever we find. Might even film it.)

 

Please do film it if you can, would be very interested to see the results: Death of Cantata, parts 1 to 4.

 

Thanks to everyone for your input to the original question. It seem that the issue is not as clear cut as I thought it might be.

[ianl]

as part of one of our training modules we do a demonstration of the difference between static load and dynamic load on a safety wire.

 

I use a 56pound weight and the thinest safety wire I could buy (so either 1mm or 2mm I would have to check) and about 600mm long

 

the safety is chocked round a bar and hooked onto the weight. it is left hanging for most of the module.

 

then the weight is held just above the bar and droped. most of the time the wire snaps somewhere along its length, not usually at the choke point.

 

I used to drop the weight from just under the bar but found embarisingly that the wire didnt break about half of the time

 

I could post some photos of the broken wires if I remember

[ramdram]

Finally did the drop tests with Cantata using a chain and a wire. Used the chain and wire repeatedly in different "modes", choke and so on. Much to my surprise/joy the bonds held firm, even after several iterations.

 

Chain has been retired and wire is now a keyring.

 

Took some pics, and video will go on YouTube as and when.

[ramdram]

Cantata tests (5) found here:

 

 

Not very scientific but the old bonds held which, frankly, was a pleasant surprise.

[dbuckley]

Theres definitely a degree ow schwing from the chain being linked to the back.

 

That is just great; theres been a ton of theory spouted here, including by me, but thats the first time I've actually seen a lantern come a cropper.