Personal Fall Arrest Systems

[PaulKruse]

PPE = Personal Protective Equipment (Hardhats, Ear Plugs, Harnesses, ect.)
PFAS = Personal Fall Arrest System
EA = Energy Absorber
MAF = Maximum Arresting Force
HLL = Horizontal Lifeline
VLL = Vertical Lifeline
SRL = Self Retracting Lanyard

I decided to start this new thread because we seemed to have enough interest in it to clutter up the Knot Testing thread, which is closely related.  Fall Protection is an extremely broad and generic term, encompassing may techniques and much equipment.  Most of it is designed to prevent a fall in the first place.  But the subject of this thread is the PFAS, which is designed to safely arrest a fall after it has occurred.  The assumption is that it was not possible to prevent the fall by other methods in the first place.

The main components of a PFAS are the full body harness, an anchor point, and a means of connecting the two together.  It is the connection that seems to be of most interest in this forum, so that is what I’ll address in this thread.

The main objective is to arrest the fall without hurting the person who has fallen.  That means that he does not hit anything on the way down, he does not swing into other hazards, and the arresting loads are below 1800 pounds (OSHA requirement).  Nearly all manufacturers of PFAS PPE cut that number in half, so the MAF is 900 pounds or less.

If you fall only six feet on a stiff lanyard, perhaps wire rope or even various synthetic ropes with little stretch, your MAF can be thousands of pounds.  You have two ways of minimizing arresting loads.  Both represent means of absorbing the fall energy slowly over a short distance rather than suddenly at the end of a sharp jerk when the rope goes tight.

The standard EA will apply an arresting force of 750-900 pounds over a distance of up to 3.5 feet.  The most common EA is simply a nylon strap lanyards folded back on itself and stitched together.  Energy is absorbed when the stitches tear apart.  The industry has generated many other EA designs, but this one is still the most common.  My normal lanyard is a 9.5 foot nylon strap, one inch and four plies, folded and stitched to a total length of six feet.  It has a double locking snap hook at each end.  The strap is very stiff, so very little of the energy will be absorbed that way.  But the stitches can absorb all the energy of a six foot fall.  The normal requirement is to rig so that your free fall distance is no more than six feet.  When you must rig for greater free fall distances, then you need special rigging. 

All this is background information so we can understand each other as we discuss the subject further.  VLL’s were of interest, so my next post will address them.  I've seen other threads on slack liining.  That is nothing more than a HLL, so we can discuss them, too, if someone is interested.  The Engineering is all the same.

Paul Kruse

[PaulKruse]

The fall protection industry has many similarities to rock climbing, which is more commonly addressed in this forum.  But when you start looking at the details, we have more differences than similarities.  The prime reason for this is that the rock climber is willing to learn and practice more complicated techniques.  You can trust him to tie a knot correctly every time.  In fact, the rigging is the main focus of his craft.

In the fall protection industry, you have to keep things much simpler because the rigging is not the main focus of the craft.  The purpose of being at heights is to weld, paint, or to do some other construction or maintenance task.  The rigging is just one of many tools, and not even the main tool of your craft.  Normally, knots are outlawed all together in favor of other “fool proof” connecting hardware.  We use a lot of anchor straps, beam clamps, pre-spliced rope, and carabineers that prevent the need to tie a knot.

But in my particular industry, we have found it impossible to buy standard hardware to fit all applications.  We therefore employ a rigging crew.  They alone are permitted to tie knots.  They install the rigging used by all the other trades, and I’m their systems engineer.

The question came up in the Knot Testing thread as to the type of rope best used for a VLL.  That depends on the application.  But in general, I prefer a very stiff rope that stretches very little.  That prevents a fall from becoming a bungee jump, with the victim perhaps bouncing off the ground.  The rope therefore absorbs very little of the fall energy.  (I’d prefer it absorb none at all.)  We therefore depend upon the EA to absorb all the energy.  The standard connection is with a three foot lanyard to a rope grab, which is similar to an ascender in rock climbing.  The lanyard will include the EA.  Sometimes, we put the EA at the top of the VLL.  We do that with permanent VLL’s, which are made of wire rope.  We do also use some very stretchy nylon rope for VLL’s, but then I’ve got to do the analysis to determine the guy won’t bounce off the ground.

Paul Kruse

[KC]

Very sorry to hear of the losses of life, time and functionality etc.  The point should be made, that these now have re-re-re-re-paid for the costs of these lessons to all; so that hopefully none will need to repeat/pay the prices of these lessons for everyone else again!

i really don't go for working over/under each other; but on some occasions you must i guess.  Here we can have one falling onto another, or their line, dropping tools, knocking loose branches etc. or simple tangling of lines etc.

The shock/impacting produced by a fall is lessened by the absorbtion of the dynamic part of the forces by the system; the rest is transffered to the body and the support by the increased line tension.  The impacting acceleration is just lots of distance input over short time; deacceleration is distance lost/ taken from system forces over time (rather than input into system).  You can not alter

So the shock body takes is simply acceleration divided by deacceleration of the devices taking the force.  This is all just distance and force over time.  So, deformations in gear, just displace distance to limit forces retained; deformations being the path of least resistancel; that allows forces to remain low; by alllowing the peak forces to escape like steam pressure from a system.  If the deformations are immediately recoverable; they are elastic.  But if stitch pack you describe, rope stretch that doesn't immediately have potential to recoil, hardware deformations etc. the deformations that give up distance are non-elastic but still divide the forces by these distances to lower line tension.  Elasticity can give bounce in certain ranges, but is a dang good buffer of forces.

To me, every thing that conducts these forces is a device; each device must be accoutned for and calcualted seperately; on how it catches, carries and relays the forces.  The forces are like the evil that never dies but just changes form as it is passed on in the Denzel Washington movie (appropriatey named for my purposes) : "Fallen"(!).  Another aspect of how the body is a device in this imagery.  There is a differance in how it catches forces passed to it by ohter devices.  Caught by side D-Rings places the forces crushing and stretching ribs- not best.  In a Tree Climber's saddle or bosun seat; we have front D-Rings that pull up at seat/ back of legs.  this is not right at organs, and gives hips as joint to isolate these forces from organs.  Belts that catch at back, can pull at many places typically; to spread out force and place most at legs again.  But if adjsuted wrong; or straps cut to modify ; the forces can go to the less desireable points of body.

Peoples should not climb alone.  There are risks that anyone can have a stroke, get hit in head etc. and just hang.  Suspension trauma/ "orthostatic intolerance" can happen several ways.  Blood pulled in legs, not getting by straps or just from hanging; can get stagnant; and poisonous or sudden change in pressure when releived.  Conter intuitively; victim found in this state; should not lie flat; any oxygen should be given wide open!!!!  This can happen in climbing, parachuting, deer hunting stands(even claimed for soldiers standing at attention tooo long) etc.; where your friend of harness/belt can kill you.  If legs feel funny, move them; ride bicycle in air etc.  The heat and perhaps lack of liquids of working/ playing etc. can tend to accelerate and thicken things, making things worse(er).  The higer away from Center of Gravity of high chest and upper back D/ support locations also make this worse, than connections closer to CG of body device; like low chest/ waist attatchmeant points (are better by this view).

Another type of suspension trauma is if stunned and folded to from pressure on diaphragm not allowing it to function.  This is where waist attatchmeant points are worse.

Cable will give less than rope.  When you say rope grab; i assume cam; which takes a longer distance to close due to arc; so takes your weight X more distance for good pinch on line.  These must be matched to rope diameter, so pinch happnes when ppressure point on rope is fairly inline with cam pivot.  Cams can be aggressive on rope; and provide no slip.  Mountain/rescue/tree friction hitches might slip an inch or so at high/high loading; this gives notification of near failure, but also drops the forces in the system to less, by this slide/ giving up of distance.  The length of line taking the impact adjusts the elastic response; doubling rope or increasing diameter makes elasticity less.

Another point is to keep the rope grab level or above worker.  If rope grab is below feet, we enter into the more dangerous arena of fall factor 2 mathematics; which is how my chart is figured; where the pulley is below the load.

[PaulKruse]

KC:

That was a well written addition to the topic. Thanks.  What you say about re-re-re-paying the cost is all too true.  All these people had been correctly trained and equipped by their employers.  The one who followed his training is completely undamaged.  One of the ones hurt is telling his story in a very vivid manner, and all his buddies are being much more careful as a result.

Nobody climbs or works at heights without a buddy.  In this case, two buddies were climbing through the steel to get to a job site.  One fell on the other.  The buddy on the bottom became the energy absorber for the one on top.  Free fall distance was about 16 feet, but it was over 400 from there to the floor.  He had rigged for a 28 foot free fall, with an intermediate platform only 22 feet below where his buddy was standing.

I've never seen a rope grab with a cam in it.  I've seen cams in various rope clams and assenders, but not in rope grabs.  They have a 4-bar mechanisim in them that clamps tightly against the rope when you pull down on them.  Most are designed to slide freely up and down the rope under normal conditions, but will quickly lock up when you fall.  They take less than a foot to do that.  I've tested rope grabs on wire rope that locked me up within an inch.  The ones I use on synthetic rope will take maybe 4-6 inches.  You can also buy a manual rope grab that locks where you put it, but can easily be released if you need to slide it up or down.  I like those not for VLL's, but rather for lanyards that need to be adjusted for length as you move around.  I'd normally use them for fall prevention rigging rather than fall arrest rigging.  An example would a low sloped roof.  Just keep the lanyard short enough so you can't reach the edge to fall off.

The only attachment point to the body that is acceptable to arrest a fall is the dorsal D-ring, located on the center of the back just below the neck.

The fall protection industry has equipment similar to the rock climbers, but they seem bent on making sure we don't buy equipment from rock climbing sources.  All the equipment is designed for 5/8 to one inch rope.  All the rock climbing stuff will be half inch max.

Paul

[Funambulist]

I'm interested in a discussion.

I use two strands of equalised 8mm (1/2 UIAA) climbing rope tied onto seperate 12mm x 104mm forged steel rings. These are taped so that they act as a single leash. The knots at the ring end are tied with the end of the line threaded through 15mm tubular webbing using a 3/4 fisherman's and to me a figure of 9.



I don't intend ever to fall onto the leash again and catch the line if I fal these days. I did take some falls a couple of years ago and they are were not too bad, but not all leash falls are made equal!

Do you know where I might be able to get a circular eyelet with an internal diameter of around 6-8cm? This is what I have been searching for to make the running connection. It would need to be forged ideally!

Cheers

[Dan_Lehman]

I use two strands of equalised 8mm (1/2 UIAA) climbing rope tied onto seperate 12mm x 104mm forged steel rings. These are taped so that they act as a single leash. The knots at the ring end are tied with the end of the line threaded through 15mm tubular webbing using a 3/4 fisherman's and to me a figure of 9.

Nowadays, a "1/2" rope might also be a "single" and further a "twin" rope,
but what is yours?  If indeed it's (only) a "half", you might want to re-think
the "equalized" part of your arrangement, as half ropes not qualifying as
twins will likely yield a too-high impact force, as they are designed to take
a fall initially alone and then with some assistance from the other "half".

I don't intend ever to fall onto the leash again and catch the line if I fal these days. I did take some falls a couple of years ago and they are were not too bad, but not all leash falls are made equal!

What do you mean, then?  Of course, you are not intending to fall,
but that remains a condition you must anticipate.

As for the "3/4 Fisherman's", I think I know what you mean (a Grapevine
loopknot but for the SPart's component being a single vs. double Overhand).
Why this, and not, say, the rockclimber's beloved Fig.8 LK (nb:  this can be
made easily untiable by "re-weaving" it starting from the SPart's end of
the knot (i.e., the opp. end from usual))?
To your harness (are you tying this first, or making a LK in the bight
and clipping in somehow (guarding against any chance of some
misorientation of the metal bit clipped to)?), I'll suggest a Bowline
with a Bight, bight-end brought around over the twin SParts
for the lock.  It can take a bit of fiddling to dress it nicely, but it
then has a gorgeous shape, kind to the rope, and easily untied
(it being a bowline with SParts turning around four strands, not two)!

Also, what's the nature/type of your main line/tape?

--dl*
====

[Funambulist]

''Nowadays, a "1/2" rope might also be a "single" and further a "twin" rope,
but what is yours?''

Indeed, mine is also rated as double and should be ok I think, although making one slighlt longer than the other is something i have considered. I think I will make further investigations into impact forces though. If I have the longer one made from 'single' 11mm then that would be even more redundant.

I saw on the testing thread mention of the barrel hitch, my friend who I  walk highline with use one to tie his rings in place of my 3/4 fishermans due to it's shock absorbancy. I worry that this equals heat and therefore potential fo melting...

Tests on dyneema have shown that due it's low melting point it can and will fail under very small loads when shock loading if knotted.

I spilt tea on my keyboard and so I'm using an on-screen one. Will post up when I have a new one ond it's not as bloomin' slow.

Thanks for you interest.

[Dan_Lehman]

''Nowadays, a "1/2" rope might also be a "single" and further a "twin" rope,
but what is yours?''

Indeed, mine is also rated as double and ...

... you've just broken the terminology base!  "single", "half", "twin"--"double"

If I have the longer one made from 'single' 11mm then that would be even more redundant.

Rather, that would make it less shock-absorbing; redundancy can't be had here so
innocently easily, as the piece is coming into play no matter, and thereby affects
system behavior--i.p., impact force on you & the horizontal line & anchors.  With a little
figuring, though, one could make one tie-in the redundant of the other:  the shortness
of the fall means that it wouldn't take much slack to provide for full elongation of the
first-loaded piece.
Hmmm, apparently yet to be part of the official UIAA standard, but under consideration,
max. elongation on first fall (max.impact force is a first-fall measure, too, btw) limit is 40%.
If your tether is 5' long, a redundant one could be 7'.

I'm not sure what the feasibility would be of constructing one's own shock absorber
from shock cord as something to embed in your tether--thinking that one might need
several strands of stout shock cord?

I saw on the testing thread mention of the barrel hitch, my friend who I  walk highline with use one to tie his rings in place of my 3/4 fishermans due to it's shock absorbancy. I worry that this equals heat and therefore potential fo melting...

I think that both are overstated--given a short fall, maybe the limited absorption by
knot compression plays a relatively bigger role (similarly, a longer loopknot eye would
be more significant in its line-doubling, absorption-halving effect), but I doubt it's much.
And with all the falling that gets done, one doesn't find much melting.  Your potential
FF could be high (2!), but adding more than mininmum rope reduces it while lengthening
it, towards a FF 1(well, using a lonnnnng tether) fall.

Tests on dyneema have shown that due it's low melting point it can and will fail under very small loads when shock loading if knotted.

What tests are these?  I know of tests with dynamic drops on HMPE tape slings,
knotted with various Girth hitches; they survived large loads (UIAA drops, IIRC).

--dl*
====

[Funambulist]

" With a little
figuring, though, one could make one tie-in the redundant of the other: "

Exactly what I was thinking! It is not hard to make one line 1 couple of foot longer the the other. To do this I would simply tape a loop back on itself on one of the line. Falling off and hitting something in the first coule of steps is a far more significant danger than than the increase in fall factor by having the leashes a sensible length.

The system itself is also very dynamic and will pull down far more than a traditional tightrope as it is made with nylon tape, not wire cable which softens the impact greatly.

There is some stuff about the tests here: http://www.ukclimbing.com/forums/t.php?n=177030

I couldn't find the other stuff I was reading about it, but a friend of mine met the bloke who did the tests discussed here, and I understand that the guy who started the thread reported it correctly.

[Dan_Lehman]

There is some stuff about the tests here: http://www.ukclimbing.com/forums/t.php?n=177030

"some stuff" started sadly as crazy rumor, which got some dismayingly, uncritical tolerance!
It amazes me what assertions gain echoes on the Net.  But enough skepticism came on to drive
the thread to clarity.  But there should be by now (then was April'06) a published reference
from Lyon Equipment to cite.  --guess that's another item to probe . . .
(I don't see anything in an quick glance of the Lyon site.)

Some of the nonsense uttered freely on the WWWeb in that UKClimbing thread:
"dyneema is the generic name ... Spectra is the trade name":  bulloney, they are both
trade names for HMPE/UHMWPE/HPPE (a trio of acronyms ending in "PE" for
"polyethylene", between which there might be some different meanings).

">> The tighter the turn the fewer strands of rope on the outside of the turn actually bear the initial load in any fall,
 >> hence they can break more easily.
>
> I had that explained many years ago, tying a fig 8 knot should be done with the loaded side of the rope
> on the outside of the knot to reduce the risk of it failing.

Rather, I believe one will find that it's the material on the inside of the curve, which gets so
tightly compressed, that breaks; this is what I've found in cases of laid rope, assessing where
each of the three strands were at the point of rupture, and which of them (often only one or two)
broke.  (I can only imagine what sort of image this commentator would point to as showing the
knot he envisioned--likely another bit of sillyness!)

I couldn't find the other stuff I was reading about it, but a friend of mine met the bloke who did the tests
 discussed here, and I understand that the guy who started the thread reported it correctly.

No, his initial report (the "OP") was nonsense; but this got beat into shape on scrutiny, and a guy
from the tester/Lyon, Pete Potter, came on to say "stay tuned ..." but I don't see the promised result.
Well, I'd like to see the "numbers" on this--the peak impact force, and the rated sling strength.
Might be that while we're waiting or seeking Lyon results.

As for the "melting", while HMPE has the lowest melting temp. of the cordage fibres
(or similar with reg. PE), it also has the lowest coef. of friction, and so makes less heat.
Melted ends seems a typical result to breakage by tensile failure, and not the cause
of it; by some rough calculation done in the above thread by a poster, the expected load
from the fall would be great enough to break the sling esp. knotted, and I concur in the
last posters' (hence, unanswered) skepticism about the usual "fall-factor" considerations
(i.e., that distance fallen is irrelevant, only FF matters--in fact, I recall that duration
at peak load(s) is greater for greater distances fallen, with dynamic ropes) for such
static materials; they're clearly wrong if there is no stretch, as clearly the impact force
of a long-falling mass is greater.

-------

Oh, on another matter, you remark that your highline material never contacts the metal
anchor device, as though this is a good thing.  In one test of 8mm low-elongation rope
(BW II, I think) tied in a Strangle Noose (aka "Barrel Hitch"), the break came in the main
line where it entered the knot, not around the 'biner.  (I'd guess the effective dia. of the
rope, given some flattening, would be approx. 6mm to 10mm metal.)  Frictional heating
of your line within its nylon? sheath might well be greater than were it against the
metal (which dissipates heat).  (I recall some testing done of rope pads, and there was
greater deletorious effect on the rope when using at least some of the pads, on cyclic
loading, then of rope on the bare concrete/rock, from the heat!)

--dl*
====