UKC

I work on a commercial zip line and not sure our system is completely redundant. We use two opposing locking krabs through the loops of two figure of 8 on the bight knots on both ends of the lanyard, to connect to the trolley and the two harness loops on a petzl fast jacket (full body harness).

so 4 knots, 4 krabs, the actual zip line is structurally redundant, there are multiple attachment points, but the trolley is the single point of failure.

obviously there are a few ways to back the trolley up, but is this necessary? IMO trolley failure is so unlikely, it is rated to 24kN and they are regularly inspected. Does anyone know of any instances of trolley failure?

Cheers.

In reply to Removed Userbennett_leather:

Is redundancy not built into the trolley design e.g. frame around the pully?

In reply to Removed Userbennett_leather:
I wouldn't call myself an expert but I have been involved in operating zip lines at outdoor centres.

What do you mean by saying the actual zip-line is "structurally redundant"? You don't surely have two cables. So if the cable breaks you have a big problem. Regular cable inspection is fairly essential.

The trolley design may/should give you some redundancy. Does it have two pulleys? Is breakage of a pulley axle critical, or is the pulley captive - they normally are.

I would think your main risks may be the cable itself, and, of course, human error in attaching the participant.

Do you have an annual independent inspection of your systems and independent expert technical advice on your procedures? Strongly recommended. Email me for details of who we use for this. You don't really want to be asking for advice on this forum I feel.

Martin

We do regular inspections of the kit and cable, and have independent technical advice. On their last visit they recommended we clip a krab over the top of the trolley to back it up, if it failed there is still an attachment to the cable.

The powers that be did not take this advice on, I am just a concerned employee, wondering how likely total trolley failure really is, and therefore if my employers are putting kids at risk.

As far as the cable being structurally redundant, I understand the cable is actually seven cables 'interwoven' around each other, each one on its own being adequate for the load involved, so any issues would not affect all seven cables.

In reply to Removed Userbennett_leather:

I work in an unrelated industry, but understanding failure modes are key to my job. Look at the system and break it up into individual components. Then ask how each component can fail, then determine the probability and severity of the failure. Also estimate the likelihood of detecting the defect before failure occurs. This is a basic FMEA, and will indicate broadly whether you are concerned about the right thing. It may be that your trolley is so over engineered that many other things will fail first.
The human element is often the biggest source of failure, so putting the extra crab in the system is one more thing to remember, with little extra benefit. Also, could it cause extra wear, or risk jamming, would it need regular checking & documenting?-again these consequences are not always good for a business.
Is the extra crab industry standard?
Have you considered that the advisor may have been over egging the pudding?

In general, you can always make something safer, even if it is plenty safe enough, and if you put 10 experts in a room you'll get at least 11 opinions on a single point. Examine the risk assessment and if you are still concerned formally discuss it with your boss. If he is happy that he would be able to explain himself to a judge, then you'll probably not get anywhere.

I don't know how many zip lines there are in the country, but if you are identifiable from this post, your boss may be the one starting the conversation.

In reply to markAut:

How would a non-expert know all the modes or the individual risks. Surely that is why you bring in expertise (and then most sensibly, from a legal perspective, follow their advice).

In reply to Removed Userbennett_leather:

If you're minded to get stuck into IRATA accident reports the near majority of incidents are the fault of the user - human error. The fewer things within a system to be set up wrong, the better.

Kit, on the whole, without being blasé about it, doesn't break.

In reply to Removed Userbennett_leather:

Not an expert of any kind, but "we had independent technical advice that we should do this safety thing but we chose not to do it" doesn't sound great.

In reply to slab_happy:


If the technical advice is concurrent with that of the manufacturer, then you'd be correct. If not, then manufacturer's guidelines trump the technical advisor.

For what it's worth, and without knowing the specifics of the set up, the addition of a carabiner may add more hindrance than it's worth and is, having seen the effects of carabiners run at speed on wire cable, cursory at best.

In reply to Removed Userbennett_leather:

speak to the trolley manufacturer, is the pulley / trolley constructed to a high enough standard for it to be considered for such use, or is the pulley designed to be a part of a system etc.. In essence is the pulley made for what it's used for.

Do you have NDT carried out on the pulley or trolley? The same for the anchor points of your cable. It is unlikely that any failings in the pulley will be visible to the eye in advance of total failure.

In reply to Removed Userbennett_leather:

You've found yourself in a difficult position. I'll be more open with you about my own involvement. I used to be an outdoor education adviser with a local authority and now, in retirement, I'm chair of trustees of an outdoor adventure centre.

Most providers in the outdoor adventure industry encourage staff to "speak-up" when they have concerns like yours. And many will have a mechanism for staff to raise concerns, without fear of adverse consequences, with someone other than their line-manager if they feel they are not being listened to. This is good health and safety practice in any workplace. Do you know if your centre has a "speaking up" policy?

As others have said, it is potentially quite serious to disregard the advice of an appropriately qualified technical adviser, particularly, of course, should an accident result. But it wouldn't be appropriate for me to comment on this specific situation in detail without knowing the full facts.

I hope this doesn't sound patronising, but I presume from your age that you are a fairly junior instructor. That doesn't mean at all that you should be reluctant to raise concerns, but I wonder if you've had the chance to discuss your concern with any other colleagues. It may be easier to raise with your management if it's a concern shared by more than one of you.

I hope that you are able to resolve this in a way that leaves you comfortable instructing the activity.

Martin

To clarify on the technical advice, it was mentioned in passing by the advisor as an alternative way of doing things, at no point we were told we need to be doing it this way.

Despite it not being an absolute imperative, clipping one karabiner already present in the system, in a slightly different way (with the cable then redirected through the karabiner), does add an extra layer of redundancy.

The reason for my original post was to add to my own brief research on zip wire accidents. I am going to raise my concerns again with the whole team, and wanted to be as informed as possible.

Thanks to everyone for the responses!

In reply to Stuart (aka brt):
I think that last time I did a zip line (as a customer) we had to clip on a karabiner and then hook it on top of the trolley so it was out of the way and not on the wire.

(Edit because auto correct thought it was Carribean!)

In reply to Removed Userbennett_leather:

If you are going to use the karabiner you need somewhere for it to sit so it doesn't wear.

In reply to marsbar:

Yeah it would definitely be sitting over the trolley, imagine sparks would fly if not!

In reply to Removed Userbennett_leather:

If you have any concerns about the safe method of operation of the zip-line, in your role as an instructor, you should voice them to your employer - via email rather than verbally.

You could point your employer to industry respected sources of advice:
Paul Capper at Capstone Inspections(www.capstone-inspections.com/) or
Steve Woods at Vertex training (www.vertex-training.co.uk/)

In reply to marsbar:

On many trolleys for commercial use there is a main point of attachment to which a carabiner will be attached.

There is then a second carabiner that on the better trolleys is attached such that should there be a right royal cluster f*ck and the main body of the trolley was to fail (massively) then this second carabiner would be deployed such that it would sit directly on the wire. It's a real last option since a steel carabiner on a wire cable will only last so long... But people like redundancy.*

Stick to the the regime outlined by the trolley manufacturer. Again, without being blasé, kit generally doesn't fail. Human error is nearly always at fault.

* Interesting when this crops up, that no one ever seems freaked out at the notion clipping all this into one loop on a harness!

OP - was the 'independent technical advisor' an MIA/MIC or somebody with specific links to the zip line industry?

In reply to Removed Userbennett_leather:

What trolleys are you using?

In reply to Stuart (aka brt):

It does seem a bit odd when you put it like that. Maybe we should wear another harness on top.

In reply to Stuart (aka brt):

Petzl tandem speed

In reply to Removed Userbennett_leather:

You can use the top hole of the pulley for an additional carabiner in a manner that is a bit too complicated for me to describe right now.

In reply to Removed Userbennett_leather:
I'm completely not an expert in these matters, but looking at the picture of those, I can't see how it can suddenly break. If it does somehow break, it does have 2 different places to break before it comes off the wire. It seems more likely to me that the only way it would fail is by someone forgetting to attach to it. Hopefully you have that scenario covered by checks.

Does it count as redundancy that it has 2 places above the wire? Or does that not count because it's the same device?

In reply to Removed Userbennett_leather:

Redundancy isn't the be all and end all. Your cable for example isn't really redundant, despite being formed of multiple strands it's still a single cable, it's just massively over specified for the intended load and of a material/construction that will degrade gracefully and detectable. Lots of thing we consider safety critical aren't redundant structures, take aircraft wing spars for example but again, they should be strong enough to survive use within specified limits.

As for your zip wire trolley it's really hard to imagine how it could have been constructed so as to not be redundantly attached to the wire assuming it has two wheels and is formed of either a single folded plate or a pair of parallel plates joined by (at the very least) the axels.

If you're still concerned then raise your concern with your manager but do be prepared to accept a reasonable explanation.
jk