Managing premature collapse of wiring systems

[Charlotte_R]

Something I've been wondering for a while is how people are managing the requirements of clause 521.10.202 of bs7671 which states:

 

 

"Wiring systems shall be supported such that they will not be liable to premature collapse in the event of a fire"

 

 

In a situation where you can run cables along the top of a box truss and through the pickups or are picked with steel strops, I feel comfortable that the requirements are met; but what do you do to prevent premature collapse of the supply cables when rigging from scaffold bars?

 

 

My understanding of the history of this regulation is because of the deaths of firefighters caught in cables which feel to head height. There's an element of common sense: if we assume that a cable will double in length due to the heating effects of a fire, if we use short cables between units then if it won't get to head height with the expansion and melting of the supports I feel that we've met the requirements as firefighters can't get caught up in it. But sometimes the cables travel odd routes or are rigged low where in the event of fire they could droop to a point where they could ensnare someone in the building.

 

 

It feels like the obvious solution is to provide extra safety cables which we just wrap around the cables and bars; but obviously that brings a cost implication. What are you doing to comply with the regs?

 

(Posted to general technical chat because I think that this equally applies to signal / data cables as to power).

[Dave m]

If there is a need to run cables (signal or supply) along bars, I tend to alternate along either side of the steel cables that hang the bar, or if it was a handrail on a balcony, I tend to use the uprights in a similar way. That way it's self supporting

Is that what you mean?

[Jivemaster]

The way I saw it explained once was that the plastic clips for T&E were restricted to places where the clip melting wouldn't cause anything to drape down, otherwise the old fashioned metal clips should be used. Plastic cable ties were not good and PVC/gaffer tape were not to be used where a failed clip/cable tie/tape might let a charred cable obstruct firefighting.

[gyro_gearloose]

Whenever I need to run some TRS along a bar, I loosely knot the socket end of the cable around the bar and support the rest of it with PVC tape. Where the cable(s) run off the end of the bar I just form a sort of half clove hitch to hang the cabling off the bar. The tape holds the cable up, and the knots will hold the cable should the tape fail.

[alistermorton]

I tend to "spiral" the cable along the bar so it can't sag too far, and tie it off at the ends. If there's more than one cable to run, spiraling the cable in opposite directions is neater. Edited January 3, 2020 by alistermorton

[david.elsbury]

Do none of you people pack out your own shows ? ? spiralling cable along a bar? It’s all about the ‘out’ :P

Also if you’ve got steel bars, wrapping cables on them will eventually lead to induced currents building up, which won’t be ideal.

Edited January 3, 2020 by david.elsbury

[alistermorton]

Do none of you people pack out your own shows ?

 

Yes. Practically every one of them. Only missed a couple in the last ten years.

 

There's a loose spiral, and there's tightly wrapped like a heating element. Guess which I use?

 

 

 

Also if you've got steel bars, wrapping cables on them will eventually lead to induced currents building up, which won't be ideal.

 

I'm not sure it does? Only if you're using single conductors, surely, otherwise the currents in the live and neutral cancel each other out.

 

 

 

[sandall]

For short (a week or less) runs the get-out starts when you hang the first light. So, as Dave M suggests, weave cables around hanging points, and/or under wing-bolts, all cables as far as possible following the same path & (as gyro suggests) loosely tied to the bar at least at the socket end, NO slack cable along the bar & absolutely NO PVC, gaffer or cable-ties. That way there is no chance of dangling loops, & come the actual get-out a series of quick shakes gets all your cables lying in a straight line on the floor, with no rats-nests & no sticky tape to spend ages cleaning off. At least that's the theory, & (providing you can drum it into your get-in crew) it mostly seems to work.

[DrV]

Do none of you people pack out your own shows ?

 

Yes. Practically every one of them. Only missed a couple in the last ten years.

 

There's a loose spiral, and there's tightly wrapped like a heating element. Guess which I use?

 

 

 

Also if you've got steel bars, wrapping cables on them will eventually lead to induced currents building up, which won't be ideal.

 

I'm not sure it does? Only if you're using single conductors, surely, otherwise the currents in the live and neutral cancel each other out.

Dead right Alister - that's how an RCD works. As you say, you'd need to be using singles to start inducing current in the bar.

[Owain]

There's a loose spiral, and there's tightly wrapped like a heating element. Guess which I use?

If the cable is in contact with a thermally conducting surface isn't the current-carrying capacity higher than if in free air?

 

 

[Simon Lewis]

Thanks for this, Charlotte.....

 

I was initially a little sceptical, but 521.10.202 is a new regulation in the 18th edition replacing 521.11.201 in chapter 52 and I don't have an 18th edition to hand (grumble grumble .... inordinate cost of updating lots of books every couple of years etc.).

 

However, it (apparently) states, " Regulation 521.10.202 requires cables to be adequately supported against their premature collapse in the event of a fire. This applies throughout the installation and not just in escape routes".

 

The various industry commentators suggest:

 

- Suitably spaced steel or copper clips, saddles or ties are examples that will meet the requirements of this regulations. (IET)

 

- Plastic clips, cable ties etc. are not outlawed, but can be used only to supplement metal fixings used in such a way that cable cannot fall in the event of a fire.

 

- The 18th Edition wiring regulations apply to all types of cable installation (and cable types) and not just escape routes such as fire exits.

 

- Commercial commentators are saying it applies to temporary installations as well as permanent (although I can't find chapter and verse for this yet, and by "temporary" they seem to mean construction sites etc. but also mention 'festivals' and similar).

 

- the drive for this arises from some instances of fire fighter deaths after becoming entangled in cables.

 

 

This poses some issues for us in the entertainment sector....

There's plenty of what we do (especially the temporary and touring sector) where we shrug shoulders and say "7671 won't apply - it's not a fixed installation". However, the regulation is in response to an identified hazard and is likely to become an issue that's raised in future.

Many, I suspect, have gone through that lengthy argument about soft roundslings "not being legal" because in the event of a fire, "the truss could fall down on firefighters". We typically argued back, 'firefighters don't usually enter a burning building, unless it's to rescue people' and forget the soft slings... even the aluminium ferrule on a steel will soften at a similar temperature to polyester slings giving way'. Again, if trailing cables (or indeed truss) is now identified as a real hazard in a fire, we may actually need to assess the risk and show mitigation.... or are we going to argue to the contrary? Even if we do, will venues start implementing 'trailing cables must be support by metal ties' rules, just as many did with "no gear comes in here without a PAT sticker" (or even the occasional "you can't take that chain hoist out of its case unless I can see its inspection certificate").

 

So - I suspect we need some industry guidance on this... fixed installs seems fairly clear cut, but to what extent would temporary systems be affected? If so, what would we do?

 

Has anyone got any comment from PLASA / ABBT / James Eade on this? I can drop James a line if not?

 

Simon

Edited January 4, 2020 by Simon Lewis

[TomHoward]

There’s a summary of the relevant 18th edition changes on James’s website with comment on this section-

 

https://eade.uk.com/wp-content/uploads/IET-Wiring-Regulations-18th-edition-updates.pdf

[Simon Lewis]

There's a summary of the relevant 18th edition changes on James's website with comment on this section-

 

https://eade.uk.com/...ion-updates.pdf

 

 

 

Excellent...! proportional and reasonable.... in fact when I penned the stuff above, I half wondered why this hadn't been more noticeably flagged when the 18th came out...

 

It does still affect the more permanent installs though...

 

 

This requirement should be taken in the context of its origin. Cables along a wall, in floor ducts,

risers, over a festival stage or similar are not likely to be encountered by firemen crawling through

the smoke-filled building, so don’t need additional measures to be taken. For temporary systems

where a risk is perceived, adequate measures could include looping cables inside the odd chord of a

truss or laying on top of box truss for example. Like all these things there is a proportional approach

to be taken for the risk presented.

[cedd]

I can foresee an invention....

 

A device similar to this Amazon Link - a simple 50mm spring clip. But with a larger area where the screw hole is - enough to hold a few lengths of H07 flex and some data. You tape your bar in the usual way and then pop a few of these on to the bar over the bundle. It holds all the cabling in place in a similar way to safeties or wrapping, but comes off in seconds when pulled. You could just have a bucket of them.

 

My experience of bars with taped cabling to them is that the lengths are relatively short - often between LED fixtures with thru-line connectors or between powercon in and outs of fixtures. With this in mind I think a risk assessed approach is probably best - as short lengths such as these won't need much additional restraint as the fixtures themselves are doing the job. Really we're talking about a long run, potentially with a taped bundle/roll of excess that would allow a loose cable to hang down - something I don't see too often.

 

Arguably it's the little cables that are going to need more thought but are most likely to get ignored. A socapex along a bar is naturally going to receive thought anyway due to its weight, whereas a single mic or data cable might just get strung along with some tape because it's so lightweight. I'd hate to come across either in a smoke filled room though.

 

 

 

[Ynot]

I can foresee an invention....

 

A device similar to this Amazon Link -..............

Didn't someone post on here a while back with a bar clamp/hook thingumabob that does just that??