tray work earth continuity, and cable ties?

[Gavin A]

I was on a fairly big new build commercial job recently, nowt too big electrically, but lots of SWA on tray all around the place by the main electrical contractor.

So anyway, I noticed a couple of things they'd been doing that didn't seem right to me, but not really my place to query it at the time, and the lads doing the install had been sound, but just wanted to check my assumption that this isn't how it should be done.
,
1 - None of the tray work had been installed as being continuous and wasn't bonded to the MET or anywhere, ie it just had gaps between one section of tray and the next

2 - All the SWA including some fairly hefty sub mains were just clipped the the tray with pretty flimsy plastic cable ties, so it'd not have a hope in hell of holding it on the tray in the event of any significant fault current, or fire.

The job was a fairly high spec build, at least I thought it was meant to be, so was a little surprised to see this done in this way.

thoughts etc?

 

[davesparks]

Read GN8 for the answer to the earthing/bonding query. The short answer is that it doesn't strictly need bonding unless it is an extraneous part.

 

[Engineer54]

Read GN8 for the answer to the earthing/bonding query. The short answer is that it doesn't strictly need bonding unless it is an extraneous part.

Which 9 times out of 10 it will be... We connect all our metal containment, including tray, basket , ladders etc, back to the MET/EMT and ensure it's continuity across any installation gaps within/along it's route.... Same goes for any air handling ductwork.

 

[tazz]

All cables, ducting and pipework should be supported by metal ties, or metal strap.to fixed metal support, ie tray ladder rack or direct support to building fabric.

 

[Gavin A]

I've been thinking about this since posting, and wondered if it makes a difference that all the cables being run were SWA, so logically any fault from a cable on the tray should already return to earth via the armour.

I still don't really see it, it just seems sloppy, it's not hard to join to sections of tray together properly. I guess some sections where it's bolted to steelwork it would be continuous anyway through the steelwork, but it was the same on the breeze block wall where the sub board was mounted.

tazz - that's also what I thought, though we do use cable ties for the bulk of the fixing points we ensure we've got metal cable ties every 800-1000mm or so, and that's mainly running 4mm2 singles for the DC side with us. On the roof on that job I'd used stainless ties on every fixing point because the tray was exposed to UV light, so the cable ties would inevitably degrade, and we'd run our DC cables down in full mechanically protected 3mm containment with class 1 and 2 surge protection at either end due to lightning risks. The risks associated with 800V DC cables without ADS fault protection are obviously different to 3 phase AC with ADS fault protection and run in SWA, but I was just a little surprised to see the AC side being run in that way on such a big job.

I was actually having a look to see if we could pick up any tips on the best fixing methods for larger SWA cables as we might well have some coming up, and had been considering subbing that side of things out if I liked what I saw...

ps one of our sparks would seem to have er borrowed our guidance notes.

 

[cliffed]

It really is all about good workmanship,these days I think it's just smash in it & leave,continuity should continue on tray,trunking etc

 

[Engineer54]

The now common use of cable ties to secure cables to tray/basket/ladders etc, is a bad practice that needs to end.

 

[telectrix]

might disagree there eng. wrt supported by metal. if the cables are sitting atop tray, the cable ties are there just to keep them in position. they are supported by the tray itself.

 

[davesparks]

might disagree there eng. wrt supported by metal. if the cables are sitting atop tray, the cable ties are there just to keep them in position. they are supported by the tray itself.

Until a fault occurs you mean?

 

[High Tower]

There are far to many people rely on metal ties!!, they are they are terrible for offering restraint and on fire alarms are a token offering at best, you rely on the plastic ties to do all the dressing.

As far as I'm concerned if it's smallish SWA then stout plastic cable ties are fine, if it's bigger and fault current is a issue then they should be cleated to the tray!

 

[shanky887614]

cable ties are more secure than clips.

we all know how little force it takes to either pull them out or bend them.

if you say no to tywraps its either back to thread or p clips/band etc

 

[ipf]

Can remember how we used to spend ages manufacturing and fitting 'U' clips of various sizes for multi pyros on tray from solid copper strip, sometimes with insulation, drill for each fixing point and fix with brass 2ba screws, nuts and washers. I bet that lot would have outlasted the fire...

 

[Engineer54]

All our permanent cable tray restraints are either from site manufactured metal strip (2 fixings) or sized metal cleats and the like. There maybe some plastic ties left on some containment, but these were used for initial cable laying, providing initial temporary support while permanent cable supports/restraints being installed. These temporary cable tie supports have a tendency not to be always totally removed during the permanent cable support installation... I catch most of them though!! lol!!


Oh, i should also state that plastic cable ties are expressly forbidden in our electrical specification for metal containment cable support!!!

 

[tazz]

Again down to designer......he would have the final say..things to consider, is cable to heavy for ties, should steel wrap be used instead or should cleats be used due to weight...every project is different