I am a bit surprised that it needs to be plug in? Why is that please?

Chris

• The reason for plug in is two fold. Firstly is for ease of maintenance and removal of fittings. The second is that the light fittings are suspended on chain and as such can swing about as the building has open sides and there can be a cross draught. I noticed a problem on a previous job where I was fitting led vapourproofs suspended on chain. I had pvc conduit running the length of the building at high level going in and out of a Wiska box at each lighting position. The flex from the light fitting entered the Wiska box via a stuffing gland and the flex was also held in position with a couple of clips prior to it entering the Wiska box also. The problem I noticed was that even with all this strain relief on the flex and cable clips, the flex moved about inside the Wiska box when the light fitting was swaying about in the breeze. Admittedly not by much but over the lifetime of the installation I thought it would eventually cause a problem. As such I considered a plug and socket arrangement would transfer any movement to an accessory that would be designed to cater for this. Obviously having the fittings fixed so that they couldn’t move would be the answer but the customer wanted them suspended so I had no choice. Also recently been fitting led hi-bays which can only be suspended so I had no choice but to try to deal with the problem

Baldyhugh said:

The problem I noticed was that even with all this strain relief on the flex and cable clips, the flex moved about inside the Wiska box when the light fitting was swaying about in the breeze.

Thank you. Isn't flex supposed to be able to, er, flex? However, I do take the point because even if the sheath is held firmly by the gland, the conductors might still be able to move (a little) independently.

I cannot see how a plug and socket would help because the flex must be clamped somewhere - I am now thinking of the cable grip in a 13 A plug.

I think that if the cable inside the box is laid in a lazy S, or perhaps even coiled once, any movement at the terminal will not be a problem.

(I now have a Hitchcock-esque vision of shadows of cattle on a barn wall moving in a howling gale. :-) )

Baldyhugh said:

The problem I noticed was that even with all this strain relief on the flex and cable clips, the flex moved about inside the Wiska box when the light fitting was swaying about in the breeze.

Thank you. Isn't flex supposed to be able to, er, flex? However, I do take the point because even if the sheath is held firmly by the gland, the conductors might still be able to move (a little) independently.

I cannot see how a plug and socket would help because the flex must be clamped somewhere - I am now thinking of the cable grip in a 13 A plug.

I think that if the cable inside the box is laid in a lazy S, or perhaps even coiled once, any movement at the terminal will not be a problem.

(I now have a Hitchcock-esque vision of shadows of cattle on a barn wall moving in a howling gale. :-) )

Hi Chris

yes flex is supposed to move but not inside a junction box as it will put strain on the terminations. That’s why I was planning to go down the plug and socket route as the cable grip in the plug will hopefully hold the flex firmly and the plug is designed to move in and out of the socket, not that any swaying of the light would move the plug about hopefully. Admittedly the movement of the conductors in the junction box was small but it didn’t sit well with me so decided to find a better alternative.

In most flexes the cores are free to move back and forth within the jacket, and are commonly dusted with chalk or talc to aid this. (and in eacch core, the strands are free to slide as well) If this is not done, then the bend radius is much larger - rather like the leaves of a leaf spring vs a solid steel triangle - the movement is small but essential to the operation. As such, when the lamps sway, the strands and then the cores are being tensed and slackened in turn. It is not the jacket moving, but the cores within.  If you really want to stop it, you have to clamp the cores individually. But some flexing at the terminals is unavoidable.

Mike.