Why metal junction box for SWA cable?

[bertoli]

Hi,

I was wondering why a galvanised metal junction box is preferred according to the advice on this board.

Has the metal j. box any advantages over an IP55 plastic waterproof junction box, or could this just be used just the same?

Also when in the junction box, could the banjo fitting on the SWA gland be attached directly onto an earth block , or should I crimp an eyelet to an earth cable and attach it directly to the banjo?

Many thanks for your help

kind regards

 

[mapj1]

The problem is purely mechanical. The SWA is quite stiff, and all but the thickest walled plastic boxes will flex alarmingly, and then may not close properly, even if they don't shatter, if the SWA decides its going to straighten out or twist due to small alignment errors. (the tail wags the dog )
It can be done of course, but a metal box is much more forgiving of small errors and off-axis forces, and it makes organising the earthing arrangements easier too. With plastic boxes the banjo can be either inside or outside, and a bolt and nut taken through the wall of the box and the smaller banjo hole to form a solid earth stud. A pair of shakeproof washers is also a good addition to the scheme.

 

[ban-all-sheds]

Also when in the junction box, could the banjo fitting on the SWA gland be attached directly onto an earth block

I can't visualise how that would work....

 

[bertoli]

I can't visualise how that would work....

Just as shown on page 3 of this thread, //www.diynot.com/forums/viewtopic.php?t=27081.

I am aware this is not too pretty, but it's the principle of the banjo connection to the earth block I'm interested in. Though on second thoughts, just using a bolt and nut as a clamp through the banjo itself would mean the earth block would not be needed.

I won't be fiiting it to a consumer box, but using this instead http://www.screwfix.com/app/sfd/cat/pro.jsp?id=11549&ts=89755, so I should have plenty of room inside.

thanks for your advice.

 

[ban-all-sheds]

I can't see why you would ever have thought that was necessary.

The banjo lies flat against the inside surface of the box.

A screw passes through a hole drilled in the wall of the box, through the hole in the banjo, through the eyelet crimped onto the end of the flying earth lead.......

Good luck with fitting a SWA gland into that cheapo plastic box, BTW...

 

[securespark]

Por l'amore de dio....

 

[pdcelec]

Is that picture meant as a joke?. It sure made me laugh. Why go to all that trouble and extra time to produce such a connection. BTW where is the shroud?

 

[Big_Spark]

I can't see why you would ever have thought that was necessary.

The banjo lies flat against the inside surface of the box.

A screw passes through a hole drilled in the wall of the box, through the hole in the banjo, through the eyelet crimped onto the end of the flying earth lead.......

Good luck with fitting a SWA gland into that cheapo plastic box, BTW...

Ban, the Bano should be on the OUTSIDE of the box not the inside, but otherwise I agree with your comments. There is no need to do what is shown in the pictures, to increase the Bond the simplest method is simply to put a crimp lug on an appropriate earth core and secure this using the nut and bolt that fastens the banjo..obviously the crimp should be on the inside of the enclosure..the olther end is terminated into the block inside the enclosure as standard.

Why make a simple Job complicated?

 

[bertoli]

First off, these are not my pictures, these are from the thread I linked to!!!! It is the reason I asked my original question!

Por l'amore de dio....

For the love of God?.......And this meaning........?

That's the reason I asked that particular question. No-one appeared to be questioning benjiman's method of connection on that thead. And from the above comments, I don't understand why nothing was said on that thread.

This is a discussion forum, is it not?

Good luck with fitting a SWA gland into that cheapo plastic box, BTW...

Thanks for your help ban-all-sheds. I'm sorry I wasn't aware this was a cheap box. The galvanised metal boxes you recommended in the thread I mentioned above ( //www.diynot.com/forums/viewtopic.php?t=27081&postdays=0&postorder=asc&start=30 ), are actually cheaper than the one's I had mentioned? Does this mean that any galvanised box, no matter how cheap, is better than a plastic junction box?

With the "tail wagging the dog scenario", I would have thought the method used to attach the junction box to wall would be more of a factor in its stability, than the cable to box connection?

I was considering using these IP55 rated boxes indoors using 4mm SWA cable. But as the boxes are suitable for outdoor use, surely SWA cable would be the first choice to use when installing outdoors, then connected to the IP55 box? So they would have to be strong enough for this purpose?

kind regards and many thanks

 

[mapj1]

Actually -that photo is someone who really made life much harder for himself than he really needed to.
If the plastic box has a block of earth terminals built in, then as was suggested, a nut, a bolt and flying lead between it and the earth block is all that is required.
If an earth block is not provided then a neater variation on this theme, where the terminal strip is bolted in on the inside wall of the box can be assembled, but even that is a second best solution.

From memory, in this picture, the guy drilled the hole first, and then found that it collided with the ribbing and other moulded parts inside the box.

As always, forward planning goes a long way - a box with enough terminals, and enough room inside for all the wires you wish to connect to plus some working room for your fingers and tools is a good start.

As I said, SWA direct into plastic boxes can be certainly done, but for the same strength are more expensive than steel, or for the same cost, are too thin walled and can tend to either flex or split as things are tightened up, especially in inexperienced hands.
P.S. when cutting the cable, leave enough slack somewhere that you can shimmy it along and make the end off a second time if it gets damaged.

 

[bertoli]

P.S. when cutting the cable, leave enough slack somewhere that you can shimmy it along and make the end off a second time if it gets damaged.

A simple and obvious suggestion, but one I certainly wouldn't have thought of!

thanks for taking the time to give your advice.

 

[mapj1]

Be aware that the 3.50 plastic box you linked to i.e. http://www.screwfix.com/app/sfd/cat/pro.jsp?id=11549&ts=89755,
has no terminals inside - usually get filled with crimps or choc bloc singles dangling in the breeze. If done nicely then a big piece of choc block can be used but attaching it to the floor of the box involves 2 or 3 M3 countersunk bolts passing through the box and the choc bloc mounting holes to hold it in place.

 

[bertoli]

Be aware that the 3.50 plastic box you linked to i.e. http://www.screwfix.com/app/sfd/cat/pro.jsp?id=11549&ts=89755,
has no terminals inside - usually get filled with crimps or choc bloc singles dangling in the breeze. If done nicely then a big piece of choc block can be used but attaching it to the floor of the box involves 2 or 3 M3 countersunk bolts passing through the box and the choc bloc mounting holes to hold it in place.

Though the same can be said for a galvanised junction box, as I assume this would need the choc bloc as well. Would this then need fixing to the floor of the box as with the plastic j.box?

I already have a long strip of 30A choc bloc, so thats not a problem.

Or are these a better solution? http://www.tlc-direct.co.uk/Products/LB9045.html

 

[mapj1]

They are a better solution, if it is big enough for what you need inside.
I'd drill the metal box to mount the choc block too, but I'm like that.

 

[ban-all-sheds]

Or are these a better solution? http://www.tlc-direct.co.uk/Products/LB9045.html[/QUOTE]
I'm by no means convinced that there would be enough clearance inside for the SWA gland...