As observe things, the use of lock rings is fairly unusual in steel conduit systems - typically limited to situations like a running thread joint (where you can't turn either length of conduit into a coupler), or occasionally where conduit is taken into a thin walled box (lock ring each side and a female bush) - but the more usual arrangement is a male bush and a coupler, so no lock ring needed.Generally where the threaded conduit is taken tight into a threaded coupler or threaded conduit box, nothing more is needed.

   - Andy.

I agree, however, in machinery with vibration, or fixed installations with reasonable thermal cycling, brass nuts do seem to work loose, or at least over the years I have found quite a few that have done. And glands for SWA which in terms of the hole mounting method are similar construction to conduit mountings,  seem particularly bad, or perhaps some 'skilled installers' have not mastered the use of a spanner.  Hence my preference for  pirahnas, especially on plastic boxes, where I am not comfortable to achieve the gorilla torque required for it not to shake loose, without risking the box.
note that the male bush/ coupler method is tighter to thread wire into than the male tube and 2 nuts in a tight spot where the bore is smooth right into the box, though I agree it is more pleasing to the eye and gives a bit of friggle room for installation adjustment. Have used something similar on pyro wire as well where the wire stops a short distance above the box, and the cores are led in the last 3-4  inches in conduit with coupler, reduces the risk of a snap-off at the glands, and allows some room for working slack.
Mike

I agree, however, in machinery with vibration, or fixed installations with reasonable thermal cycling, brass nuts do seem to work loose, or at least over the years I have found quite a few that have done. And glands for SWA which in terms of the hole mounting method are similar construction to conduit mountings,  seem particularly bad, or perhaps some 'skilled installers' have not mastered the use of a spanner.  Hence my preference for  pirahnas, especially on plastic boxes, where I am not comfortable to achieve the gorilla torque required for it not to shake loose, without risking the box.
note that the male bush/ coupler method is tighter to thread wire into than the male tube and 2 nuts in a tight spot where the bore is smooth right into the box, though I agree it is more pleasing to the eye and gives a bit of friggle room for installation adjustment. Have used something similar on pyro wire as well where the wire stops a short distance above the box, and the cores are led in the last 3-4  inches in conduit with coupler, reduces the risk of a snap-off at the glands, and allows some room for working slack.
Mike

mapj1 said:

machinery

If 'machinery' as defined in the Supply of Machinery (Safety) Regulations, then other standards apply, such as BS EN 60204-1.

Where conduit is the CPC I'm a lot more comfortable with flange couplers with the lead gasket. The lead allows a bit of movement and that's where I fear things would otherwise go wrong. With locking nuts once the locking nut comes lose the main nut will surely also come lose.

For conduit in general, if constructed properly nothing should be able to fully unscrew other than maybe the bushings.

With SWA I wouldn't use anything but a pirahna nut. I've never quite seen the point in the silly banjo rings, if the nut has came loose then the glands potentially going to fall out anyway so the banjo is doing absolutely nothing at all. 

Back to conduit, nothing you can really do to lock the bushings because of the gender of termination. Maybe someone should make fittings that are threaded but also have a space for a grub screw - like a conlok but threaded. I think the couplers don't have enough metal to do that but the flanged fittings are cast so I might drill and tap one to say I did it first!