Electricity + Control October 2017

CABLES + ACCESSORIES

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Take Note!

• The Adjustable Cable Gland was invented by George Montague Prat- ley. The use of Adjustable Cable Glands for termi- nating armoured cables makes good sense when considering technical advantage, installation cost savings, and the extra margin of safety. The design brilliance of the Adjustable Cable Gland lies not in its com- plexity but in its simplici- ty.

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Most South Africans are aware that Pratley Putty is the only South African manufactured product on the Moon.

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G eorge Montague Pratley should however be remembered more for the brilliant engi- neering he applied to his patented ‘Adjust- able Cable Gland’. Never regard a cable gland as just a lowly nut at the end of a piece of cable. A cable installation risk analysis puts things in perspective. Assuming the cable is properly sized, made by a reputable manu- facturer and intelligently routed, it is apparent that the real risk lies at its ends where it's terminated. The risks include, power failure, fire, corrosion, water ingress, electrocution and the most common one; the financial risk of having to redo, the installation. The quality of a cable installation is also materially in- fluenced by the quality of the gland. If it's easy to fit, it's generally installed more professionally. So what's so special about Monty Pratley's Adjustable Cable Gland?Well, lots really, but two things in particular. Firstly, it's adjustable. By simply turning the nipple by hand, the gland can be adjusted to grip absolutely any diameter of cable armour wire. This is unique and it means that unlike the case with or- dinary glands typically with fixed or captive cones, the electrician will never find that he is unable to

mate the gland threads due to oversized armour- ing. Similarly, he will never tighten the gland only to find that the armouring or braid has not been gripped because it is too thin for the gland. Al- though it's seldom necessary, this adjustability also means that large adjustable glands have the ability to terminate small cables as well. Secondly, the gland has a virtual thread. A cable gland's primary function is to grip the cable (its ar- mouring). Other things being equal, the better the grip, the better the gland. So, the Adjustable Cable Gland is already adjust- ed for the exact size of armouring as described. Now comes the really clever bit, a virtual thread! It's well known that for any given torque, the clamping force exerted by a thread is greater if it's a fine pitch thread. Problem – fine pitch threads are expensive to make, easily cross threaded, easily damaged and difficult to engage. Solution – a gland body; or 'differential nut' in this case, with two coarse threads each of slightly different pitch. So, how does this work? When the gland is tightened, it does so accord- ing to the pitch of the coarser of these threads. However, whilst doing so, it simultaneously loos-

George Montague Pratley ‘Monty’.

Figure 1: Adjustability.

Figure 2: Differential thread action.

26 Electricity + Control

OCTOBER 2017