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Mortice, rim and deadbolt aren't a firm of solicitors, but locks - just some of the outsourced specification data that architects regularly include in their ironmongery schedules without giving it a second thought. Three lever or five lever? Pin tumbler versus cylinder latch? Lever handle or the cheeky knobset? The third in our series of NBS Shortcuts looks at what they are and how they work.

Whichever crime statistics you choose to believe, the chances of an average household being burgled are around once every 35-50 years. Not bad odds, but still we try to devise more and more ingenious ways to keep over-hyped intruders out of our houses.

Domestic front-door locks haven't changed much for 150 years; literally true if you examine the Easter Island display at the Science Museum's magnificent lock exhibition. Even in the West, the materials are different and their appearance is more varied, but the lock's essential operation remains unchanged. Yale even promotes itself as emulating the traditions of Egyptian locks and keys, which date back to the city of Nineveh, 4,000 years ago.

At the turn of the 19th century, British locksmiths Robert Barron and Joseph Brammah developed the idea of having a small key that didn't reach the bolt but acted through intervening moving parts. In these, the lock turns a cam that pulls a latch back, allowing the door to open. A spring usually pushes the bolt back out again. A non-spring-loaded latch called a deadbolt (requiring a key to open and close the bolt) is usually more secure as it is more difficult to push the bolt in from the side of the door.

Jeremiah Chubb of Portsmouth patented the detector lock in 1818, which won a government challenge for a lock that could not be opened by any other than its own key. Even a professional locksmith who was imprisoned on a ship in Portsmouth Docks - incentivised with the promise of a pardon and £100 - failed to open it after trying for three months.

The cylinder pin tumbler lock, by Linus Yale of New York in 1848, uses Barron's double tumbler principle invented 70 years earlier. It has rows of two sets of pins one above the other - the double tumblers - which stop the cylinder from moving. The correct key raises the pins such that the joint lines between them line up with the cylinder edge. This frees the cylinder to move within the barrel, allowing it to move and turn the cam. The cam moves the locking bolt - or latch - in and out. With minor modifcations, this remains the basis of Yale locks today (see drawings overleaf, which also list some of the definitions commonly associated with locking devices).

EXTERNAL DOOR SECURITY A mortice lock (the name is taken from mortice and tenon joints in woodworking) is one that shoots a deadbolt into a notch, or 'keep', in the doorframe. A deadlock is a mortice lock that operates by key only, whereas a sashlock combines a deadlock with a separate spring-loaded latch. A spring-loaded latch is operated by a doorhandle, which enables the door to be secured without always having to use a key.

The majority of locks fitted to external doors in the UK are to BS 3621: 2004, simply because most British insurance companies insist on it. In short, this standard requires that the locks be provided with the following: at least five levers, anti-pick notching, locking devices separated from any handle operation, minimum 14mm bolt projection, anti-drill plates, concealed screw fixings and the locking mechanism manufacture must be one of 1,000 keying variations. (A three-lever lock is unlikely to have more than 100 'differs', as they're called).

A common Ingersoll lock boasted ten levers over 50 years ago and, until recently, Chubb manufactured locks with seven levers. They were discontinued in 2004 because the more levers they have, the thinner they must be to fit the lock case. Today, no mainstream manufacturer produces locks with seven levers.

These levers are not the same as the lever handles. Levers - as in 'five-lever latch' - are the devices within the casing that lock the bolt in place and need to be raised to allow the deadbolt to move backwards and forwards. In its simplest form this is done by the indentations (step heights) of the key bit. Turning the key lifts the levers differentially, depending on their pattern, in order to align a slot, or 'gate', in each of the brass levers. This allows an obstruction, or 'stump', to pass through and the bolt to travel forward (see drawings). Each lever is spring-loaded so that the gate shuts after the stump has passed, capturing it on the other side of the lever and maintaining the bolt in its extended position. The reverse process is needed to open the gate and retract the deadbolt.

Notches are regularly cut into the lever to try to fool lockpickers about the location of the gate.

It is questionable how many of today's opportunist burglars have the professional integrity to carefully pick a lock.

Insurers will undoubtedly tick-off kitemarked British Standard 'thief resistant' locks but may reject your claim because of your flimsy doorframes or large glass panes. Sturdy specification is the first defence against breaking and entering (rather than lockpicking), and Secure by Design criteria often request additional safety features such as anti-jemmy protection, steel-faced solid-core doors, restrictors on letter plates, and external grillage, depending on the level of security required.

The Door and Hardware Federation (DHF) 'Best Practice Guide' on mechanically operated locks highlights the 11 classifications (taken from BS EN 12209: 2003) to be graphically marked on doors, identifying everything from the door mass to the ironmongery's corrosion resistance. The seventh digit (or seventh DHF icon - indicated by a silhouetted padlock) lists seven grades of security and drill resistance: from Grade 1 'minimum security and no drill resistance' to Grade 7 'very high security with drill resistance'. The ninth digit identifies the keying operation (Grade 0 has no locking; Grade H is automatically locked) and the tenth digit identifies the spindle operation. (The Guild of Architectural Ironmongers' technical manual Locks and latches - Part 1, written a few years before BS EN 12209, contains only the first seven-digit/ icon classification system).

Locks that operate with keys from both sides have a symmetrical arrangement of levers, meaning that the number of differs reduces dramatically. Effectively, any lockpicker need only identify the first three lever gate levels because the remaining two are mirror images. Therefore, it should be realised that not all fivelever locks provide the necessary 1,000 differs to show compliance with BS 3621. Additional protection is normally afforded by a 'ward' and/or 'bolt thrower' which shrouds the levers from lockpicking tools, as well as pushing the bolt without direct contact from the key bit. Cylinder locks perform a similar security function, with the added bonus that cylinders can be replaced without having to change the entire lock case.

As we have seen, the risk of burglary is fairly remote.

Statistically at least, if you left your doors open you'd probably find that no one has bothered to take advantage.

Note: Definitions are taken from BS EN 12209: 2003 (which supersedes BS 5872: 1980).

Thanks to The Guild of Architectural Ironmongers and particularly to Master Locksmith, Mr Lewis Beadle of Abilock Lock Technicians For information on NBS Shortcuts visit www. thebuildingregs. com

REFERENCES:BS EN 12209: 2003 Building hardware - Locks and Latches - Mechanically Operated Locks, Latches and Locking Plates - Requirements and Test Methods (AMD Corrigendum 16436) BS 3621: 1998 Specification for Thief Resistant Locks 'Metalwork - External Elements: Ironmongery and Special Fittings', Historic Buildings Factsheet T 7.03, DE Door and Hardware Federation, (2005), 'Mechanically Operated Locks, Latches and Locking Plates to BS EN 12209: 2003 - Best Practice Guide', DHF Door and Hardware Federation, (2006), 'Cylinders for Locks to BS EN 1303:2005 - Best Practice Guide', DHF

KEY REFERENCES:Door and Hardware Federation, (2006), 'BS 3621 and BS 8621: 2004 Thief-Resistant Lock Assemblies - Best Practice Guide', DHF Guild of Architectural Ironmongers, (2000), 'Locks and Latches - Part 1', Technical Manual 1.4, GAI Guild of Architectural Ironmongers, (2001), 'Locks and Latches - Part 2', GAI

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