Windows are the eyes of a building, often it’s most defining feature and one of the elements most vulnerable to attack.

Intruder protection can be achieved using bars, shutters or grilles and if blast resistance is required the building can be provided with robust window frames supporting heavy laminated glass panes. However, many building owners would prefer not to introduce obvious physical barriers as they can be visually intrusive and create abnormal environments for people to live and work within. It may also be difficult to fit new strengthened windows as many sensitive buildings are Listed or within Conservation Areas and leaseholders may not be able to alter the fabric of the building.

Purpose made secondary windows installed independently to the inside of the existing window frame can provide enhanced protection whether to a single vulnerable window or to a complete building. Protection is available from simple low level deterrence to significant resistance against a determined attack. When people’s safety is at stake or when property has to be safeguarded, it is essential that products offering protection are suitably tested and certified.

Secondary windows are designed to complement the external window style and are very discrete to avoid drawing attention to the protection offered. They can usually be installed with little disturbance to the fabric of the building or its occupants.

Physical Attack is assessed at 3 levels, Low, Medium and High Risk. Low Risk guards against an opportunistic attack by a thief who wants to move quickly and with the minimum chance of detection. Although there are no test standards at this level, a simple secondary window incorporating a lock and toughened or laminated safety glass acts as a deterrent by creating an additional barrier to entry that is both visual and physical. Medium Risk is when the thief will be prepared to spend more time attacking the frame but may not break the glass as this will attract attention. The window frames and locks need to be robust and accredited to “Secured by Design”, the crime prevention initiative promoted by The Association of Chief Police Officers (ACPO). This Standard does not cover an attack on the glass but Selectaglaze, the UK’s leading secondary glazing specialist, recommends a minimum of 6.8mm laminated glass, tested to BS6206 Class  A.  And if the risk of glass breakage is higher, Anti-Bandit laminated glass tested to BS5544 or BSEN356 should be considered.  When the building is known to be High Risk, where any attack will be planned and very aggressive, both the frame and glass will come under determined attack via a range of tools. Products should be specified that are certified to the Loss Prevention Certification Board’s LPS1175 standard and listed in The Red Book. The standard has a number of increasingly challenging test levels. Tools for Level 1 include a screwdriver, lever, pliers and knife and the test duration is 10 minutes, with an actual attack time of 1 minute. This rises to 20 minutes test time and 5 minutes attack time at Level 3 with tools such as a claw hammer, small axe, battery drill and a 700mm long crowbar.

Frames must be very strong with significant locking to opening panels and incorporate specialist laminated glass.

Blast Protection is also assessed as Low, Medium and High Risk. Standard annealed glass is likely to shatter even 100m away from the explosion and flying shards of glass can lead to many casualties wreaking the most devastating personal injuries and destruction to equipment and services. Whilst the majority of buildings will not be direct targets, the vulnerability of glass at such a large distance demands that building owners or managers must give serious consideration to glass containment.  Buildings classified as Low Risk are those considered unlikely to be a direct target but if it has commercial or public use there is a duty of care and glass should be protected. Conventional secondary windows glazed with 6.8mm class “A” laminated glass will provide an effective barrier against a vehicle bomb detonated 40-45m away - the laminate glass incorporates a thin plastic sheet which will stretch under the blast load and hold the glass together. Medium Risk properties are those close to “sensitive” buildings where windows should, as with Low Risk, be treated with 6.8m laminated glass but the frames, locks and fixings will be more robust and offer protection at a distance of 30-35m from a vehicle bomb. A High Risk building is one classified as a potential direct target because of its usage or ownership.

Selectaglaze’s units have been tested to Standard ISO16933 at the Spadeadam test facility, which is a world leading centre for trials involving the use of high explosives. This standard tests the effect of explosive devices of different sizes from small packages to a vehicle bomb at various distances from the target.

Fire Resistant secondary glazing may need to be specified for windows adjacent to a fire escape route. A fully developed fire reaches temperatures above the melting point of aluminium. To address this problem Selectaglaze has developed a specially adapted fixed frame glazed with Pilkington PyrodurTM glass, in conjunction with fire rated mastics and intumescent seals, which offers a fire rating of 30 minutes.

Testing to BSEN 1363-1,1364-1 requires samples to be built into a frame mounted to the front of a furnace. The temperature is raised in a controlled manner until all combustible material in the chamber has ignited - the sample continues to be tested for the specified period of time.

Ballistic Attack Protection has traditionally relied on thick multi laminated glass which is very heavy and difficult to handle. However, the development of composite materials using glass and polycarbonate provides thinner, lighter panels which are practical to install within secondary frames.  Tests are carried out to Standard EN1522/1523 which covers handguns, rifles and shotguns. In practical terms secondary glazing will generally cover to handgun level FB4.

Article courtesy of Selectaglaze Ltd. More information and literature is available on: 01727 837271
E-mail: enquiries@selectaglaze.co.uk or visit: www.selectaglaze.co.uk

The website also shows footage of products being tested against physical attack conducted at the Building Research Establishment.