What is Safety Glass? Implications and Insights

There are different types of glass, only some of which can be classified as ‘safety glass’. For instance there’s float glass, also described as “annealed glass”, which has undergone a slow cooling process to remove impurities and natural stresses in the material. Annealed glass is not a safety glass and cannot be used in areas where the chance of human contact is high, because it breaks into dangerous shards which can cause serious injury.

Other types of glazing can be labelled ‘safety glass’ however. “Toughened” or “tempered glass” is 5 times stronger than annealed glass and breaks into small pieces of 15mm or less. It is produced by returning the sheet of glass to the oven, heating it to around 620C and rapidly cooling it, putting the outer surface into a state of compression and the interior surface under tension to create the strength – and the resulting safety. 

Toughened/tempered glass may be classified as a safety glass, however we now need to apply the “level of acceptable risk test”. This glass is strong, which is good for impact; it breaks safely, which is good for the risk of injury; but once it has broken it loses all of its structure – it’s all well and good that the glass won’t cut anyone, but it’s not brilliant news if you’re standing or leaning on it when it breaks. When the risk of injury due to falling is unacceptably high, toughened glass alone is no longer a safety glass – for example: balustrades; escape routes; and any location higher than 5 metres above ground level.

“Laminated glass”, however, refers to the inclusion of a plastic or resin interlayer between two or more panes of glass. This effectively removes any risk of injury if the glass breaks as the interlayer holds the broken panes together – when specified correctly laminate glass will also remove the risk of falling as well.

(If you’d like to know more about the different types of safety glass, we’ve written a blogpost dedicated to the subject here.)

Having established the correct glass to specify in terms of safety requirements, it now needs to be assessed for impact – not just to see whether it breaks safely, but also to test the level of impact the glass can take before it breaks, and after it has broken.

Glass safety regulations were subject to significant changes back in 2006. BS EN 12600 replaced BS 6206 as a European-wide performance standard for the impact resistance of glass.

The concept of safety glass has been around for decades but the application of standards became more complicated with BS EN 12600. It incorporates a more modern test – often informally known as the pendulum test – which defines the level at which glass will ‘break safe.’

Previously, measuring the BS 6206 standard involved a series of tests, where a lead-filled leather bag (weighing 45kg) was dropped like a pendulum from different heights onto the glass. At Cantifix, we prefer ball bearings to leather bags, but the effect is very much the same. 

Now, the BS EN 12600 is designed as a more modern test, although the performance levels are very similar. It involves a weight being dropped at three different heights – 190mm, 250mm and 1,200mm. These weights are dropped onto a standard sized pane of glass, and its resulting classification will depend on at which point the glass breaks.

Two ‘safe’ broken states are defined as follows: when there is no hole large enough to push a fist through (and only a limited amount of glass fragments come loose) or when the glass leaves no large, dangerous fragments after shattering.

In order for glass to be classified as an impact safe material, the glass must undergo the Safety Standard Test as outlined above. This test is used to define the level at which glass will ‘break safely’, meaning there will be no large shards of glass – only small fragments that will not cause injury. It will then be classified into a group, which will signal its performance and capabilities.

BS EN 12600 uses a three-part classification system forming a three digit code representing (in order):

• Strength as measured by the aforementioned drop height Class 1, 2 or 3
• The breakage type A, B or C
• The containment aspects of the glass when broken (or the highest drop height class at which the glass does not break or does not allow penetration – again classed as 1, 2 or 3).

Therefore, classifications within BS EN 12600, starting with the highest performance are:

• Class 1B1
• Class 2B2
• Class 3B3

When using the BS EN 12600 classifications, class 1 is used for testing the glass for critical applications with a drop height of 1,200mm. Class 2 is used as an equivalent to more than an adult walking into the glass but less than if the adult were to force their way through by running at it. The drop height for this classification is 450mm. The last classification, class 3, is equivalent to an adult pushing firmly against the glass or a child running into it. The drop height for this class is 190mm.

All three classifications have a type B breakage which is demonstrated by the fragments being held together upon breakage. This is the typical breakage mode for glazing products such as laminated glass, or glass where a safety film has been applied.

Toughened safety glass should always obtain a Class 1B1 classification, making it one of the safest glazing products. Laminated safety glass with a polyvinyl butyral (PVB) interlayer normally achieves a Class 2B2 classification, depending on the glass and interlayer thickness. Wired glass will obtain a Class 3B3 classification.

The thicker the glass the greater the impact resistance and this would be the first consideration for any glazing specialist when assessing a project. After this, the next step would be to calculate the required strength to decide if toughened glass is required, and whether a standard or high strength laminated interlayer is necessary. All these options have cost implications, which often exposes a serious issue – when a quote is requested and there are no glass or performance specifications supplied, how can prices be compared?

At Cantifix we feel our responsibility as experts is to price for the optimum glass solution; however we are aware we are competing with others on price who have not applied the same high standards. But we don’t believe safety should ever be compromised, and we have no intention of changing our policy simply to win more business.

Toughened or tempered glass can, on very rare occasions , spontaneously break (1m2 in every 10,000m2 manufactured). Glass has many impurities (though low iron glass removes the naturally occurring iron particles) and the toughening process can make these impurities “active”. The most common is Nickel Sulphide inclusion (NiS) and even after the glass has been heat soaked the danger of spontaneous breakage still exists (1m2 in 1,000,000m2). NiS is described by glass manufacturers as a “phenomenon” and not a defect, so it is not covered by any warranty and is therefore hard to insure against.

If the full strength of the toughened glass is not required and the glass is laminated, then an alternative to consider if the risk of NiS is too high would be heat strengthened glass. It is twice as strong as annealed glass, compared to toughened glass which is 5 times as strong, but heat strengthened glass virtually eliminates the chances of spontaneous breakage.

There is the whole area of bomb blast and balistic glass, but we will do this another time  – when we have the time to go into what is meant by “an acceptable level of collateral damage”.

If you’d like to discuss BS EN 12600 in relation to the specific requirements for your project, or anything else raised in this article, please contact Cantifix’s technical team for expert advice here.