According to written standards developed by an independent testing laboratory. The preeminent testing laboratory in the United States is Underwriters Laboratories (UL). UL 687 provides specific ratings based upon the types of burglary tools used and the amount of time that a safe can delay an attack. These ratings are designated using an alphanumeric code, such as "TL-15", "TRTL-30", etc. The following codes are used within the ratings:
TL =Tool-Resistant: designed to resist attack from common hand tools and power tools.
TR =Torch-Resistant: designed to resist attack from cutting torches.
TX =Explosive and Torch-Resistant: designed to resist attack from explosives and torches.
15 =Designed to resist attacks for a minimum of fifteen minutes.
30 =Designed to resist attacks for a minimum of thirty minutes.
60 =Designed to resist attacks for a minimum of sixty minutes.
The codes are combined to form the safe's rating.
The basic rating system is based on the safe's ability to provide protection against an attack on the safe's door and door face, the most common points of forced entry. An additional rating is provided for safes that provide protection on all six sides (face, top, bottom, back, left side, and right side.) These safes receive the additional designator "X6", which is appended to the basic rating.
Don't be shocked to read that some UL rated burglar safes can be penetrated in as little as sixteen minutes. Keep in mind that UL conducts its tests using expert safe crackers in a laboratory setting, and that these safe crackers study construction blueprints of each safe before they crack it. In simple words only Tom Cruise in Mission Impossible could crack a safe that fast. If Tom would be able to crack a safe in 16 minutes , it would probably take the average burglar an hour or more, even if he was reasonably skilled. TL-30 and TL-60 safes are even more difficult to crack, putting them beyond the capabilities of all but the most professional of burglars.
Construction ratings are based upon the types of tools that a burglar would need to penetrate your safe. Common construction ratings include "B-rated safes", "C-rated safes" , and "B/C-rated safes". Construction rating methods were used by the safe industry in the past, but this method of rating has become less popular in recent years. Drawback to this rating system is that there is no independent testing or validation of the ratings - manufacturers can simply claim a rating based upon the way that the safe is constructed. Also, there is no guarantee that the safe will resist any specific type of attack for any specific amount of time, although this may be implied.