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Underwriters Laboratories (UL)

We often refer to UL certification when writing about fireproof safes in our reviews and articles. So what is UL? And what are the UL ratings used for?

UL stands for Underwriters Laboratories. It is one of few companies known as Nationally Recognized Testing Laboratories. They are approved by the U.S. federal agency (OSHA) to perform safety testing of various products and solutions.

During it’s more than a hundred years history the company expanded into an organization with 64 laboratories. It established offices in 46 countries. As of right now UL has its head quarter in Northbrook, Illinois. Underwriters Laboratories participated in the safety analysis of many new technologies though out 20th century.


From the company’s history:

Underwriters Laboratories Inc. was founded in 19th century by an electrical engineer William Henry Merrill. His job at the time was to access risks and issue fire insurance. He then realized that in order to execute his tasks he needed to develop a set of specific standards and conduct various tests. That was the reason behind creating Underwriters Laboratories.

What is UL rating and why is it important?

UL Class rating provides information of how long a safe container will preserve a certain temperature while being exposed to a set heat levels.

Let’s look at UL Class 125 1-hour rating, for a example.

UL Class 125 1-hour rating means that, the temperature inside the safe will not exceed 125° F (52°C) for at least 1 hour while the temperature outside the safe can reach up to 1700°F (926°C). This basically means that whatever you store inside your safe-box won’t heat up to more than 125° F (52°C) even if the safe is exposed to fire of 1700°F. But the guaranteed duration of such protection is 1 hour. It can last longer but it won’t last less.


Let’s take another example – UL Class 350 1-hour rating.

UL Class 350 1-hour means that, the temperature inside the safe will not exceed 350° F (177°C) for at least 1 hour while exposed to external temperatures of over 1700°F (926°C). As you can see in both cases the temperature outside the safe can reach 1700°F (926°C).  But UL Class 350°F 1 hr. certified model will have it’s inner chamber heating up more. It can go as hot as 350° F (177°C) compared to 125° F (52°C).

You need to pay attention to these numbers (125°F vs 350°F) because they suggest what exactly you can safely store in the safe and not worry about it in case of fire emergency. If you keep hard cash or paper documents in safe, then UL Class 350 model is enough. Paper will not burn while the temperature is under 350°F. But if you would like to store a hard disc in your safe, then UL Class 350 is not enough. A device will be damaged under that temperature.