How Long Does a Fire-Resistant Safe Last: 843, 927 and 1010 °C Explained

Short answer: How long a fire-resistant safe lasts is determined by three values read together: the duration (30, 60 or 120 minutes), the external temperature the test furnace reaches at the end of that time (843, 927 or 1010 °C), and the interior temperature limit the safe keeps its contents below (177 °C for paper). The ratings you see on product pages, "30 minutes at 843 °C", "60 minutes at 927 °C" and "120 minutes at 1010 °C", are in fact three points on the same standard fire test curve.

Three Numbers, One Curve: Where Do 843, 927 and 1010 °C Come From?

In safe fire tests the furnace is not held at a constant temperature. It heats up progressively, following the time–temperature curve defined in standards such as ASTM E119 and UL 72. On this curve, the furnace temperature is approximately:

• 843 °C at 30 minutes (1550 °F)
• 927 °C at 60 minutes (1700 °F)
• 1010 °C at 120 minutes (1850 °F)

So "2 hours at 1010 °C" does not mean the safe was held at a constant 1010 °C for two hours. The safe is exposed to a steadily rising temperature along the two-hour curve, and the furnace reaches 1010 °C at the end of the test. In practice, the temperature figure should never be read on its own but together with the duration: 1010 °C describes 2 hours of protection, 843 °C describes half an hour.

The Real Measure: The Interior Temperature Limit

What matters is less how hot the outside of the safe gets and more which temperature the inside never exceeds. Paper begins to char and scorch at around 177 °C, so the goal for document safes is to keep the interior below 177 °C for the full test duration.

The UL 72 standard defines different classes depending on the contents: Class 350 for paper (interior below 177 °C), Class 150 for magnetic and photographic media (below 66 °C) and Class 125 for digital media (below 52 °C). The more heat-sensitive the contents, the lower the interior temperature the safe has to maintain.

The Mühlen range specifies two different interior temperature limits:

• Below 177 °C: Fire Bag, Fire Safe and Panzer CK series — for paper documents, title deeds, promissory notes and cash.
• Below 80 °C: most models in the MP and MR series (e.g. MP 53B, MR 85A) — leaving a much wider safety margin than paper requires. As the value can differ from model to model, check the limit stated on the product page.

Note: 80 °C is not one of the UL 72 digital media classes. If you plan to store flash drives, external hard drives or a photo archive, separately confirm that the relevant class (Class 125 or 150) is explicitly stated on the product certificate.

Water Resistance: What Do 8 Hours and 24 Hours Mean?

In a fire, the safe is threatened not only by heat but also by firefighting water. On models with a stated water resistance, the value shows for how many hours the safe keeps its contents dry when fully submerged. Mühlen specifies 8 hours of water-tightness for the Fire Safe, Panzer CK and Fire Bag YG20xx series, and 24 hours for the Fire Bag YG75 and YG85. In basements, ground-floor archives or flood-prone areas, this value is just as important as the fire rating.

Mühlen Models Side by Side

The table below brings together the values as stated on the product pages:

US UL certification is stated for all models in the table. You can browse all fire-resistant models in the fire-resistant steel safe category, and the water-protected ones in the fire- and water-resistant safes range.

Which Rating Is Enough for You?

1. What will you store? For paper documents and cash, the 177 °C limit is the key benchmark. For photos, film or digital storage media, look for a lower interior temperature and a matching certification.
2. How long do you need protection? How long it takes to put out a fire depends on the building's construction, the fire load and how quickly the fire brigade can arrive. Because of this uncertainty, the 60- or 120-minute classes offer a safer margin for irreplaceable documents such as document archives, company records or title deeds.
3. Is there a water risk? In basements, on ground floors or in areas with sprinklers, choose a model with a stated water resistance.
4. Portable or anchored? The Fire Bag series is easy to carry and grab in an emergency, while larger safes are anchored in place to protect against theft. For a comparison of safe types, see our steel safe guide.

Before Opening the Safe After a Fire

• Do not open the safe until it has cooled down completely. A sudden rush of air into the hot interior can cause scorched paper to ignite.
• Electronic keypads can be damaged by fire, so keep the emergency override key in a secure place away from the safe.
• Photograph wet documents before drying them and keep a record for use in the insurance claim.

Frequently Asked Questions

How long does a fire-resistant safe last?
It depends on the safe's test class. Mühlen models are rated for 30 minutes (843 °C), 60 minutes (927 °C) and 120 minutes (1010 °C); throughout that time the interior is kept below 177 °C or, depending on the model, below 80 °C.

Does "2 hours at 1010 °C" mean the safe stayed at 1010 °C for 2 hours?
No. The test furnace heats up following the standard time–temperature curve and reaches 1010 °C at the 120-minute mark. The value refers to the temperature at the end of this two-hour curve.

Why 177 °C?
Paper starts to char at around 177 °C. Class 350, the UL 72 class for paper, is based on this limit.

Can I store flash drives and external hard drives in a fire-resistant safe?
Digital media are damaged at much lower temperatures than paper. UL 72 defines Class 125 (52 °C) for digital media; for this kind of content, confirm that the relevant class is stated on the product certificate.

Is a fire-resistant safe also waterproof?
Not every model. Water resistance is a separate feature and is stated in hours on the product page; Mühlen models are rated for 8 or 24 hours.

Related Content

• Steel Safe Guide: Types, Security Indicators and Selection Criteria
• Fire-Resistant Steel Safes · Fire- and Water-Resistant Safes
• Bag Safes · Electronic Steel Safes
• How to Choose a Gun Safe: 7 Criteria for Rifles and Handguns

To find the fire rating that suits your needs, use the form on the product pages or get in touch with us.