Would the Fire Department have trouble entering a house with this lock in case of a fire?

I have experience breaking doors. I’ve put locks like this or better on my house knowing it will slow the fire dept down a bit but like others have said we have tools that will break even this. A halligan tool in the right hands will get through this pretty quickly, however with a lot of noise and certainly will deter the average criminal. 800lbs is a good amount of force to withstand, I think a average 200lb fireman with a halligan tool is probably capable of 1000lbs Of force applied to a certain spot on the door. If you put the lock high or low that will make breaking That more difficult. But the hydraulic tools fireman carry go up to 5000lbs.

Does it only work when someone is inside?

I installed this on my front door. As others have said, you must be inside to lock it, and it cannot be locked or unlocked from the outside. So you have no access to the door from the outside when the lock is in place. I keep it locked all the time, even when I'm not home, because I go in and out through my back door. For extra safety and strength, I installed two of them on my front door, one about chest high and one about two feet from the floor. I did this because I have a glass panel on the top half of the door, so if a burglar breaks the window, the bottom blocker is out of reach. As extra security for my back door, I installed a security door (like a screen door except it's heavy gauge metal with its own handle lock and deadbolt).This door blocker is extremely well-made (heavy metal) and is a fantastic product. I highly recommend it.

do the screws come with this, or if not, what size do I need to buy

Yes screws come with this device don't use them they're garbage go buy yourself some good heavy duty three to four inch screws from the hardware store so you can get into the frame of the house not just a door frame it'll make this device a lot more sturdy

I bought this but cannot open it. Is mine defective or is there a trick to opening it?

Sometimes you are right. I had one defective out of three I received. The spring was loaded improperly which made the latch always to be in the open. Or if I forced it, the spring would dislodge and make it difficult to even close. And once closed it would not open. Now if that is the case then you need replacement. However, if you are trying to understand how it works, all you have to do is when the latch lodged in the recess, basically in its closed position, pull out as if you are navigating around the latch notch that stops it. So the idea is pull laterally first and then turn the latch around the notch. Hope that helps.

Manufacturer	Prime-Line Products
Part number	U 10827
Item Weight	168 g
Product Dimensions	6.35 x 3.51 x 7.62 cm; 167.83 g
Item model number	U 10827
Size	3 in.
Color	Satin Nickel
Style	Lock
Finish	Satin Nickel
Material	Aluminum
Shape	Rectangular
Item Package Quantity	1
Number Of Pieces	1
Plug Profile	Door Mount
Special Features	Weather Resistant
Included components	1 door lock
Batteries Included?	No
Batteries Required?	No

PRIME-LINE Defender Security U 10827 Door Reinforcement Lock – Add Extra, High Security to your Home and Prevent Unauthorized Entry – 3” Stop, Aluminum Construction (Satin Nickel Anodized Finish)

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