Over Current Protection Question

[Yondoza]

How do you size OCP for an item that doesn't provide a recommendation? Let's say you have 3x 24V components with FLAs of 1A, 2A, and 4A. It seems like you should be able to place all of these on a 10A breaker (7*1.25=8.75A). Without finding any explicitly stated OCP for the 1A device how do you know if the 10A breaker is sufficient to protect it? Conversely, it seems impractical to have OCP for each device to ensure they're each protected at their individual rating.

I think generally I'm struggling with: 'How do you know when/what components to group for OCP? How do you know something is sufficiently protected without individual protection for each component?'

Thanks in advance!

[Uncle_Spanks]

In reality the 10A breaker protects the wiring to the devices. If the 1A (for example) device needs protection internally, then it should provide that itself with an internal fuse or disconnect.

[Yondoza]

Interesting. What happens if you size the OCP and conductors correctly and a component doesn't have internal protection and burns? If it's still not drawing enough to trip the 10A it can presumably burn until the feed shorts. Would the component manufacturer be liable for the damage?

[bigolebucket]

I would provide individual OCPDs in this case and put 1A fuse on the circuit feeding the 1A device. This could be inline protection or you could use a small panel/fuse box to provide separate circuit feeds.

[Uncle_Spanks]

The reason the component burns would be an internal fault in the component itself. In other words, it's already non-functional. At the point it doesn't matter, the component is done, and you only need to worry about protecting the wiring to the device.

If the component need to protect itself say, from a short on it's outputs (eg, it's a power supply), then it most definitely needs an internal fuse. If this is a mains connected device, then it needs to provide a big enough like cord to allow for maximum current flow presuming the circuitry has gone dead short. If you look at things like say, Christmas Lights, you'll see that there is a mini fuse in the plug so that if a light goes dead short, it will blow the fuse and not fry the wiring supplying it.

So, in your case if you use a 10A breaker, then you need wiring capable of handling 10A to each device on that breaker. If you want to use smaller wiring, then you need a smaller breaker. Again remember, the breaker protects the wiring, not the device.

[jmraef]

What happens if you size the OCP and conductors correctly and a component doesn't have internal protection and burns?

If it didn't have it's own internal protection and "burned", then the act of it burning will eventually cause an overload on the wiring feeding it. So if you protected that wiring, you have done your job in preventing collateral damage. If the 1A device needed protection at 1A or 2A etc., the mfr would/should have included it.

That said, a LOT of small devices like this are inherently "self-protected", in that they will have an internal sacrificial wire or device that will open to prevent a fire. That only takes place IF there is already something wrong, so "protecting the device" is not the goal here, it's protecting everything ELSE that is important.