to decide fuse rating of a device????please guide

i m making a project of gps gsm based tracking device.thThe maximum current required from device can reach upto 2 A
during GSM transmit burst. The sleep curren will be less than 1mA
The device will be designed to take power from voltage range from 10v to 40V.

so please help me how to decide the fuse rating??please elaborate it??

Regards

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please b fast??

I've not known any hard and fast rule for deciding the fuse rating, generally I do it with experience. So for maximum current of 2A, I would have used a fuse of 2.5 to 3 A rating. I generally take it as 1.5 to 2 times of the maximum current, but at times people use even 4-5 times of maximum rating.

what I do is look at the current behaviour and how susceptible the circuit is to current overload

if it can handle current overload for a while, then use slow-blow else fast-blow

if the current pulses then more to slow-blow

if the circuit is easily damaged then a rating close to the maximum

it is a balancing act between how easily the circuit is damaged and the pain of replacing a fuse

for short circuit case its simply ohms law

V = 40 V DC
Load = 10.0 Ohm [obviously connected in parallel to supply]

then Current I = 40/10 = 4A
now if an abnormally low resistance path is formed in circuit in parallel to our load (may be because of component failure) then circuit current will be substantially increase like for example, if short circuit path has a resistance of 0.1 Ohm then majority of the current will flow through this low resistance path and current sourced by power supply would be around 40 V /0.1 Ohm = 400 A according to Ohm's law. resulting in damaging power supply, pcb. Assuming designer has not used power supply rated for 16 KW operation for 160W operation.

Other case is of Overload and it has plenty of details like in rush, normal circuit current, I^2 *t for circuit, DC transients etc. for this you can either refer to opti-fuse guide or paper written by Cynthia Cline on fuse selection both are short 6-7 pages and a good read . It specifies various parameters for fuse selection for both AC and DC operation. Otherwise i would suggest select 2 A fuse if selecting local or just next value if selecting any standard part do consider ambient temperature as most fuse wire go off because of I^2*R losses.