1756-if16

hi guys

we want put a fuse protection before the signal go into IF16 analog module(4-20mA), how to decide the size of the fuse?

thanks

I have never seen it done......and why would you want to fuse it?...
The circuitry of a non-isolated analog channel is self contained; the module itself regulates the voltage/current depending of the initial setup of the module.
The modules are overvoltage protected and they themselves establish and control the electrical load (current); there really is no need for "extra" current protection.
If you really want to install a fuse, the Ohm's law applied for a 10.5 VDC voltage over a 249 ohms input circuit resistance returns approximately 40 mA.
You will also have to consider that the addition (yes even of a fuse) to an analog circuit will alter the quality and resolution of a pretty "feeble" signal to begin with.

Lightning problem? Induced signal noise from adjacent AC power circuits? Probably the smallest amp rating that you can find will work. No, I see that a 40 mA glass fuse is available from Newark Electronics:

http://www.newark.com/fuses_cartridge/fuse-current/40ma/pg/810115180

I probably would use at least a 80 or 100 mA fuse just to prevent nuisance trips. It is doubtful if the current could ever come from the signal generator, but from some outside source.

Any fusing would serve to protect the wiring, power supply, and instrumentation from a short in the circuit before it reaches the module. The fuse should be between the instrument power supply and the instrument and must be in the same enclosure as the instrument power supply. The max size for the fuse according to NFPA79 is
18 AWG = 7A
20 AWG = 5A
22 AWG = 3A
24 AWG = 2A
or not greater than the power supply output, whichever is less.

For example, if I had a power supply that was rated at 5A but my circuit had 22 AWG wire in it then I could not use a fuse larger than 3A. Or if I had a power supply with a 1A output but used 18 AWG wire I could not use a fuse larger than 1A.

You may use a smaller fuse if you wish, but the purpose of the fuse is to protect the wiring, not the 4-20mA loop. If you have adequate power supply protection and are placing a fuses on each of the loops to allow you to interrupt/isolate a single 4-20mA loop then any fuse that is not larger than the primary circuit protection fuse will do.

You may use a smaller fuse if you wish, but the purpose of the fuse is to protect the wiring, not the 4-20mA loop.

Click to expand...

That is the traditional inside-the-box usage, but apparently this case is not that. A fuse or circuit breaker is a two-way street. It can protect in both directions. For example, about 2 years ago a squirrel jumped on top of the transformer feeding my house, causing a surge on the power line. The 200 Amp main breaker in my panel tripped out (unknown to me), and the 100 Amp pole cut-out fuse on the 7200 volt power line also tripped, causing a power outage on the street. When my neighbor's power was restored, mine was still off. The breaker had reverse-tripped, not from a short downstream, but from a surge from upstream. Fuses can work the same way and protect against lightning surges introduced at any point on the line, or induced voltage, or a surge caused by a cut line.