Fuse sizing !

Dear experience control engineer, tech and panel builder,

I'm reading this page (still learning), I wonder how people sizing the fuse, some has and some does not need fuse at all !?

In this first page of electrical diagram, I see

1 DI, 1 DO, 1 AO modules

DI(1756-IB16) has 1A fuse,
DO(1756-OXBI) has 5A fuse,
AO(1756-IF6O) has no fuse, 2A fuse just for 2 Pressure transmitter surge protector.

how they decide 1A for DI (24V power) while 5A for DO (24V power)
DO has 4 CR, what if I install more CR, will I increase 5A fuse !?
(each point I/O of DO has 1A fuse)

#AnythingButFuse

ganutenator said:

#AnythingButFuse

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The fuse vs. circuit breaker question is like Coke vs. Pepsi. No clear answer. Both have their strengths and weaknesses.

Getting back to the question of the OP, the simple answer is that the fuse has to be sized large enough to let the expected current pass through it and small enough to blow when a short to ground occurs.

Since you're listing AB part numbers, they specify fuse sizes for the inputs of all of their devices, so that's probably where they got 1A for the DI and 2A for the AO. The DO points will have a rated output and the load they are driving can be known as well, so the 5A is either the maximum rated output of the card or the maximum expected load plus a safety factor.

Fuses are sort of the dark ages and fusing the input power of every card is a waste anyway.

CapinWinky said:

Fuses are sort of the dark ages and fusing the input power of every card is a waste anyway.

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Agreed but its not uncommon. It helps the maintaining electrician locate the trouble. I've had to design panels that had a specification of only a short duration of down-time allowed due to a failed device and a blown fuse indicator can quickly narrow the "usual suspects"; small fuses are less expensive than CBs so its more attractive to the machine builder.

jstolaruk said:

Agreed but its not uncommon. It helps the maintaining electrician locate the trouble. I've had to design panels that had a specification of only a short duration of down-time allowed due to a failed device and a blown fuse indicator can quickly narrow the "usual suspects"; small fuses are less expensive than CBs so its more attractive to the machine builder.

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I'll second this notion. With properly maintained lighted fuse blocks we have, on more occasions than I can count, found a fuse to an input or output, went to the field and known we were looking for a short to ground. Changing fuses can be a pain sometimes, but that type of system works great. Especially in larger cabinets.

To answer your question more in-depth: when doing sizing for any kind of circuit interrupt device you need to look at your max load. This takes into account any type of overload, fuse, circuit breaker or anything else that might open a circuit in the case of a current over-draw. Everything in the entire circuit is sized for the load of the device it is running to prevent fires and explosions.

You can check in the NEC or IEC codebooks (NEC for USA and IEC for everyone else) to determine the proper sizing for any such component. This, in my opinion, is probably the single most important topic when it comes to building electrical circuits. This topic is what causes the majority of fires, explosions and casualties when dealing with electricity.