Allen Bradley PLC output Fuse Sizing

[shoon]

I work at a liquid nitrogen plant. When the plant was constructed one of the PLC panels utilized only a single 5A fuse to protect all of the 16 outputs on the relay cards.
The issue we have encountered (3x now) is that some of the pneumatic actuated valves are solenoid controlled, and we have had solenoids short out and wipe out all outputs on that card wreaking havoc to the process / automation. Not sure why the solenoids are failing, it could be partly due to the harsh environment they are installed.

I was just curious if there was a standard for sizing fuses for these outputs? they are all 24VDC @ 10 watts. (417mA)

Thinking of leaving the 5 AMP branch fuse in place and adding 1 amp fuses to each of the 16 outputs to minimize downtime and speed up troubleshooting. Is it worth it using the time delay fuses? My thoughts are that 1 AMP fast acting is more than adequate for each of the individual outputs, as the coil current should in theory never change even if the valve jams.

 

[richwaskowitz]

Could you provide the catalog number for the output card please? Have you considered a fused wiring arm (if your card offers one)? For instance, we use fused swing-arms for the 1771 series, where the output cards have one 10 amp fuse on-board and the individual fuses are 3 amp.

Otherwise, you will want to coordinate the fusing so that you are assured that the smaller/faster fuse will always open before the module's on-board fuse and branch circuit fuse.

 

[shoon]

Could you provide the catalog number for the output card please?...

Thanks!
Pretty sure It's a 1756-0W

 

[richwaskowitz]

I see that that module is a relay-contact type and will not have any on-board fuse nor will any fused swing arm available. So fuse speed and rating coordination is all you have to work with.

 

[shoon]

I see that that module is a relay-contact type and will not have any on-board fuse nor will any fused swing arm available. So fuse speed and rating coordination is all you have to work with.

Thanks for looking into it for me, and for the fusing advice! I'll use a Time-Delay fuse for the branch (module fuse) and the fast acting fuses will serve each individual output. I would have overlooked that aspect for sure :thumbsup:

 

[Ingenieur]

Thanks for looking into it for me, and for the fusing advice! I'll use a Time-Delay fuse for the branch (module fuse) and the fast acting fuses will serve each individual output. I would have overlooked that aspect for sure :thumbsup:

if these are dry contacts also consider the load served
sounds like an inductive load, a pneumatic solenoid valve
you want to make sure the fuse doesn't pop when energized

any info on the solenoid valve?

 

[rlundsrud]

I would suggest putting an intermediate relays that are controlled by the plc outputs. This way you protect your outputs. Use something that can withstand the fault current, but still fuse it.

 

[Jraef]

I would suggest putting an intermediate relays that are controlled by the plc outputs. This way you protect your outputs. Use something that can withstand the fault current, but still fuse it.

A more common term is "interposing" relay; the PLC turns on a small relay, the small relay, hopefully with more robust contacts, turns on the solenoid. That way if a solenoid takes a dump, it doesn't wipe out an expensive Output card taking out 15 other solenoids from being under control, it maybe wipes out one cheap little relay.

You can get thin little interposing relays like this that are essentially just like terminal blocks; that's what I use. They are more expensive than cheap little "ice cube" relays, but take up a LOT less room and act AS the field terminal block, eliminating wiring. The contacts can be rated 6A each, far more than what the relay on the I/O card is rated.

 

[ptonsparky]

A more common term is "interposing" relay; the PLC turns on a small relay, the small relay, hopefully with more robust contacts, turns on the solenoid. That way if a solenoid takes a dump, it doesn't wipe out an expensive Output card taking out 15 other solenoids from being under control, it maybe wipes out one cheap little relay.

You can get thin little interposing relays like this that are essentially just like terminal blocks; that's what I use. They are more expensive than cheap little "ice cube" relays, but take up a LOT less room and act AS the field terminal block, eliminating wiring. The contacts can be rated 6A each, far more than what the relay on the I/O card is rated.

View attachment 16722

:thumbsup: Use them frequently. Reasonably priced.

Drove 90 miles to get a box last week. We are supposed to have them on hand. Grrr.
Lost were found less then five minutes after I got back. Backseat of my pickup in a Walmart bag...grr.

 

[Jraef]

:thumbsup: Use them frequently. Reasonably priced.

Drove 90 miles to get a box last week. We are supposed to have them on hand. Grrr.
Lost were found less then five minutes after I got back. Backseat of my pickup in a Walmart bag...grr.

You found them because you no longer needed them. That's a well known corollary of Murphy's Law.

 

[tom baker]

All my PLC outputs, even if a relay type, to to an interpoising relay,unless its for a local pilot light in the cabinet. I use the type shown above, the terminal block with cartridge relays.
These are 24 vdc operated, and often they drive a 120V AC relay in a separate cabinet. The ac loads, motors, fans, heaters all have different ac sources and those different sources are all in one interpoise relay cabinet for foreign voltages