Fusing question

wes said:

I am working on an automation project, it is a simple press to install molded rubber inserts in a die-cast aluminum plate. I am using an Omron CQM1 PLC (24 VDC input, relay output (2Amp)).

This PLC was originally purchased for a project that never materialized. It consisted of an Omron PLC in an enclosure with a 24 VDC power supply and 10 pneumatic solenoid valves (120 VAC 1.6 W coils).

The existing circuit protection consists of a 5 amp breaker on the incoming 120 VAC line, a 1 amp fuse in the 120 VAC line to the 24 VDC power supply, a 2 amp fuse in the 120 VAC line to the PLC.
It was not stated what the power supply output rating was, a 1 amp input can allow a 5 amp output...if its a 3A Power supply this could be a problem.

I am not familiar with OMRON but check the specs for it, a 2A may be high unless its the same AC source for relay, 120vac, outputs. In this case it appears to be high...personally I would use separate fusing for the PLC and the outputs...that way a shorted valve etc would not affect the PLC.

In my new application I will be only using 3 solenoid valves.

My question is the 5 amp breaker the correct size? Also should there be a fuse (1or2 amp?) on the 24 VDC output line form the power supply to the to PLC input common. There will be about 12 prox switches for inputs. What do you think?
Do not worry about fusing the PS output but to simplify troubleshooting you may want to fuse each valve or device...check the current ratings for them and use accordingly. You can get terminal block fuse holders and fuses rated in milli-amps.

The 5A may be OK but determine your total current requirements and go from there. A 5A main may be too high...it depends.

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CaseyK said:

It is possible to have too many fuses, but not common.

Fuses will blow pretty quick with a short circuit, but if the current is brought up slowly, it might not blow until the current is roughly twice the rating. The old adage, if five is good, then ten is better doesn't work.
Fuses or CB's work depending on type used and matching too the application. Time can be a factor...ie time delay but fast blow etc should never reach double rating.

Circuit breakers often will trip if the current is continuous, at a level lower then the rated current, such as 80% or 90%.
Technically if the CB is working properly then 80-90% should have no effect IF the system is designed properly...ie running at 80-90% with inductive loads energizing that could exceed the rating.

I prefer fuses for most applications, with enough to isolate the inputs, outputs, relays, solenoids, etc seperately. On large progects, banks or racks fused seperately.
Fuses or CB's, just make sure it matches the application.

When I started in the dark ages, a single fuse protected the whole generator starting panel. It did not protect short circuits that burned up small runs on the pc board relay sockets. Sometimes it would blow, on a panel that required a positive signal to the engine shut down solenoid, and the engine would keep running. The older engine controls were a good example of how not to doe a lot of things.
I go back to D333 and older models that had pony motors and manual throttle control, you had to shutdown the engine manually. CAT with the S4 did use 1 fuse for the control with an energize to shutdown solenoid but the control, both the original tray and the black box, had a type relay system (electronic) that when a failure occured would send power to the solenoid, if not aware the shut-off solenoid would stay continuously powered and possibly burn up due to a failure. It was powered by the batteries.

Even if the control panel did "fail" the excitation should be removed so the gen-set would be non-operational.

What I never liked was CAT's mechanical governor system which could run away...and depending on situation, turn the gen-set into a fireball. I preferred Woodward governors.

Cummins and Detroit with Stewart/Stephenson used an energize to run system for the fuel shutoff, this method required replacing the shut-off solenoid more often.

regards.....kc

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