We have an 8 postion switch connected to DI module in PLC. There is a fused terminal block between the PLC and switch. Some times when the rotary switch is operated like suddenly rotated from position 1 to 4 some fuse in in between channel is blown.A ny one have any ideas to verify the problem is it any sort of surge or transient generated? Please advice.

thanks in advance

 

I would check the rotary switch for earth fault. Exchange the switch with a new one to see if the problem persists.

 

dear mr helge,

this has happened randomly in different switches and in each switch different channels. My doubt was some sort of transient currents generated during fast switching of contacts i.e rotating the switch quickly ...but in test condition the current was only 10 ma maximum. the fuse is 100 ma ...

thank you

 

I think that I would look at the switching with a scope (a recording one if possible) to see what is going on.

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

Sr... fuses fail because of excess current flow. But, they also fail if when exposed to vibration, especially if they are very small!

I suggest you first determine the mode-of-failure, that is, did the fuse "open" by "fusing" electrically, or "break" because of mechanical reasons?

Regards, Phil Corso

 

Sr... further to my earlier suggestion, I would test for a higher-than-normal inrush current to "input" capacitance. Smaller fuses have a low tolerance to high di/dt transients!

Phil

 

I agree with Mr. Corso. I suspect there is some capacitance to the input channels.

Why would you fuse individual inputs to the PLC anyway? Fusing outputs is common for obvious reasons, and of course the input power will be drawn from a protected circuit, and I might even understand a common fuse per input module or something like that, but I don't believe I've ever seen a machine with each input fused.

 

And you can read a lot from a blown fuse. Sagging and a narrowed break is usually a slow overload. No sag and a clean break is often mechanical. Balls on the ends of the fuse wire show there was enough energy to arc. And metal blasted all over the glass means high fault current. If the fuse wire looks OK but the fuse is open, that's often temperature the solder melts before the wire a low currents.

Regards
cww Who used to do failure analysis.

 

Further to Curt's comments... if an opaque-type, and impossible to keep intact for analysis, then have it x-rayed!

Phil

 

Hi,

That great analysis. I am very much thanks for sharing your knowledge.

 

Sr... how many of the previous posted actions did you investigate?