Ok,How about a real simple, basic one that has me stumped.

Recently, I built two identical panels for two of our machines.
These have a disconnect, a 500va control transformer, a line starter, and a control relay. Machine runs fine. But on one machine, when the mechanics turn the disconnect off, one fuse in the disconnect will blow. The plant has 240 volt 3 phase delta and the fuse in question always the one on the high leg. Only thing drawing power is the transformer. The fuses in the disconnect are 30 amp class cc slow blows and the transformer has 3 amp class cc slow blows on its primary side.
Any ideas?
Olias

Let's see...

The only load is the transformer, and its 3A primary fuse does NOT blow... Can't be the transformer

30A fuse blows when disconnect is switched OFF... Gotta to be an internal short to ground (or maybe another phase) within the disconnect mechanism. Did they happen to drill the pilot hole for the knockout directly above the disconnect? It might be full of chips!...

My first step would be to yank out the disconnect and inspect/replace... :nodi:



-Eric

P.S. Good to see you back here!...

Yea, thats what we've been thinking. I punched the hole before mounting the disconnect, but one never knows what fell in since. My field rep. will be at the plant tue. I'll have him check it out/replace. I've never had a bad disconnect before. Either they work, or don't.
Thanks for missing me, but I've been lurking. Our Co. got real busy in the past 2 years, and now I do everything. Doom on me.
Thanks for the reply.
Olias

BEFORE tearing into it:

Think I would run a set of resistance checks (phase to phase and phases to ground) on both line side and load side of disconnect with switch open and shut.

In addition to above (IF EASY to force the relays shut by hand) add shuting the relays to the above.

The part I scratch my head over is
1. with it energized and disconnect open the fuses are OK.
2. Then after shutting disconnect the fuses are OK.
3. After opening the disconnect you blow the fuse on hi leg.

Have I got the sequence right??

Dan Bentler

Dan Bentler

olias said:

I've never had a bad disconnect before.

Click to expand...

There's a 'first time' for everything...

What brand is the disconnect?



-Eric

The plant has 240 volt 3 phase delta and the fuse in question always the one on the high leg. Only thing drawing power is the transformer.

Click to expand...

I would like to get a few details clarified. To have a high leg on 240 3ph delta you would have to center tap one winding to ground to create a neutral.


If this is the case why do you need a 500va control transformer, you have 120vac available.

Or are you refering to the high leg as the leg that IS NOT connected to the control circuit transformer?
How exactly are you using the term "high leg"?

I think the first question is;


Is the motor running when they shut the disconnect off? If so, what is motor size? If not, are you saying machine can be at resting state and mechanic turns off disconnect and fuse blows? If it is something shorted (the disconnect arm, etc.) you should hear a pop or something. Of course it could be so loud you can't hear it.

My guess is the motor is running when they shut it off and if they do it at just the right time the fuse blows. This can be from back pressure in hydraulics or a couple of other things. If this is the case make sure your fuses are 175% of the rating of the motor as it appears in the NEC. Of course this only means something if you are using NEC as guidelines.

Remember; Is motor running when it happens? Is motor heavily loaded at that time? If not, do you hear a pop? If so, kill the power and meg out the disconect switch blades from phase to phase and each to ground.

Eric, Its an Allen Bradley 194 series.