Maintenance Detective: 6 Hours of Downtime

Let's play a game for all the maintenance players out there.

You come in one morning and there is a note in the passdown from night shift: 6 hours of downtime, they changed 4 components:

control transformer, a fuse, a contactor and a relay.

There are many possibilities here... but what assertions could we make? What is the most likely explanation?

And what do you do about it? (There is no right answer here, but there's a single scenario I've seen produce this exact outcome about 6 times).

Answer:

1. Unless there is evidence of a cascade where a component failing destroys other components, we would typically assume that a single component will be found bad in each troubleshoot.

2. It's very unlikely that 4 electrical components all failed at the same time.

3. So when we look at those 4 components, the question is which one would make someone replace the other 3.

4. The answer I've found in my career is that the answer is: fuse.

Fuses on single phase systems tend to defeat technicians because they test the top of the fuse against ground and then test the bottom against ground.  If they get the expected voltage on top and bottom, they think the fuse is good.

This test will render the same result regardless of the state of the fuse because single phase electricity (as long as there is no neutral) typically has the same voltage on each side of the circuit.

The scenario I'd expect here is that the technician incorrectly found the fuse to be good.

Then asserted that if the fuse is good and electricity is getting to the transformer then the transformer is the reason nothing has electricity. - Change transformer.

Problem persists, but now the tech knows the fuse and transformer are good and has no idea why the circuit isn't working, so we change a couple other bystanding components because we don't know what else to do.

Eventually they change the fuse and everything starts working again.

1. When this happens you need to catch it and teach proper fuse check methods.  

   1. A power-on check of a fuse is to check across the fuse.  If there is voltage drop across a fuse, it is bad.

2. Transformers can be almost as reliably tested by smell as with a meter.  They make smoke when they go.

   1. If someone is changing a transformer that isn't smoked, be suspicious.

      
      

Even beyond the ~6 times I've seen my techs do this, I've found original transformers unhooked in cabinets over the years that predated me. 

I like to continuity test and meg them to see if this is how they got abandoned.  I have yet to find one that doesn't work.

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