Intermittent short in Safety Loop

[PoolInWinterfell]

I have an old (circa 1991) Teledyne Laars EPC 1 325 type heater. In the safety loop, first thing off of the 24V transformer is an inline 1A slowblow fuse. Recently this fuse blew on me. I checked for direct short to ground, found none, replaced the fuse, and a week later, it blew again.
Every time I check for a short with a meter, I can't find one. Visual inspection of all of the wires in the front look fine (no rubbing or touching metal. I can't quite visually inspect the wires to the high-limit switches (they run through to the back, right side, of the unit) - but with the heater running (after a new fuse), I've wiggled, jiggled, pushed, and pulled on those wires to see if I can get a short (like if the insulation had rubbed off someplace) to no avail.

I tried replacing the transformer - just because it looked pretty old and worn - fuse blew again shortly after that.

Problem is, every time after the fuse is blown, I can never catch a short to ground (looking for a low ohm-reading between a point in the safety loop and the ground lug on the transformer.

Sometimes after I put in a new fuse it will runs for days before failing. I haven't tried it yet, but I thought about temporarily removing switches from the loop - but I can't walk away from the heater in that condition - so that doesn't seem like a good way to go. I could stare at the heater for hours and not know if I've really bypassed the culprit.

Do temperature control boards sometimes develop shorts? Or the main controller?

Any advice would be most appreciated. Do I need to just start replacing switches - maybe starting with the cheapest or more likely ones first? ouch!

 

[ps0303]

Intermittent shorts will drive you crazy. Only thing you can try to do is replace a few of the items one at a time and see if that changes anything.

 

[Pool Clown]

+1, Do you have automation connected to the heater? If you do, try disconnecting the automation and run the heater on it's own thermostat. Then try removing the high limits and inspecting the underside(s) of them for corrosion and replace if they looked so. then try the pressure switch, fusible link, and so on. Temp control boards, and Fenwalls do fail. But i would check/replace all other "cheaper" parts before considering buying/replacing the aforementioned parts.

Something else to consider...

This is a pretty old heater, if you decide to spend the coin and purchase the parts and actually get it going, remember, these are new parts in an old heater that may give you worse grief sooner rather than later!

 

[PoolInWinterfell]

Thanks for the tips everybody. I went ahead and disassembled the top of the heater so I could really check the wires going back to the limit switches. A critter had nibbled off the insulation on the wires, without biting through the wire (which would have opened the circuit). That left the exposed wires close to one of the sheet metal edges - vibration would occasionally make the grounding contact and blow the fuse.

Instead of replacing the wires, I added high-temp shrink wrap over the individual wires, then a high-temp over-braid shrink wrap over the pair. That should stop the critters for a long time!