Key Takeaways
- Systematic Approach: Always isolate power issues by checking between the breaker and disconnect before diving into the unit itself
- Control Circuit Isolation: Remove the R wire before applying power to prevent equipment startup during electrical diagnostics
- Voltage Verification: Document all voltage readings at multiple points – unusual configurations like 575V/480V setups require careful measurement
- Safety First: Working with high voltage (575V) requires extreme caution – the “contactor bump” technique should only be performed by experienced technicians
It was a warm August day, and my co-worker and I were digging into some PM work when we discovered a problem that would turn into an interesting diagnostic challenge.
We found one of the Carrier RTUs without power and the breaker tripped. This is never a good sign, especially when you’re already knee-deep in preventive maintenance schedules. I returned a couple of days later to unravel the mystery.
Initial Power Isolation and Testing
First step was to shut down the RTU from the local disconnect and reset the main breaker. After the reset, the breaker stayed on and didn’t trip. This told me something crucial: the problem didn’t lie between the breaker and local disconnect – it was hidden somewhere in the package unit itself.
A visual inspection didn’t reveal anything obvious at first, and none of the main loads showed any ground faults. This is where systematic troubleshooting becomes essential, much like when working with complex BMS control systems where problems aren’t always visible.
The Diagnostic Approach
Before applying main power to the unit, I removed the R wire from the control circuit. This critical step prevents any operation of fans, compressors, or other components, allowing me to check for power in the primary and secondary locations without anything starting up unexpectedly.
You may crucify me for this, but I did the bump – the contactor bump, that is. This technique, while controversial, can reveal important information when performed safely by experienced technicians. And in this case, it led me in exactly the right direction.
