Ice Breaker: High-Pressure Switches
Troubleshooting Compressor Shutdown Controls Requires Care
High-pressure switches are used on many refrigeration systems. They are designed to shut down a compressor if the system’s high pressure rises above an unsafe level, not allowing the compressor to restart until the pressure returns to a safe level. High-pressure switches are available with either an automatic or manual reset.
An automatic reset will automatically restart the compressor when the pressure falls to a safe value. A manual-reset high-pressure switch needs to have an operator manually press a reset button in order to restart the compressor. They are also available with either an adjustable cut-out setting that could be adjusted by a technician, or fixed settings, which cannot be adjusted by a technician.
When making field adjustments to the adjustable high-pressure switch, it is always best to follow the directions or requirements of the system manufacturer. When the manufacturer’s guidelines are not available, the cut-out setting can be set at a saturation pressure equivalent to a 155?F condensing temperature, or 60? above the normal maximum ambient.
When troubleshooting, it is important to not allow a system to run with its high-pressure switch electrically bypassed as it could potentially lead to a very dangerous operating condition.
I can personally attest to this potential safety hazard.
Early on in my career I was called out to repair a system. When I arrived on the job, I noticed it had tripped on its high-pressure switch. I attached my gauges, reset the high-pressure switch, and, sure enough, the high-side pressure steadily increased to the trip point of the high-pressure switch and shut down the system again.
Not knowing the true cause of the problem, I decided to electrically bypass the high-pressure switch. (That was not a very smart decision, as you will see shortly.) I restarted the system and the high-side pressure again started to rise. I watched as it continued to rise well beyond the trip point of the pressure switch, so I decided to shut down the system. Again, being an inexperienced technician, I decided to start the system up again. Not sure why I needed to see the pressure rise again, but I did.
Just as I restarted the system for a second time, my pager went off (this was back in the day, before we had cell phones). So I made mistake No. 2 and left the system running while I went to find a phone to call the office. A few minutes into my phone conversation, I heard a loud explosion and I, along with everyone in the area, immediately ran out. Needless to say, the explosion was actually an 8-inch section of the discharge line (from the unit I was servicing) blowing out of the system. Apparently the system pressure had built up so severely that this section of discharge line broke free and was jettisoned across the room. It actually took down several lights and got wedged into a wall close by.
If I had been there or another person was walking close by we could have been severely injured. Lucky for me, my company and my customer emerged uninjured. But it was definitely a lesson well-learned.
Publication date: 3/31/2014