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HVAC School - For Techs, By Techs

Mar 19, 2019

Today's short podcast episode is all about surge protection on HVAC/R equipment. Lightning poses a severe threat to equipment, and surge suppression may or may not help.

Unfortunately, surge protection cannot protect HVAC equipment from direct lightning strikes. Surge suppression strategies connect to the high-voltage line and will protect your equipment from surges from the utility. Large, instantaneous spikes in voltage can mess up your equipment quite severely.

In general, we install MOVs (metal oxide varistors) as surge protectors in residential and light commercial equipment. Series mode (SM) acts as a low-pass filter that blocks higher frequencies, but MOVs are our main go-to for surge protection. When the voltage is within the clamping voltage, the metal oxide varistor shunts or redirects current to ground instead of the device; these devices have very high resistance, and they can fail when they get too hot. In thermal runaway, the MOV is very hot but continues shunting the current; as a result, the MOV is at risk of catching on fire. Thermal protection can exist for MOVs. MOVs also need a strong, secure ground connection to operate correctly. Make sure the MOV is connected, and a good way to do that is to test from leg to ground.

MOVs also require careful consideration during installation. These surge protectors can fit inside an ICM493 box that prevents catastrophic fire in the case of thermal runaway. Overvoltages below the clamping voltage can occur on MOVs, especially on inverter-driven compressors. ICM493s can also control overvoltages because they have voltage-monitoring capabilities (however, there is no published Joule rating). Both the LBK10 and ICM493 can shut off the equipment in the case of MOV failure.


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