The suction line of a refrigeration system is normally insulated with a flexible, closed-cell elastomeric material. Insulating the suction line is done to prevent condensate from developing and dripping off. It also limits the amount of heat added to the refrigerant as it travels back to the compressor.
A popular electrical component used on many control circuits is the step-down transformer. It is the power source for a separate lower-voltage circuit which typically controls the operation of a system.
Checking the acidity and/or moisture content of refrigerant oil is one of the methods used to test if a refrigeration system is clean and uncontaminated. But how can you take an oil sample without an oil plug?