External sources of contamination can generally be minimized by maintaining a sealed
system, but on some types of equipment this is not possible. Examples of external sources of contamination are moisture, oxygen, and solid debris introduced during maintenance of the equipment or during oil processing.
Internal sources of contamination can be controlled only to a limited extent because these sources of contamination are generally chemical reactions, they cannot be stopped, but their rates are determined by factors that are well understood and often controllable.
Internal sources of contamination
• Non-metallic particles such as cellulose particles from the paper and pressboard
• Metal particles from mechanical or electrical wear
• Moisture from the chemical degradation of cellulose(paper insulation and pressboard)
• Chemical degradation products of the oil that result from its oxidation (e.g.,acids,aldehydes, ketones)