The schematic of the M59 ballast you are using is below, from the attached datasheet.

The coil in this design is really just a transformer turning one voltage into another; the purpose of the capacitor is to turn the output of the transformer from the voltage source to a current source:

• At a given frequency and input voltage, only a finite amount of current can get through the capacitor at 60Hz; this is relatively independent of the lamp voltage

 

These oil filled capacitors fail in two ways:

• They can dry out, causing the capacitance to fall

o This results in a reduction in lamp current and dim lamp 

• They can punch-through (dielectric breakdown) causing them to gradually turn into short circuits; resulting in a lot more current flowing through the lamp than the design spec, resulting in:

o Power factor for designed MH lamp goes out of spec (below 0.9) as current increases – see our app note on ballast life here: http://www.lunera.com/wp-content/uploads/2014/01/Lunera-Power-Factor-and-Ballast-Life-Overview-App-Note-4-29-15.pdf

o Ballast heating increases – this may trigger the thermal protector and cause lamp to cycle on/off – you often see this in street lights with EOL ballasts

  • In extreme cases you’ll hear this – the ballasts hum gets extremely loud with rising current 

o If no thermal protector, ballast will continue to get hot until one or more components suffers thermal failure

• UL made a change to the capacitor quality spec around 1995 to help address this (UL810 - attached)

o Mandatory addition of a short circuit protector (embedded fuse) to cause the capacitor to fail open rather than short in a punch-through case

o If your installed capacitors are from approx. 1988, they may predate this change to UL810

o Many modern ballasts have moved to film capacitors as they are not subject to dry-out the way oil filled capacitors are