What is OSR Protection or Oil Surge Protection

What is OSR Protection:

OSR protection is used to protect the internal fault in the On Load Tap Changer Circuits. It is a single float switch bucholzz relay model. It just monitors the oil level on the OLTC Conservator tank and how force oil enters into or leaves from the OLTC.

OSR provided with the single element oil surge relay has been specifically designed to operate with OLTC. OSR does not operate under normal gas developed during changing the transformer taps. It will response when the surge is developed in the OLTC.

Types of OSR relay:

  • OSR with mercury switch (Increase of bucket types relay)
  • OSR with magnetic switch (Flap with reed contact)



OSR relay will be installed in between the conservator oil tank and head of the OLTC. OSR is fitted with special Reed switch which assures rust free contacts even after decades after installation thereby is alert for operation. This OSR is designed and tested for very intensive seismic levels

  • OSR should be installed between the OLTC diverter champer & OLTC conservator champer
  • Arrow provided in the OSR should be towards the conservator tank
  • The Pipe should not be contain any right angle EL Blows
  • The Pipe should be arranged towards the conservator tank at an angle of 2 to 5 degrees
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Working Principle of OSR Relay:

Under normal condition, the OLTC OSR relay reads normal pressure, since the relay become in active. A heavy fault inside of the OLTC incidentally generates pressure wave or Oil surge or Oil move in the direction of OLTC tank. If these flow rate exceeds the operating threshold of the damper, then the flap move the flow direction Due to this movement the reed switch will be actuated and it give trip signal to the transformer. Since the fault will be removed. The main purpose of the OSR is to limit the damage to the OLTC during fault condition.


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