Buchholz Relay

Buchholz 2

Product Range

Buchholz Relay - Product Range

TypeABCDELSd1d2d3N˚ HolesGas Vol.Weight
BS2521516085115140200172514684125 cm³2,9 kg
BS502541781251651401951850181024235 cm³4,9 kg
BS802931931602001401951880181384/8235 cm³ 5,8 kg


Description and General Specifications

The gas and oil actuated (Buchholz) relay is designed to detect faults as well to minimize the propagation of any damage which might occur within oil-filled transformers, capacitors and reactors supplied with oil conservator. The relay is therefore particularly effective in case of:

  • Short-circuited core laminations
  • Broken-down core bolt insulation
  • Bad contacts
  • Overheating of some part of the windings
  • Short-circuits between phases
  • Earth faults puncture of bushing insulators inside tank

Furthermore the relay can prevent the development of conditions leading to a fault in the transformer, such as the falling of the oil level owing to leaks, or the ingress of air as a result of defects in the oil circulating system.


Casing : non porous weatherproof compact casting of light aluminium alloy painted.

Cover : non porous weatherproof compact casting of light aluminium alloy painted. On the cover are located : the terminal box, the valve of pneumatic test, the breather cock, the button for mechanical test of alarm and trip circuits.

Inspection windows : special tempered glass with graduated scale in cm³

Contacts : they can be mercury switches or magnetic actuated switches (reed contacts). On request itís possible to supply change-over switches.

Switches characteristics :

rated voltage : 24 ÷ 250V AC or DC

rated current : 0,5A (10000 tests)

breaking capacity : 2A AC (cos= 0,4 ± 25% – 50Hz) 2A DC (T=L/R=40msec).

Mechanical protection degree : IP 54

Insulation : 2000V 50Hz between terminals and earth for a 60 seconds time.

Working temperature : oil temperature range: – 25 / +115°C.

Vibration test (in normal operative conditions) :

  • oscillation amplitude : 2mm
  • time diagram :

0Hz ÷ 100Hz                                        30 sec
100Hz (200 vibrations/sec.)           60 sec
100Hz ÷ 0Hz                                        30 sec

Contacts capacity to withstand vibrations :

  • Mercury sw : 150 horizontal vibrations/sec. (75Hz) first signals of closing contacts 120 vertical vibrations/sec. (75Hz) first signals of closing contacts.
  • Magnetic sw : 200 horizontal vibrations/sec. (100Hz) no one signal of closing contacts 200 vertical vibrations/sec. (100Hz) no one signal of closing contacts.


Slight faults : when a slight fault occurs in the transformer, the small bubbles of gas, which pass upwards to the conservator, are trapped in the relay housing, thus causing its oil level to fall. As a result, the upper float rotates on its hub and operates the alarm switch, thus operating an external alarm device.

Serious faults : when a serious fault occurs in the transformer, the gas generation is violent and causes the oil to rush through the connecting pipe to the conservator. In the relay this oil surge hits the flap fitted on the lower float (located in front of the hole for the oil passage) and causes the rotation of the float itself, thus operating the tripping switch and disconnecting the transformer. The float remains in the trip position even if the oil flow comes to a stop (the reset is done by means of the push-button). The tripping device is regulated in such a way that in transformers having forced oil cooling, the surges resulting from the starting of the oil circulating pump will not cause mal-operation of the relay. An oil leak in the transformer causes the oil level in the relay to fall, thus operating first the alarm (upper) float and then the tripping (lower) float. The ingress of air into the transformer, arising from defects in the oil circulating system or from other causes, operates the alarm float.

Checking after actuation of the relay

When the signal is given, the colour of the gas should be observed through the inspection-windows. Draw a gas sample and analyse it by means of the proper analyser. It should be noted that, usually :

  • whitish gas is caused by electric arcing in contact with paper, cotton or silk
  • yellowish gas is due to wood and cardboard
  • greyish gas is caused by a breakdown of the magnetic circuit
  • black gas is caused by free arcing in the oil.

If the relay disconnects the transformer, similar checks should be made to determine composition and quantity of gas collected by the equipment In any case never put back the transformer into service without having detected the cause of fault and having repaired it.

Operating Instructions and Maintenence

Mounting instructions : the Buchholz relay is mounted on the connecting pipe between the transformer and the conservator : the arrow marked on the relay cover must point toward the oil conservator. The pipe should not contain any right-angle elbow. Pipe diameter should correspond to the diameter of the relayís hole for the oil passage. The pipe must be arranged to slope upwards towards the conservator at an angle of 1 up 5 degrees to the horizontal.

Electric wiring diagram : inside junction box cover itís possible to find electric wiring diagram that consents the operator to effect the connection of the relay to alarm and trip circuits and to earth. Once electrical circuits are connected it is advisable to effect the mechanical test of the alarm and trip circuits by means of the apposite button located on the relayís cover.

Caution : gradually press the button until first alarm contact is obtained, and afterwards also the trip contact is activated. The push-button is self returning thereby freeing the signalling devices of the relay as soon as the pressing action upon the button is discontinued.

Signalling :

  • in case of gas accumulation : alarm
  • in case of lack of oil : trip
  • in case of oil surge : trip

Maintenance : No particular maintenance is required. Only, on regular basis – typically once a year, schedule inspections to verify that there are no oil leakages and to check the correct working of the contacts. The check can be done in two ways :

1 ) MECHANICAL TEST : by pushing the mechanical test button – pos. 8 on Buchholz drawings – on the top of the relay casing – after unscrewing its cover

2 ) PNEUMATIC TEST : by injecting air into the relay with a suitable pump (can be supplied by TERMAN in case on request) through the valve for pneumatic test – pos. 7 on buchholz relay drawings – until the floats rotate thus causing contacts actuation. After testing discharge the injected air by means of the breather- discharge cock on the cover of the relay casing – pos. 6 on Buchholz drawings . In case the window transparents are dirty, thus reducing the readability of the oil level inside of the buchholz casing, you can clean them with a soft cloth, water and soap.

Finished Product Quality Control Tests

Hydraulic test for tightness for the body : the aluminium alloy casing is filled with oil at the temperature of 90°C and at the pressure of 300 kpa. After 4 hours no leakages must be observed. The same test is carried out for floats and switches.

Dimensional check.

Hydraulic test for tightness of the finished relay : the test is carried out by filling the Buchholz relay with oil at the temperature of 90°C and at the pressure of 200 kpa. Said test lasts 4 hours. During this period no oil leakage is admitted.

Manual and pneumatic inspection of the electric contacts actuation : This test is carried out both on alarm and trip circuits. Itís performed by means of visual indicators mounted on a panel-board.

Check of tripping velocity : performed by mounting the relay on a oil filled pipeline. By means of a calibrated flow meter and of suitable visual indicators mounted on a panel-board.

Check of instrument mechanical protection degree : IP 54.

Insulation test : carried out by means of a microprocessor controlled testing unit.