Buchholz relay BB 25

The type 22 (BB 25) of the Buchholz relays is especially used for rail vehicles. Find out more about this special type.

The Buchholz relay BB 25

The Buchholz relay BB 25 was especially developed for use in rail vehicles and ensures operation free of interferences even with changing angles of inclination. In addition, the EMB Buchholz relays are ideally suited for the monitoring of traction transformers due to their high shock resistance.

The Buchholz relay BB 25 is installed in the pipe between the transformer tank and the conservator. During normal operation it is filled completely with insulating liquid. Due to buoyancy both floats of the Buchholz relay are at their top position.

    In addition to the Buchholz relay type 22, the following types are also suitable for use in rail vehicles:


    The Buchholz relay BB 25 is installed in the pipe between the transformer tank and the conservator. During normal operation it is filled completely with insulating liquid. Due to buoyancy both floats of the Buchholz relay are at their top position.

    If a fault occurs inside the transformer, the Buchholz relay responds as follows:

    Gas accumulation

    Fault: Free gas is available in the insulating liquid.


    Response: The gas in the liquid moves upwards, accumulates in the Buchholz relay and displaces the insulating liquid level. As the liquid level  falls, the upper float moves downwards. Movement of the upper float actuates a switch contact of the upper switching system so that an alarm signal is  tripped. The lower float is not affected as from a certain gas volume the gas flows through a piping to the conservator.

    Insulating liquid loss

    Fault: Insulating liquid loss due to leakage.


    Response: As the liquid level falls the top float moves downwards. An alarm is tripped. If the liquid loss continues, conservator and piping as well as the Buchholz relay will be emptied. As the liquid level falls, the lower float moves downwards. The moving lower float of the lower switching system actuates a switch contact so that the transformer is disconnected.

    Insulating liquid flow

    Fault: A spontaneous incident generates a pressure wave moving in the direction of the conservator.


    Response: The liquid flow reaches the damper arranged in the liquid flow and held by a permanent magnet. If the flow rate exceeds the operating value of the damper, the latter moves in flow direction. Due to this movement a switch contact of the lower switching system is actuated so that the transformer is disconnected. After response the damper is locked and retained in this position even after reduction of the flow rate. Now the damper and, hence the lower switching system have to be unlocked by turning the test key anticlockwise.

    Technical data

    VoltageAC 5 V - max. 250 V
    DC 5 V - max. 250 V
    CurrentAC 0,01 A - max. 6 A
    DC 0,01 A - max. 6 A
    Cos φ > 0,5
    L/R < 40 ms
    Switching capacityAC max. 1500 VA
    DC max. 1250 W
    Dielectric strengthAC 2500 V
    AC 2000 V (NO contact, NC contact)
    AC 1000 V (CO contact)
    Between electric circuit and earth
    Between open contacts

    Temperature range:

    • Ambient temperature

    - 40 °C to +55 °C
    - 40 °F to +131 °F

    Climatic testing acc. to
    DIN EN 60068-2-78

    Others on request

    (Insulating liquid based
    on esther)

    Temperature range:

    • Operating range
      • Temperature of the insulating liquid

    - 40 °C to +115 °C
    - 40 °F to +239 °F
    till +135 °C requires code 21

    Temperature range:

    • Operating range
      • Viscosity of the insulating liquid

    1 mm²/s to 1100 mm²/s
    Insulating liquidMineral oilOthers on request
    Resistance against vibration

    Vibration: 2-200 Hz, 2 g
    Shock:     25 g, 6 ms

    Acc. to class 4M6 following 
    DIN EN 60721-3-4
    Resistance to pressure0,25 MPa 
    Resistance to vacuum< 2,5 kPa 
    Insensitivity to magnetic fields25 mTStatic magnetic field of any
    direction and polarity
    Switching system:
    • Number of switching contacts
    • Switching element
    • Damper


    Response time of damper

    Magnet contact tube
    Held by magnets


    < 0,1 s

    More on request
    Response of switching system
    in case of:
    • Gas accumulation
    • Insulating liquid flow
      Pipe diameter DN: 25 mm

    200 cm³ to 300 cm³
    01 = 0,65  ± 15%
    02 = 1,00  ± 15%
    03 = 1,50  ± 15%

    Cable glandM20x1,5; M25x1,5Others on request
    IP codeIP 56Others on request: IP 56, IP 66
    Casing colourTwo-component texture paintOn polyurethane basis


    Cable gland *

    M20x1,5: 1 cable gland and 1 dummy plug1
    M25x1,5: 1 cable gland and 1 dummy plug2
    M20x1,5: 2 cable gland3
    M25x1,5: 2 cable glands4
    M20x1,5: 1 Harting connector and 1 dummy plug5

    * Mandatory order data

    Casing colour *

    Casing colour RAL 7001 (silver-grey)41
    Casing colour RAL 7012 (basalt-grey)42
    Casing colour RAL 7022 (umber-grey)43
    Casing colour RAL 7033 (cement-grey)44
    Casing colour RAL 7038 (agate-grey)45
    Casing colour RAL 7035 (light-grey)46
    Casing colour RAL 7016 (anthracite-grey)47
    Casing colour RAL 9002 (grey-white)48
    Casing colour RAL 7032 (siliceous-grey)49

    * Mandatory order data


    With oil drain plug (only double-float Buchholz relays)28
    With premounted Harting connector
    (The option is indicated by a letter after the code.
    For further information, please ask for special reference material.)


    Special designs

    Climate-proof version/IP Code

    Climate-proof version for extreme frigid open-air conditions till - 40 °C34
    Climate-proof version for offshore36
    IP code 5638
    IP code 6639


    Insulating liquid

    Insulating liquid silicone oil20
    Insulating liquid based on esther21


    Switching system

    Lower switching system eqipped with two magnet contact tubes25


    Special request

    Special request (on special agreement with customer)29



    Please contact us! We are glad to advise you and help with your selection.

    Otto-von-Guericke-Allee 12
    39179 Barleben
    Phone +49 (0) 392 03 / 79 - 0
    Fax +49 (0) 392 03 / 53 30


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