Monitoring Relays

EMB monitoring relays feature easy operation, high reliability and a very long life.

Monitoring relay for tap changers

The monitoring relay for tap changers, also known as the protection relay for tap changers or oil flow relay, is a monitoring device for insulating liquid-filled tap changers with conservators. It protects the tap changer and the transformer from damage. The monitoring relay responds to excessive oil flow in the direction of the conservator and generates a signal disconnecting the tap changer and the transformer immediately from voltage supply.


The monitoring relay is suitable for open-air as well as indoor installation.


We provide more than 45 years experience in producing monitoring relays and other protection devices for liquid-cooled and liquid-insulated appliances. It ranks among the most distinguished manufacturers of this type of equipment.

EMB monitoring relays feature easy operation, high reliability and a very long life.

Our staff of highly qualified engineers and experienced skilled workers ensures top quality and highprecision products. The casings are machined on modern CNC controlled machine tools. All products are subjected to final inspection when all functions are checked using special test equipment.

Profound experience and expertise in this special field are a sound basis for high product quality. Extensive references from reputed tap changer and transformer manufacturers as well as other users are proof of the high qualitative level of the products.



The casing is made of weather-resistant cast aluminium alloy provided with a paint coat. To check the switching system for proper function, the casing is provided with sightglasses arranged opposite each other and protected by hinged lids (1).


The cover is made of weather-resistant cast aluminium alloy provided with a paint coat. The upper section of the cover accommodates the terminal box (1). The test and reset key covered by a cap nut (2), a screw plug (3) or, alternatively on customer’s request, a bleeder valve (see section 9. „Explanation on code 97“) are arranged in front of the terminal box. In addition a plate (4) with instructions for actuating the test and reset key is arranged there. The terminal box (1) accommodates the earth terminal (5) and the bushings (6) for the terminals provided in the base of the cover. The terminal box is sealed by an aluminium cap (7) so that it is safe to touch and protected against pollution. If the cap is opened the graphic symbol and the connection diagram (8) are shown. The cable is inserted through the cable gland (9).


The monitoring relay is installed in the pipe between the tap changer and the conservator as close as possible to the tap changer head. During normal operation of the tap changer gases are generated which are collected in the gas dome of the monitoring relay. If the gas volume exceeds the volume of the dome, the gas flows in the direction of the conservator.

Fault: An incident causes an oil flow in the direction of the conservator.

Response: The liquid flow reaches the damper. If the flow rate exceeds the operating value of the
damper, the latter moves in flow direction.

This movement actuates a switch contact, and the tap changer and the transformer are

Technical data

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

Temperature range:

  • Ambient temperature

- 40 °C to +55 °C
- 40 °F to +131 °F

Climatic test acc. to
DIN EN 60068-2-78: 2002-09

others on request

Temperature range:

  • Operating range
    • Temperature of insulating liquid

- 40 °C to +115 °C
- 40 °F to +239 °F
Up to + 135 °C requires version 21

Temperature range:

  • Operating range
    • Viscosity of insulation liquid

1 mm²/s to 1100 mm²/s
Insulating liquidMineral oilOthers on request
Resistance to vibrationVibration: 2-200 Hz, 2 g
Shock: 10 g, 11 ms
Resistance to pressure0,25 MPa 
Resistance to vacuum< 2,5 kPa 
Resistence to magnetic fields25 mTMagnetic balance in any direction and
Switching system:
  • Number of switching contacts
  • Switching element
  • Damper


Response time of damper

Magnet contact tube
Held by magnets


< 0,1 s

See ordering data

Cable glandM20 × 1,5; M25 × 1,5Others on request
Nominal installation position2° to 4°Ascending towards conservator
IP codeIP 56Others on request
Casing colour2-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 glands3
M20x1.5: 2 cable gland and 1 dummy plug (enclosed)3B
M25x1.5: 2 cable glands4
M25x1.5: 2 cable gland and 1 dummy plug (enclosed)4B
1/2“ NPT: 1 cable gland and 1 dummy plug6
1/2“ NPT: 2 cable glands7
Cable gland: on customer’s request9

* 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


Metal name plate15
With bleeder valve97
With gasket (ÜRF 25/10-26 only)98


Special designs

Climate-proof version/IP code

Climate-proof version (extreme frigid open-air conditions below - 40°C34
Climate-proof version for offshore36
IP 6639


Insulating liquid

Insulating liquid, silicone oil20
Insulating liquid on ester basis21


Switching system

Switching system equipped with two magnet contact tubes25
Switching system equipped with three magnet contact tubes99


Customer's request

Special request (as agreed 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