Premium Membership ♕

CYBER MONDAY OFFER 💥 Save 20% on PRO Membership Plan and Video Courses with the coupon CYM23 and learn from experienced engineers.

Home / Technical Articles / D.M.C.R. Protection Relay for Oil Transformer
D.M.C.R. Protection Relay for Oil Transformer
D.M.C.R. Protection Relay for Oil Transformer


Introduction to D.M.C.R.

The DMCR is a protection relay designed for the hermetically sealed oil immersed transformers without gas cushion. This device enables complete control of the tank’s internal parameters, i.e. pressure, temperature, oil level and gas detection.

Fitted directly to the transformer cover, this relay ensures protection against internal faults, prolonged overvoltages and fire risks associated with the use of inflammable dielectric fluids.

The accessory continuously monitors:

  1. Dielectric fluid level,
  2. Tank internal pressure,
  3. Dielectric fluid temperature at two different thresholds.

To operate properly, the protection relay mustbe fully filled with fluid (level higher than the float visible in the transparent section of the unit). If this is not the case, check the instructions which appear inside each casing.

To ensure optimum protection, the following action and adjustments are recommended:

FindingRecommended adjustmentFault detectedAction to be taken
Gas emitted or drop in levelLarge float at the topSerious faultDe-energize unit
Pressure switch0.20 barSerious faultDe-energize unit
Thermostat threshold 190°COvervoltagesActivate alarm
Thermostat threshold 2100°COvervoltagesDe-energize unit

There are two types of transformers:

  1. The air-cooled transformer, also called dry transformer
  2. The oil immersed transformer. The transformer is immersed in an oil-filled tank.
The DMCR is a protection relay designed for the hermetically sealed oil immersed transformers without gas cushion. This device enables complete control of the tank’s internal parameters, i.e. pressure, temperature, oil level and gas detection.

Go to Content ↑

Faults detected on live transformer unit

1. Dielectric fluid level is detected as low

The protection relay is empty and the large float is at the bottom. May be due to:

1.1 Air entering

There must be a dielectric fluid leak and thus oily stains should be visible on the ground. Accurately localise the fault, carry out repair, then recheck the level with the body of fluid at a temperature of 20°C, before switching the unit live again.

1.2 Internal gas emission

Gas should be sampled using a syringe and then analysed. Whilst awaiting results, under no circumstances should the transformer be switched live again because a risk of total destruction exists.

Go to Content ↑

2. Overheating is detected

This may be due to:

  1. Improper cooling of the transformer (insufficient air flow around unit or plan-troom ventilation),
  2. Continuous overvoltage.

Go to Content ↑

3. Excess pressure is detected

This may be due to:

  1. Overheating
  2. Internal gas emission
  3. Topping up of dielectric fluid with the body of fluid below 20°C; drain the over-flow with the transformer de-energised and the dielectric fluid at 20°C.

Go to Content ↑


This protection relay has been designed according to the European standard EN 50216-3, specification which came into action on the 5th of June 2002.

This standard applies to protection relays for hermetically sealed oil immersed transformers, (in accordance with the EN 60076 standard) and induction coils (in accordance with the EN 60289 standard) without gas cushion for an indoor or outdoor use.

The DMCR relay is an IDEF Systemes design, made in France. It has two French and one European patents.

DMCR relay dimensions
DMCR relay dimensions

Go to Content ↑

Possibilities of D.M.C.R.

1. Oil level control and gas detection

The DMCR enables to control both the oil level and the presence of gas inside the transformer’s tank. The DMCR body is a small see-through tank fitted on to the transformer’s tank. Should gas form inside the transformer, it will then accumulate inside the DMCR and cause the oil level to drop.

Visual information

The level drop is first visible through the lowering of the small red float inside the upper part of the DMCR, followed by the lowering of the main red float.

The 360 degree visibility is a specifically designed and patented system.

Electrical information

A circular magnet is fitted in the main float and it acts upon a magnetic changeover contact (REED switch) sitting inside the brass tube that runs through the float. The lowering of the float triggers the activation of an electrical contact, through the magnet’s motion.

A bleeding system facilitates gas collection inside the relay so that one can analyze it and understand the reason for its presence. The bleeding system has a male G1/8’’ thread, accordingto the standard.

For the oil level and gas detection control, the contacts have been chosen in order to use the REED switch’s working contact when the float is in a high position. This means that, in such a position, i.e. with a normal oil level and therefore normal conditions of use, the contact has already switched.

This is a positive safety system. With this method, any potential damage to the switch is immediately spotted.

The DMCR below is filled in with oil: the main float and the secondary float are both in high position.

The DMCR - Oil level control and gas detection
The DMCR – Oil level control and gas detection

Go to Content ↑

2. Pressure control

Pressure inside a transformer’s tank can increase significantly when:

  1. There is a temperature rise due to the transformer charge: oil expands and pressure increases
  2. An internal short-circuit occurs and provokes an oil temperature rise.

An adjustable pressure captor detects overpressure in the transformer’s tank. It features a changeover contact actuated by a soft membrane which deforms under pressure.

The DMCRs pressure captor
The DMCRs pressure captor is accessible from above, once the top removed

Go to Content ↑

3. Temperature control

Visual check:

A needle thermometer indicates the temperature inside the transformer.

Electrical check

Two identical adjustable thermostats detect potential over-heating inside the transformer.

The thermostats feature a changeover contact actuated by a diaphragm linked by a capillary tube to a temperature probe sitting deep inside the central brass tube, which is immersed in the transformer’s tank. The capillary tube and probe are filled in with a liquid which expands proportionally to the temperature surrounding the probe.

The ALARM thermostat detects a primary temperature threshold. The TRIPPING thermostat detects a secondary temperature threshold, superior to the first.

DMCR - Tripping and alarm treshold settings
DMCR – Tripping and alarm treshold settings

Go to Content ↑


  • MINERA Transformer Installation guide – Schneider Electric
  • DMCR relay – IDEF Systems

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Daniel Rdmond
    Nov 21, 2023

    Good morning
    This is Daniel Marketing for ANA Group Iraq.
    Regarding the transformer power, we are interested in supplying Transformer power accessories (D.M.C.R. Protection Relay for Oil Transformer).

    could you please feedback?
    I am waiting for your response.
    Best Regards
    Marketing for ANA Group

  2. Waqas Ahmed
    May 20, 2022

    due to high ambient temperature cracks are occurring on the DMCR, how to repair.
    if you not have a solution. please tell me the material type of DMCR jar.

    Sep 23, 2020

    Buen día estimados quisiera adquirir 2 reles me podria ayudar con algun contacto para la compra gracias

  4. David Alexander
    Apr 20, 2020

    Please can you provide the part number for the large o-ring (200mm long round) that goes under the plastic cover you remove to get access to the electrical connections.
    I have a DMCR2744.

  5. Hennie
    Dec 30, 2019

    How to reset the DMCR relay please.

  6. Hennie
    Dec 12, 2019

    How do I reset the DMCR BUCHOLTZ RELAY. The relay is full of oil and no air /gas bubble available.

  7. Muhammad Imran
    Aug 01, 2019

    On my site gsu tripped on Pressure Trip signal receive from DMCR .But visual inspected GSU temperature and PRD Working normally.DMCR pressure trip active.Please suggest me what i do against this matter.

  8. premkumar
    Jan 09, 2019

    who is the supplier for DMCR in india? any agencies in Chennai?

  9. Jorge García
    Jan 29, 2018

    Buenas tardes.
    Estoy en México y necesito la cotización de un Relé tipo DMCR.
    por favor enviarme el contacto de quien pueda cotizar.
    o bien enviarme la cotización lo antes posible.

    Jorge García
    Cel: 55 4909-0792

  10. behram khan
    Jan 23, 2018

    Dear Sir,

    I have different situation small float is at lower level and main float is at maximum level .

    what is the caes .

    • ben youssef rekik
      Oct 17, 2020

      purger le tube /faite un petit appoint 20°C

  11. John
    Dec 12, 2017

    Can extreme cold ambient temperature cause the unit to trip on over pressure ?

  12. amin
    Apr 11, 2015

    Thanks TeamWork

  13. Teguh Santoso
    Oct 20, 2014

    DCMR is the same as DGPT, right?

  14. Olarotimi
    Jan 29, 2014


Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

84  ⁄    =  fourteen

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge