Metered Ring Main Unit Testing and Commissioining Method Statement -
Metered Ring Main Unit T&C method Statement

Metered Ring Main Unit or MRMU is one of the major items that can be found nowadays in construction projects.

In this article, I will explain step by step how to perform the testing & commissioning for a metered ring main unit – MRMU In your project.

1- Introduction

System Description: 

In an electrical power distribution system, a Metered Ring Main unit (MRMU) is a factory-assembled, metal-enclosed set of switchgear used at the load connection points of a ring-type distribution network. It includes in one unit two switches that can connect the load to either or both main conductors and a circuit breaker and switch that feed a 13.8 KV MVSGR. 


The scope of work is to conduct Testing and commissioning for Metered Ring Main Units (MRMU) to confirm readiness to be placed safely in service, ensuring the project performance requirement and preserving a log of the testing measurements taken during commissioning for future reference.


The purpose of this method statement is to define the step-by-step procedure to implement the correct practices for the Testing & commissioning of Metered Ring Main Units (MRMU).

This is through the guidelines contain here to ensure that the job execution complies with the project specifications and serves the intended function to a satisfactory level.

2- Prerequisites to Procedure

Site Accessibility and Safety:

Free access is provided to officially appointed personnel. Only a Safe environment is provided in electrical rooms.

 Documentation for inspection:

A method statement is approved, including test forms.

Quality Control Test reports and submittals:

  • WIR for installation are cleared from major items
  • WIR for testing all the cables connected to the Metered Ring Main Units is approved.

Power on requirement

All Electrical Pre-commissioning to be completed, witnessed, and approved.

3- Design Parameters

  • Relevant manufacturer recommendations
  • Relevant design and document approvals.
  • Relevant materials approvals.

4- Reference Documents/Drawings 

Specification Reference:

Specification of the project 16320

Relevant Codes / Standards:

  • NFPA 70
  • NEMA
  • IEC 265
  • IEC 282
  • IEC 296
  • IEC 529
  • SASO
  • UL

Approved Drawing Reference:

Related approved schematic drawings references to be attached to WIR & test package

Approved Schematic Drawing Reference:

Related approved schematic drawings references to be attached to WIR & test package

Relevant Approved Material Submittal:

Related approved Material Submittal references to be attached to WIR & test package.

5- Pre-Start Checks/Pre-commissioning

List all pre-commissioning checks:

The following Checks shall be carried out for Metered Ring Main Units (MRMU):

  • Ensure that the Metered Ring Main Units installation is completed & all Electrical and Mechanical comments on installation completed and approved.
  • Check and ensure that no wet services are running near or above the Metered Ring Main Units.
  • Check the Metered Ring Main Units are free from any foreign / construction debris.
  • Ensure that no damage has occurred on the component and enclosure of the Metered Ring Main Units.
  • Ensure that the Metered Ring Main Units are properly tagged and identified as specified in the drawings.
  • Ensure that the field power & control cables are properly tight and secure into the Metered Ring Main Units.
  • Ensure that the Earthing of Metered Ring Main Units has been completed.
  • Ensure that the rating of Breakers is as per the approved shop drawing.
  • Ensure that the interlocking is done as specified in the drawing and ensure that its operation.
  • Ensure that the cable gland is securely tightened and adequate bend radii are allowed for all cables.
  • Check lugs are crimped rigidly.

6- Commissioning/Test Procedures

Insulation Resistance Test:

  • Disconnect all control Fuses, MCBs trip units, and Neutral wires Prior to performing the Test.
  • Clean all the supporting insulators.
  • Connect lead wires and apply the rated DC voltage between the `Brown’ Phase to ‘Black’ Phase, ‘Black’ to ‘Grey’ phase, and ‘Brown’ to ‘Grey’ Phase.
  • Conduct the Test between Phase (All phases shorted) to Neutral and Phase (All Phase & Neutral shorted) to Earth.

Current Transformer

  • Ensure the physical condition of CT is OK before starting the Test.
  • Keep the Secondary circuit of CT in an open condition suitable to connect the Testing Equipment.
  • Check the CT’s secondary winding resistance using a Multimeter to ensure the CT is not Open Circuit.
  • Input the CT ratings in the test kit.
  • Connect the Primary Injection of the Kit output to the CT primary (Busbar) as per the correct polarity (P1-P2).
  • Connect the Input of the Kit to the Secondary of the CT as per the correct polarity (S1-S2).
  • Inject the Current and note the readings / actual ratio.
  • Ensure the polarity is OK in the Test.
  • Repeat the Test in another Phase CT’s also.
  • Calculate the percentage error with respect to the results.

Over Current/Earth fault Relay:

  • Check the over current & Earth fault relay settings and verify with the approved settings.
  • Check that the necessary protection functions are enabled as per the relay coordination study.

Over Current/Earth fault Relay Primary Injection:

  • Make sure that the CT connection is done as per the schematic diagram.
  • Ensure that the Protective Device (Relay) is powered up, and the protection function (51, 50n, 51N, etc.) under Test is enabled, and all other functions are disabled.
  • Start injecting current and ensuring that the reading on the protective device matches the one on injection equipment.
  • Keep increasing the current until the circuit breaker trips.
  • Repeat the above steps for phases R, Y, and B.
  • Repeat the above for the remaining protective functions.


  • Checks that all current meters are working properly.
  • Checks that all voltage meters are working properly.
  • Check that all power meters are working properly.

Busbar Torque Test (as per attached table)

  • Refer to the table below for the Torque Wrench setting concerning the hardware size. 
  • Ensure the proper tightness of the busbar joints with the torque wrench. 
  • After confirming the correct tightness, apply a red mark on the hardware for indication. 
  • Check for outgoing wire/cable continuity for each circuit as per the approved schematic wiring diagram.

7- Interface Procedure

  • Verify that the required connection to BMS is available.
  • All connections made between MRMU and BMS will be tested during BMS System Pre-Commissioning.
  • Permanent power should be provided.

The following video is about the Factory acceptance test of RMU manufactured by ABB, have a nice watching.

8- Documentation

List all relevant test sheets.

  • T&C- MRMU Pre-Commissioning Check Sheet.
  • T&C- MRMU Commissioning Check Sheet.
  • Instrument Log.

9- Demonstration Procedures

  • After completing the procedures mentioned in section 6.0, the Inspection request with all relevant documents must be sent to Main Contractor for review and inspection.
  • After initial verification by the Commissioning Manager, the PMCM will be invited to witness the works.
  • After attaining approval, test/report sheets will be signed off from all mentioned in the signature block.
  • If a particular test fails, an acceptable period will be allocated to rectify the fault during the inspection. 
  • Test/report sheets to be distributed as per Project Specification. 

10- Instrumentation

Testing Equipment:

A- Mega ohmmeter.

An instrument for insulation resistance measuring

B- Torque Wrench.

It’s a tool for setting and adjusting the tightness of nuts and bolts to the desired value.

C- Multimeter.

It’s An electronic measuring instrument that combines several measurement functions in one unit.

D- Digital Clamp Meter

An instrument for measuring

E- Hand Tool Box

Assorted hand tools


  • All instruments, testers, and tools will be calibrated and have a calibration certificate not older than 12 months on the Test date.
  • A copy of the certificate will be submitted prior to the Test, and it will be available at the site with the device.

11- Staffing

  • Testing & Commissioning Engineer
  • Facilitates all Testing and commissioning of MEP installations of the project, Testing, and commissioning, start-ups, and documentation

Senior Electrical/Mechanical Engineer:

Controls all the electrical/mechanical installation management processes.

Safety Engineer:

Responsible for ensuring the safety of the workers as well as the workplace during Testing and commissioning.

Commissioning Manager 

Verify and approve the entire Testing and commissioning of electrical/mechanical installations and documents before requesting inspections to PMCM/ Client representative.

PMCMs / Client Representative:

Verify the entire Testing and commissioning of electrical/mechanical installations to witness and approve the Test on site.

QC Engineer:

  • Coordinates with the site Engineers and Client Representatives with regards to Testing and commissioning, Hand-over process.
  • He is responsible for witnessing the Test and will assure all installation and Testing are approved base on specifications and standards.
  • He should attend during Inspection, Testing & Commissioning, and Hand-over.

Site Foreman/ Electrician:

To assist with the Testing and commissioning for electrical/mechanical works.

Supplier’s Representative:

To assess the Testing and commissioning on their owned/supplied electromechanical equipment.

12- Safety Aspects & Permit to Work Requirements

12.1- Health and Safety

12.2- Introduction

  • All sub-contractors will comply completely with the approved HSE plan / Ref.
  • The contractor must adhere to the client his health Safety& environment Policy.

12.3- Safety Aspects & Permit to Work Requirements

  • All employees involved in the activity shall undergo a site HSE induction course prior to the worksite.
  • According to the approved “Project Health, Safety and Environment Plan, ” site activities shall be carried out safely.”
  • The commissioning team must follow the client’s permit to work system (Internal) and Client LOTO procedures.
  • The commissioning team must read and understand and follow the Client LOTO procedures.
  • Designated permit issuer/receiver or coordinator shall be appointed to control the commissioning area and commissioning works.

12.4- Training

  • All commissioning & testing personnel will attend a Safety Commissioning
  • A meeting and pre-commissioning safety training prior to work commencing.

12.5- Hazards Related to Testing & Commissioning

The following HSE training shall be delivered to the personnel but not limited to:

  • Working at height.
  • Hazards from electricity.
  • Use of Power/hand tools.
  • Night working.

12.6- Risk Assessment and Control

  • HSE Hazards involved in the activity have been assessed through the Hazards & Effects Management Process (HEMP) for every activity, and that the outcome is recorded. 
  • The contents in HEMP Register shall be disseminated within the shift supervisor/foreman’s teams through toolbox talks.
  • Any new hazards identified other than in the HEMP regarding site conditions shall be assessed, and the Site Engineer/Supervisor shall take control measures prior to commencement of any activity.

The following control measures generally apply to all hazards that shall be adopted as a proactive measure, in addition, the specific controls identified in the HEMP register:

  • Correct use & Maintenance of tools and equipment.
  • Deployment of competent personnel for the task.
  • Conduct daily toolbox talks prior to work commencement.
  • Good Housekeeping.
  • Adequate and effective supervision.
  • Provisioning appropriate PPEs based on the nature of the works.
  • Awareness about emergency response.

12.7- First Aid / Medical Facility / Emergency Response

With ready access to first aid boxes kept at the site, the Availability of First Aiders shall be ensured at the worksite.

13- Attachments with the Method Statement

  • Test Sheets.
  • T&C- Metered Ring Main Units (MRMU) Pre-Commissioning Check Sheet.
  • T&C- Metered Ring Main Units (MRMU) Commissioning Check Sheet.
  • Risk Assessment.

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