Ring Main Unit Testing and Commissioning Method Statement

In a ring main unit distribution system feeder covers the whole area of supply in the ring fashion and finally terminates at the substation from where it is started. The unit forms a closed loop and looks like a ring. The advantages of RMU is that less conductor material is required as each part of the ring carries less current than in the radial system. Other benefit is less voltage fluctuations. Although it is difficult to design when compared to the designing of a radial system.

Below is the list of required tools for ring main unit testing and commissioning, that should be arranged before starting the testing and commissioning:

  • Primary Injection kit
  • Contact Resistance kit
  • High Voltage test kit
  • 5.0 KV Megger
  • Multi meters (Fluke Meters)

RMU Commissioning Steps

The following sequence will be carried out for testing.

  • Check the Bus bar tightness check with torque table mentioned below
  • Check the Earthing Busbar tightness check with torque table below as well as measure and record corresponding contact resistance
  • Check the contact resistance for the bus bar I breaker as per the procedure mentioned below Check the Breaker timing test.
  • Connect the auxiliary supply to the respective cubicles as per the schematic drawing
  • Check the ratio/ circuit for the current transformer by primary injection.
  • Check the secondary injection for relays if required. g. check the functional test for RMU.
  • Check the remote signals up to our Panel terminal blocks only.
  • Check the High voltage test for the switchboard
  • Insulation resistance test of CT and switchgear.
  • When Installation is completed, carryout testing and commissioning as per the procedure below.

Torque Table for Bolt Tightening




N – m

Ft – lb N – m Ft – lb N – m

Ft – lb


9 19 14 28 20.6 25



11 37.5 27.7 50 36.9 50



14 62 45.7 75 55.3 79 58.2
14 16 98 72.3 120 88.5 125


Measurement of Contact Resistance on Busbars

Inject successively a DC current of 100A for each phase at level of connections and note down the Low resistance value which shall be in Micro ohms. Or use an apparatus with direct reading (micro ohmmeter), injection 100A DC current.

Inject current from 100A DC output to both sides of the test object. Input VDC measures the voltage drop at the test object, the current is measured internally, and from these values the software calculates the test object’s resistance.

Primary / Secondary Insulation Resistance Measurement

  • Remove CT earth connection on secondary terminals Measure insulation resistance between core & ground Equipment to be used: 5 KV Insulation testers.
  • CT primary is insulation resistance measured by applying 5KV DC. When meggering each phase other phases will be kept earthed.
  • Secondary circuit Insulation resistance measured by applying 500V DC  to each core with respect to earth and core to core .
  • Insulation between Primary and Secondary winding

Circuit Breaker Testing

Insulation Resistance Measurement

When the breaker in closed position measure the insulation resistance Phase to earth with 5.0 KV DC. Then open the breaker and measure the insulation resistance between pole to pole. The above, will be carried out for all the phases.

Contact Resistance Test

The breaker will be in closed position before starting the test.

Test: Inject successively a DC current of 100 A for each phase top and bottom, and measure the milli voltage drop by the multimeter. Otherwise connect the contact resistance kit and measure the resistance values in micro ohms directly.

High voltage output Clearances

The  high  voltage  output  from  the  unit  Is  from   a  screened  cable  with  an  un-screened termination,  and adequate clearances  must be maintained between the following parts and any other conducting object (whether earthed or not):

  • HV output connection
  • Non-screened part of output cable (between the bulge In the cable and the output clip)
  • Non-grounded parts of test object

Any part of the test object not connected to earth should be considered live at the test voltage. Particular attention should be paid to clearances between any parts of the test object at test voltage potential and the test enclosure or barriers.