Main Distribution Board (MDB) testing and commissioning

MDB is panel under power distribution system which consists of a fuse, circuit breakers and ground leakage protection units. Where electrical energy is taken from a transformer or an upstream panel to distribute electrical power to numerous individual circuits or consumer points.


After installing the panel including proper terminations into each breaker, the same shall be offered up for an inspection from consultants. Earth fault loop impedance test & earth leakage test for LV Distribution Board shall be done & recorded in the prescribed format.

There are different cases in testing the MDB:

Case 1: Only interconnecting cable between transformers and MDB’s

  • Physically inspect cable length for any damage and peel off of outer sheath

  • Inspect and confirm all cable endings (Breaker / Busbar / Terminal Blocks cables) including phase sequence/rotation and isolation sleeve.

  • Clean the panel before the tests, conduct the Megger test to check the insulation resistance. Minimum Insulation Resistance is 20 MΩ. Rectify problem if reading obtained is below 20 MΩ.

  • After physical inspection and megger test are passed or rectified, Turn ON the panel respectively while at the “OFF” position receiving the end incoming breaker.

  • Record the instrument reading.

  • Pad Lock receives the end Panel Incoming circuit breaker in the “OFF” position and returns the key to the administrator who keeps the key inside the key box.

  • Clean the area of the panel from conducting materials. Arrange and lay approved rubber mat around “energized” panel.

Case 2:

  • Repeat the steps in Case 1 for all cables coming to the panel.

Case 3:

  • In order to minimize unnecessary shutdown and or deprive others of the opportunity to access the “Upstream” panel, all work including cable laying and terminations removed from the “Upstream” panel must be completed before applying for PTW.

  • Physically inspect each cable length from the “upstream” panel to each sub-system such as MDBs, SMDBs, Lift and Escalator, AHUs etc. is properly terminated and free of cable skirmishes requiring repair work. Upon completion, proceed to step (iii).

  • Apply for a work permit (PTW) describing in detail the type of work to be carried out, the “extent” of the required electrical insulation, the place of work and the expected duration (time to complete work).

  • Confirm Administrator had an upstream and downstream circuit breaker Shutdown / Open Incoming and an appropriate “Warning Sign” displayed against these two circuit breakers.

  • Keep the key/s given by Administrator and repeat steps (i) through (iii) as in Case 1 until work is completed. Turn “Off” all feeder circuit breakers.

  • When satisfied with the work, withdraw the men and equipment from the site and return PTW.

  • Arrange a Final Inspection for “Complete” Panel/Switchboard.

  • Arrange Permit To Run (PTR) if you need to test run downstream sub-system like AHU, FCU, Pumps etc.