Is there any contract/agreement with the contractor ? If so, terms and conditions have to be seen. I am not aware of full facts of your issue. However, I am giving general information.
General information for your knowledge
Exemption from electricity is conditional. It is exempted only if transmitted or distributed by an electricity transmission or distribution Utility (i.e. Electricity Board, Central Electricity Authority, State Transmission Utility or Central Transmission Utility ). Read Serial no.25 of Notification No.12/17-CT(Rate) dated 28.6.17. No exemption is available to you.
996912 Electricity Distribution services
This service code includes services involving distribution of electricity; maintenance of electric meters.
Rate of tax is 18% vide Notification No.11/17-CT (Rate) dated dated 28.6.17 as amended up to 30.9.19. It may be composite supply.