Perancangan instalasi proteksi differensial pada transformator bay sangat diperlukan untuk mengamankan transformator yang merupakan peralatan utama dalam sebuah GI dari gangguan hubung singkat yang terjadi didalam daerah pengamanan transformator. Relay differensial bekerja dengan cara membandingkan arus yang masuk dan arus yang keluar dan relay ini bekerja instant. Dalam skripsi ini akan dibahas perancangan instalasi proteksi differensial ini meliputi perancangan sistem proteksi differensial dimana terdapat peralatan listrik yang harus dipilih agar aman memenuhi persyaratan K3. Sehingga instalasi itu aman dari ancaman-ancaman yang mungkin terjadi. Pemilihan peralatan listrik harus memperhatikan persyaratan yang diatur dalam PUIL. Pemilihan peralatan listrik yang di atur dalam PUIL meliputi faktor-faktor dari segi baik, berfungsi, standar, harus mampu dialiri hubung singkat minimal satu detik, sesuai pembebanannya, penggunaannya harus sesuai, dan segi lingkungan instalasi tersebut dipasang. Pada perancangan instalasi proteksi differensial harus dapat diandalkan, selektif, cepat beroperasi dan sensitif. Pada instalasi proteksi differensial peralatan listrik hanya boleh dipasang dan dipelihara oleh karyawan yang memiliki sertifikat ahli listrik, karena instalasi tersebut berada di ruang kerja listrik terkunci dan hanya boleh dimasuki minimal dua orang.

The design of a differential protection installation on the transformer bay is necessary to secure the transformer which is the main equipment in a GI of short circuit interference that occurs within the transformer security zone. Differential relay works by comparing the incoming and outgoing currents and these relays work instantly. In this thesis will be discussed design of this differential protection installation includes design of differential protection system where there are electrical equipment that must be selected to be safe meet the requirements of K3. So the installation is safe from possible threats. Electrical equipment selection should take into account the requirements set forth in PUIL. The selection of electrical appliances set in the PUIL includes factors in terms of good, functioning, standard, must be capable of running short circuit at least one second, according to the loading, its use must be appropriate, and the environment of installation is installed. In the design of the installation of differential protection should be reliable, selective, fast operating and sensitive. In the installation of differential protection electrical appliances should only be installed and maintained by employees who have electrical expertise certificates, because the installation is located in the locked electrical workspace and should only be entered at least two people.