The best designed machinery in this universe is the human body. It has an excellent built-in self-defense and self-repair system. Even that highly intelligent system needs occasional repair and maintenance. And so does every manmade system, including solar PV installations. Within the solar installation is the inverter which receives Direct current (DC) from solar strings as input and puts out Alternating Current (AC) to the grid on the output end. During installation, routine maintenance and emergencies it is necessary to isolate the panels from the AC side, and hence, a manually operated isolating switch is placed between the panels and the inverter input. Such a switch is called a DC isolator because it provides DC isolation between the photovoltaic panels and the rest of the system.
This is an essential safety switch and is mandated in each photovoltaic power system according to IEC 60364-7-712. The corresponding British requirement comes from BS7671 – Part 712.5184.108.40.206, which states “To allow maintenance of the PV convertor, means of isolating the PV convertor from the DC side and the AC side must be provided”. Specifications for the DC isolator itself are given in the “Guide to the Installation of PV Systems”, section 2.1.12 (Edition 2).
A DC isolator can be single pole or double pole. A single pole isolator can be used when a grounded array is to supply a transformer based inverter (inverter feeds the grid through an isolating transformer). When an ungrounded array is required to feed a transformer-less inverter, a double pole DC isolator switch is required. Models of isolators are also available which have an AC isolator in addition to a DC isolator.
The switch must be rated to handle approximately 20% above the open circuit array voltage, and 25% above short circuit current of the array. This margin is considered necessary because interrupting DC is more difficult than interrupting AC. To learn more : https://www.quora.com/What-is-the-difference-between-an-AC-isolator-and-a-DC-isolator-Can-they-be-used-interchangeably