EN 50539-11 is the European standard that specifies the requirements and tests for surge protective devices used in photovoltaic applications. A photovoltaic installation is unlike a conventional electrical installation in ways that matter for surge protection: the source is direct current and it cannot be switched off, so the array remains live while it is illuminated; the modules are mounted outdoors over a large area and the direct-current cables between them are long and act as an efficient receiving antenna for lightning; and the operating voltage of a string is high enough that a device intended for a low-voltage alternating-current circuit cannot be used. The standard therefore defines the specific characteristics of a photovoltaic surge protective device, including the maximum continuous operating voltage appropriate to the array, the impulse current and the voltage protection level, and it specifies the test programme, which combines the impulse current tests with the tests that establish the behaviour of the device under the continuous direct-current voltage of the array.