You are buying 1x Mains Uk socket Tester.

Easily test sockets and trailing mains sockets to ensure they are wired with the correct polarity. 
This is a simple but effective plug , ideal to keep in your toolbox.
Test new and existing wiring in one safe and simple operation.

Test live neutral and earth circuits
 
This plug checks for 

Instruction:
• There is no requirement of setting the plug tester. Check the function of the tester by inserting into a proper socket outlet
with all three indicator lamps light up. The tester is ready for testing.
• Make sure the mains is turned on.
• Insert the plug tester into the socket outlet that requires testing.
• If all the three indicator lamps light up, the socket is wired correctly.
• If other situation happens, please refer to the chart on the plug tester.
Consult a qualified electrician to rectify the situation.
• Unplug after use.