Martindale Electric PC15250 Mk2 Instruction Manual page 6

The socket tester is not intended to determine if circuits being
tested are dead. The correct testing methods should be used to do
this.
If the LED indicators do not illuminate, this does not necessarily
mean all three phases under test are dead. Two phases could be
open circuit, but one phase could still be live.
Caution
Avoid severe mechanical shock or vibration and extreme
temperature.
3