Testo 770-3
One centimetre here and there is really not inconsequential, use an electric measuring tool to be safe. There's no cord to restrict you, just make sure you've charged the battery.
Conrad Electronic
testo 770-3 Clamp meter, Handheld multimeter Digital CAT III 1000 V, CAT IV 600 V Display (counts): 6000
£256.56
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