I have had a few questions recently about how you NULL an earth lead on a Martindale HPAT tester. Interestingly it is missing from the manual?

So here is the instructions……

[From Martindale]

Occasionally the HPAT500 may call for the earth test lead to be nulled (or zero’ed) in order to remove its nominal internal resistance from the overall test calculation.

This may occur if the lead becomes bad or damaged in use, or if a non-standard replacement test lead is used. The unit can be recalibrated with the following procedure.

• Power off the pat tester.

• Attach the test lead.

• Connect the test lead probe to the EARTH pin on the front panel 13amp test socket.

• Hold down the Class One and Class Two buttons while powering up the device*

• The NU symbol on the right of the display will light up.

• Within 5 seconds – Press the LEAD button to initiate the calibration.

The unit will run through the calibration check and if the lead passes will return with OK – you can then carry on

PAT testing as normal.

* Note – activating this procedure can be tricky on software versions earlier than v5.06 – you may need several attempts at entering NULL mode.